Add --no- prefix for bools (#455)

Implements support for --no- prefix negation on boolean and flag DU fields
when marked with [<ArgumentNegateWithPrefix>]. This allows both --flag and
--no-flag forms to be accepted, with --no- variants negating the value.

Changes:
- Extend ArgParserGenerator to generate --no- prefix handling
- Add conflict detection for overlapping --no- prefixed arguments
- Update help text to display both forms (e.g., --verbose / --no-verbose)
- Add test examples in ArgParserNegationTests.fs demonstrating:
  - Boolean field negation
  - Flag DU negation
  - Multiple ArgumentLongForm with negation
  - Combined features (defaults, help text)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.config/dotnet-tools.json

@@ -9,10 +9,10 @@
       ]
     },
     "fsharp-analyzers": {
-      "version": "0.33.1",
+      "version": "0.34.1",
       "commands": [
         "fsharp-analyzers"
       ]
     }
   }
-}
+}

.github/workflows/dotnet.yaml

@@ -167,12 +167,12 @@ jobs:
     - name: Pack
       run: nix develop --command dotnet pack --configuration Release
     - name: Upload NuGet artifact (plugin)
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v5
       with:
         name: nuget-package-plugin
         path: WoofWare.Myriad.Plugins/bin/Release/WoofWare.Myriad.Plugins.*.nupkg
     - name: Upload NuGet artifact (attributes)
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v5
       with:
         name: nuget-package-attribute
         path: WoofWare.Myriad.Plugins.Attributes/bin/Release/WoofWare.Myriad.Plugins.Attributes.*.nupkg
@@ -182,7 +182,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Download NuGet artifact (plugin)
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: nuget-package-plugin
           path: packed-plugin
@@ -190,7 +190,7 @@ jobs:
         # Verify that there is exactly one nupkg in the artifact that would be NuGet published
         run: if [[ $(find packed-plugin -maxdepth 1 -name 'WoofWare.Myriad.Plugins.*.nupkg' -printf c | wc -c) -ne "1" ]]; then exit 1; fi
       - name: Download NuGet artifact (attributes)
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: nuget-package-attribute
           path: packed-attribute
@@ -209,7 +209,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
       - name: Download NuGet artifact
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: ${{ matrix.artifact }}
       - name: Compute package path
@@ -249,7 +249,7 @@ jobs:
       contents: read
     steps:
       - name: Download NuGet artifact
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: nuget-package-attribute
           path: packed
@@ -268,7 +268,7 @@ jobs:
       contents: read
     steps:
       - name: Download NuGet artifact
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: nuget-package-plugin
           path: packed
@@ -294,7 +294,7 @@ jobs:
           extra_nix_config: |
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
       - name: Download NuGet artifact
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: nuget-package-attribute
           path: packed
@@ -332,7 +332,7 @@ jobs:
           extra_nix_config: |
             access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
       - name: Download NuGet artifact
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: nuget-package-plugin
           path: packed
@@ -368,7 +368,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
       - name: Download NuGet artifact
-        uses: actions/download-artifact@v5
+        uses: actions/download-artifact@v6
         with:
           name: ${{ matrix.artifact }}
       - name: Compute package path

CHANGELOG.md

@@ -1,5 +1,16 @@
 Notable changes are recorded here.
 
+# WoofWare.Myriad.Plugins 9.1.1, WoofWare.Myriad.Plugins.Attributes 3.8.1
+
+Adds the `[<ArgumentNegateWithPrefix>]` attribute, which can be placed on a boolean or flag-valued field when using the `ArgParser` generator.
+This causes the boolean to be specifiable with the `--no-` prefix to negate its value.
+(For example, `Foo : bool` is normally specified as `--foo`; this new attribute lets the user additionally give `--no-foo` to get the same semantics as `--foo=false`.)
+
+# WoofWare.Myriad.Plugins 9.0.1
+
+Converts the `static member Empty` field on each generated mock (from `GeneratedMock`) into a function, so as to permit the `GeneratedCapturingMock` to have the same signature.
+(`GeneratedCapturingMock` contains mutable state, so must be created afresh each time.)
+
 # WoofWare.Myriad.Plugins 8.1.1
 
 Adds `GenerateCapturingMock`, which is `GenerateMock` but additionally records the calls made to each function.

ConsumePlugin/ArgParserConflictTests.fs

@@ -0,0 +1,124 @@
+namespace ConsumePlugin
+
+open WoofWare.Myriad.Plugins
+
+// This file contains test cases for conflict detection in the ArgParser generator.
+// These are expected to FAIL at build time with appropriate error messages.
+// Uncomment each section one at a time to test the specific conflict detection.
+
+// ============================================================================
+// Test 1: Field named NoFooBar conflicts with FooBar's --no- variant
+// ============================================================================
+// Expected error: Argument name conflict: '--no-foo-bar' collides with the --no- variant
+// of field 'FooBar' (which has [<ArgumentNegateWithPrefix>])
+
+(*
+[<ArgParser>]
+type ConflictingFieldNames =
+    {
+        [<ArgumentNegateWithPrefix>]
+        FooBar : bool
+        NoFooBar : bool
+    }
+*)
+
+// ============================================================================
+// Test 2: ArgumentLongForm "no-foo" conflicts with Foo's --no- variant
+// ============================================================================
+// Expected error: Argument name conflict: '--no-foo' collides with the --no- variant
+// of field 'Foo' (which has [<ArgumentNegateWithPrefix>])
+
+(*
+[<ArgParser>]
+type ConflictingLongForm =
+    {
+        [<ArgumentNegateWithPrefix>]
+        Foo : bool
+        [<ArgumentLongForm "no-foo">]
+        Bar : bool
+    }
+*)
+
+// ============================================================================
+// Test 3: Multiple ArgumentLongForm, one conflicts
+// ============================================================================
+// Expected error: Argument name conflict: '--no-verbose' collides with...
+
+(*
+[<ArgParser>]
+type ConflictingMultipleLongForms =
+    {
+        [<ArgumentLongForm "verbose">]
+        [<ArgumentLongForm "v">]
+        [<ArgumentNegateWithPrefix>]
+        VerboseMode : bool
+
+        [<ArgumentLongForm "no-verbose">]
+        Quiet : bool
+    }
+*)
+
+// ============================================================================
+// Test 4: ArgumentNegateWithPrefix on non-boolean field
+// ============================================================================
+// Expected error: [<ArgumentNegateWithPrefix>] can only be applied to boolean
+// or flag DU fields, but was applied to field NotABool of type string
+
+(*
+[<ArgParser>]
+type InvalidAttributeOnNonBool =
+    {
+        [<ArgumentNegateWithPrefix>]
+        NotABool : string
+    }
+*)
+
+// ============================================================================
+// Test 5: ArgumentNegateWithPrefix on non-flag int field
+// ============================================================================
+// Expected error: [<ArgumentNegateWithPrefix>] can only be applied to boolean
+// or flag DU fields
+
+(*
+[<ArgParser>]
+type InvalidAttributeOnInt =
+    {
+        [<ArgumentNegateWithPrefix>]
+        NotAFlag : int
+    }
+*)
+
+// ============================================================================
+// Test 6: Complex conflict with custom names
+// ============================================================================
+// This tests a more complex scenario where a custom ArgumentLongForm creates
+// a conflict with a different field's negated form
+
+(*
+[<ArgParser>]
+type ComplexConflict =
+    {
+        [<ArgumentLongForm "enable">]
+        [<ArgumentNegateWithPrefix>]
+        FeatureA : bool
+
+        [<ArgumentLongForm "no-enable">]
+        DisableAll : bool
+    }
+*)
+
+// ============================================================================
+// Test 7: Valid usage - no conflicts (this SHOULD compile)
+// ============================================================================
+
+[<ArgParser>]
+type NoConflict =
+    {
+        [<ArgumentNegateWithPrefix>]
+        EnableFeature : bool
+
+        [<ArgumentNegateWithPrefix>]
+        VerboseMode : bool
+
+        NormalField : string
+    }

ConsumePlugin/ArgParserNegationTests.fs

@@ -0,0 +1,48 @@
+namespace ConsumePlugin
+
+open WoofWare.Myriad.Plugins
+
+// Test types for ArgumentNegateWithPrefix functionality
+
+type TestDryRunMode =
+    | [<ArgumentFlag false>] Wet
+    | [<ArgumentFlag true>] Dry
+
+[<ArgParser true>]
+type BoolNegation =
+    {
+        [<ArgumentNegateWithPrefix>]
+        EnableFeature : bool
+    }
+
+[<ArgParser true>]
+type FlagNegation =
+    {
+        [<ArgumentNegateWithPrefix>]
+        DryRun : TestDryRunMode
+    }
+
+[<ArgParser true>]
+type MultipleFormsNegation =
+    {
+        [<ArgumentLongForm "verbose">]
+        [<ArgumentLongForm "v">]
+        [<ArgumentNegateWithPrefix>]
+        VerboseMode : bool
+    }
+
+[<ArgParser true>]
+type CombinedFeatures =
+    {
+        [<ArgumentNegateWithPrefix>]
+        [<ArgumentDefaultFunction>]
+        Verbose : Choice<bool, bool>
+
+        [<ArgumentNegateWithPrefix>]
+        [<ArgumentHelpText "Enable debug mode">]
+        Debug : bool
+
+        NormalBool : bool
+    }
+
+    static member DefaultVerbose () = false

ConsumePlugin/ConsumePlugin.fsproj

@@ -89,6 +89,17 @@
     <Compile Include="GeneratedArgs.fs">
       <MyriadFile>Args.fs</MyriadFile>
     </Compile>
+    <Compile Include="ArgParserNegationTests.fs" />
+    <Compile Include="GeneratedArgParserNegationTests.fs">
+      <MyriadFile>ArgParserNegationTests.fs</MyriadFile>
+    </Compile>
+    <!-- Not compiled, because by design they *don't* compile. That makes them very hard to test in an automated way! -->
+    <None Include="ArgParserConflictTests.fs" />
+    <!-- To run the conflict tests:
+    <Compile Include="GeneratedArgParserConflictTests.fs">
+      <MyriadFile>ArgParserConflictTests.fs</MyriadFile>
+    </Compile>
+    -->
     <None Include="swagger-gitea.json" />
     <Compile Include="GeneratedSwaggerGitea.fs">
       <MyriadFile>swagger-gitea.json</MyriadFile>

Directory.Build.props

@@ -10,7 +10,7 @@
     <WarnOn>FS3388,FS3559</WarnOn>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="Nerdbank.GitVersioning" Version="3.8.118" PrivateAssets="all" />
+    <PackageReference Include="Nerdbank.GitVersioning" Version="3.9.50" PrivateAssets="all" />
     <SourceLinkGitHubHost Include="github.com" ContentUrl="https://raw.githubusercontent.com" />
   </ItemGroup>
   <PropertyGroup Condition="'$(GITHUB_ACTION)' != ''">

WoofWare.Myriad.Plugins.Attributes/ArgParserAttributes.fs

@@ -100,3 +100,24 @@ type ArgumentFlagAttribute (flagValue : bool) =
 [<AttributeUsage(AttributeTargets.Field, AllowMultiple = true)>]
 type ArgumentLongForm (s : string) =
     inherit Attribute ()
+
+/// Attribute indicating that this boolean or flag field should accept `--no-` prefix for negation.
+/// When this attribute is present on a boolean or flag DU field, the generated parser will accept
+/// both --field-name and --no-field-name as argument forms.
+///
+/// For boolean fields:
+///   --field-name (or --field-name=true) sets the value to true
+///   --no-field-name (or --no-field-name=true) sets the value to false
+///   --field-name=false sets the value to false
+///   --no-field-name=false sets the value to true
+///
+/// For flag DU fields with [<ArgumentFlag>]:
+///   --field-name (or --field-name=true) sets to the case marked with [<ArgumentFlag true>]
+///   --no-field-name (or --no-field-name=true) sets to the case marked with [<ArgumentFlag false>]
+///   --field-name=false sets to the [<ArgumentFlag false>] case
+///   --no-field-name=false sets to the [<ArgumentFlag true>] case
+///
+/// This attribute can only be applied to bool fields or flag DU fields (two-case DUs with [<ArgumentFlag>]).
+[<AttributeUsage(AttributeTargets.Field, AllowMultiple = false)>]
+type ArgumentNegateWithPrefixAttribute () =
+    inherit Attribute ()

WoofWare.Myriad.Plugins.Attributes/SurfaceBaseline.txt

@@ -13,6 +13,8 @@ WoofWare.Myriad.Plugins.ArgumentHelpTextAttribute inherit System.Attribute
 WoofWare.Myriad.Plugins.ArgumentHelpTextAttribute..ctor [constructor]: string
 WoofWare.Myriad.Plugins.ArgumentLongForm inherit System.Attribute
 WoofWare.Myriad.Plugins.ArgumentLongForm..ctor [constructor]: string
+WoofWare.Myriad.Plugins.ArgumentNegateWithPrefixAttribute inherit System.Attribute
+WoofWare.Myriad.Plugins.ArgumentNegateWithPrefixAttribute..ctor [constructor]: unit
 WoofWare.Myriad.Plugins.CreateCatamorphismAttribute inherit System.Attribute
 WoofWare.Myriad.Plugins.CreateCatamorphismAttribute..ctor [constructor]: string
 WoofWare.Myriad.Plugins.GenerateCapturingMockAttribute inherit System.Attribute

WoofWare.Myriad.Plugins.Attributes/Test/WoofWare.Myriad.Plugins.Attributes.Test.fsproj

@@ -17,8 +17,8 @@
     </ItemGroup>
 
     <ItemGroup>
-        <PackageReference Include="ApiSurface" Version="5.0.2" />
-        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.0"/>
+        <PackageReference Include="ApiSurface" Version="5.0.3" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.1"/>
         <PackageReference Include="NUnit" Version="4.3.2"/>
         <PackageReference Include="NUnit3TestAdapter" Version="5.2.0"/>
     </ItemGroup>

WoofWare.Myriad.Plugins.Attributes/version.json

@@ -1,5 +1,5 @@
 {
-  "version": "3.7",
+  "version": "3.8",
   "publicReleaseRefSpec": [
     "^refs/heads/main$"
   ],

WoofWare.Myriad.Plugins.Test/TestArgParser/TestArgParserNegation.fs

@@ -0,0 +1,372 @@
+namespace WoofWare.Myriad.Plugins.Test
+
+open System.Threading
+open NUnit.Framework
+open FsUnitTyped
+open ConsumePlugin
+
+[<TestFixture>]
+module TestArgParserNegation =
+
+    [<Test>]
+    let ``Boolean field with ArgumentNegateWithPrefix: --foo sets to true`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = BoolNegation.parse' getEnvVar [ "--enable-feature" ]
+
+        result.EnableFeature |> shouldEqual true
+
+    [<Test>]
+    let ``Boolean field with ArgumentNegateWithPrefix: --foo=true sets to true`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = BoolNegation.parse' getEnvVar [ "--enable-feature=true" ]
+
+        result.EnableFeature |> shouldEqual true
+
+    [<Test>]
+    let ``Boolean field with ArgumentNegateWithPrefix: --foo true sets to true`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = BoolNegation.parse' getEnvVar [ "--enable-feature" ; "true" ]
+
+        result.EnableFeature |> shouldEqual true
+
+    [<Test>]
+    let ``Boolean field with ArgumentNegateWithPrefix: --foo=false sets to false`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = BoolNegation.parse' getEnvVar [ "--enable-feature=false" ]
+
+        result.EnableFeature |> shouldEqual false
+
+    [<Test>]
+    let ``Boolean field with ArgumentNegateWithPrefix: --foo false sets to false`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = BoolNegation.parse' getEnvVar [ "--enable-feature" ; "false" ]
+
+        result.EnableFeature |> shouldEqual false
+
+    [<Test>]
+    let ``Boolean field with ArgumentNegateWithPrefix: --no-foo sets to false`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = BoolNegation.parse' getEnvVar [ "--no-enable-feature" ]
+
+        result.EnableFeature |> shouldEqual false
+
+    [<Test>]
+    let ``Boolean field with ArgumentNegateWithPrefix: --no-foo=true sets to false`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = BoolNegation.parse' getEnvVar [ "--no-enable-feature=true" ]
+
+        result.EnableFeature |> shouldEqual false
+
+    [<Test>]
+    let ``Boolean field with ArgumentNegateWithPrefix: --no-foo true sets to false`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = BoolNegation.parse' getEnvVar [ "--no-enable-feature" ; "true" ]
+
+        result.EnableFeature |> shouldEqual false
+
+    [<Test>]
+    let ``Boolean field with ArgumentNegateWithPrefix: --no-foo=false sets to true`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = BoolNegation.parse' getEnvVar [ "--no-enable-feature=false" ]
+
+        result.EnableFeature |> shouldEqual true
+
+    [<Test>]
+    let ``Boolean field with ArgumentNegateWithPrefix: --no-foo false sets to true`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = BoolNegation.parse' getEnvVar [ "--no-enable-feature" ; "false" ]
+
+        result.EnableFeature |> shouldEqual true
+
+    [<Test>]
+    let ``Flag DU with ArgumentNegateWithPrefix: --dry-run sets to Dry`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = FlagNegation.parse' getEnvVar [ "--dry-run" ]
+
+        result.DryRun |> shouldEqual TestDryRunMode.Dry
+
+    [<Test>]
+    let ``Flag DU with ArgumentNegateWithPrefix: --dry-run=true sets to Dry`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = FlagNegation.parse' getEnvVar [ "--dry-run=true" ]
+
+        result.DryRun |> shouldEqual TestDryRunMode.Dry
+
+    [<Test>]
+    let ``Flag DU with ArgumentNegateWithPrefix: --dry-run true sets to Dry`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = FlagNegation.parse' getEnvVar [ "--dry-run" ; "true" ]
+
+        result.DryRun |> shouldEqual TestDryRunMode.Dry
+
+    [<Test>]
+    let ``Flag DU with ArgumentNegateWithPrefix: --dry-run=false sets to Wet`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = FlagNegation.parse' getEnvVar [ "--dry-run=false" ]
+
+        result.DryRun |> shouldEqual TestDryRunMode.Wet
+
+    [<Test>]
+    let ``Flag DU with ArgumentNegateWithPrefix: --dry-run false sets to Wet`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = FlagNegation.parse' getEnvVar [ "--dry-run" ; "false" ]
+
+        result.DryRun |> shouldEqual TestDryRunMode.Wet
+
+    [<Test>]
+    let ``Flag DU with ArgumentNegateWithPrefix: --no-dry-run sets to Wet`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = FlagNegation.parse' getEnvVar [ "--no-dry-run" ]
+
+        result.DryRun |> shouldEqual TestDryRunMode.Wet
+
+    [<Test>]
+    let ``Flag DU with ArgumentNegateWithPrefix: --no-dry-run=true sets to Wet`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = FlagNegation.parse' getEnvVar [ "--no-dry-run=true" ]
+
+        result.DryRun |> shouldEqual TestDryRunMode.Wet
+
+    [<Test>]
+    let ``Flag DU with ArgumentNegateWithPrefix: --no-dry-run true sets to Wet`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = FlagNegation.parse' getEnvVar [ "--no-dry-run" ; "true" ]
+
+        result.DryRun |> shouldEqual TestDryRunMode.Wet
+
+    [<Test>]
+    let ``Flag DU with ArgumentNegateWithPrefix: --no-dry-run=false sets to Dry`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = FlagNegation.parse' getEnvVar [ "--no-dry-run=false" ]
+
+        result.DryRun |> shouldEqual TestDryRunMode.Dry
+
+    [<Test>]
+    let ``Flag DU with ArgumentNegateWithPrefix: --no-dry-run false sets to Dry`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = FlagNegation.parse' getEnvVar [ "--no-dry-run" ; "false" ]
+
+        result.DryRun |> shouldEqual TestDryRunMode.Dry
+
+    [<Test>]
+    let ``ArgumentNegateWithPrefix works with multiple ArgumentLongForm: --verbose`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = MultipleFormsNegation.parse' getEnvVar [ "--verbose" ]
+
+        result.VerboseMode |> shouldEqual true
+
+    [<Test>]
+    let ``ArgumentNegateWithPrefix works with multiple ArgumentLongForm: --v`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = MultipleFormsNegation.parse' getEnvVar [ "--v" ]
+
+        result.VerboseMode |> shouldEqual true
+
+    [<Test>]
+    let ``ArgumentNegateWithPrefix works with multiple ArgumentLongForm: --no-verbose`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = MultipleFormsNegation.parse' getEnvVar [ "--no-verbose" ]
+
+        result.VerboseMode |> shouldEqual false
+
+    [<Test>]
+    let ``ArgumentNegateWithPrefix works with multiple ArgumentLongForm: --no-v`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = MultipleFormsNegation.parse' getEnvVar [ "--no-v" ]
+
+        result.VerboseMode |> shouldEqual false
+
+    [<Test>]
+    let ``ArgumentNegateWithPrefix works with multiple ArgumentLongForm: --verbose=false`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = MultipleFormsNegation.parse' getEnvVar [ "--verbose=false" ]
+
+        result.VerboseMode |> shouldEqual false
+
+    [<Test>]
+    let ``ArgumentNegateWithPrefix works with multiple ArgumentLongForm: --no-v=false`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = MultipleFormsNegation.parse' getEnvVar [ "--no-v=false" ]
+
+        result.VerboseMode |> shouldEqual true
+
+    [<Test>]
+    let ``Help text shows both standard and negated forms for boolean`` () =
+        let getEnvVar (_ : string) = failwith "do not call"
+
+        let exc =
+            Assert.Throws<exn> (fun () -> BoolNegation.parse' getEnvVar [ "--help" ] |> ignore<BoolNegation>)
+
+        exc.Message
+        |> shouldEqual
+            """Help text requested.
+--enable-feature / --no-enable-feature  bool"""
+
+    [<Test>]
+    let ``Help text shows both standard and negated forms for flag DU`` () =
+        let getEnvVar (_ : string) = failwith "do not call"
+
+        let exc =
+            Assert.Throws<exn> (fun () -> FlagNegation.parse' getEnvVar [ "--help" ] |> ignore<FlagNegation>)
+
+        exc.Message
+        |> shouldEqual
+            """Help text requested.
+--dry-run / --no-dry-run  bool"""
+
+    [<Test>]
+    let ``Help text with multiple long forms shows all variants`` () =
+        let getEnvVar (_ : string) = failwith "do not call"
+
+        let exc =
+            Assert.Throws<exn> (fun () ->
+                MultipleFormsNegation.parse' getEnvVar [ "--help" ]
+                |> ignore<MultipleFormsNegation>
+            )
+
+        exc.Message
+        |> shouldEqual
+            """Help text requested.
+--verbose / --v / --no-verbose / --no-v  bool"""
+
+    [<Test>]
+    let ``Multiple occurrences error: --foo and --no-foo both supplied`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let exc =
+            Assert.Throws<exn> (fun () ->
+                BoolNegation.parse' getEnvVar [ "--enable-feature" ; "--no-enable-feature" ]
+                |> ignore<BoolNegation>
+            )
+
+        // Should report as duplicate argument
+        exc.Message |> shouldContainText "supplied multiple times"
+
+    [<Test>]
+    let ``Multiple occurrences error: --no-foo and --foo both supplied`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let exc =
+            Assert.Throws<exn> (fun () ->
+                BoolNegation.parse' getEnvVar [ "--no-enable-feature" ; "--enable-feature" ]
+                |> ignore<BoolNegation>
+            )
+
+        // Should report as duplicate argument
+        exc.Message |> shouldContainText "supplied multiple times"
+
+    [<Test>]
+    let ``Multiple occurrences error: --foo=true and --no-foo=false both supplied`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let exc =
+            Assert.Throws<exn> (fun () ->
+                BoolNegation.parse' getEnvVar [ "--enable-feature=true" ; "--no-enable-feature=false" ]
+                |> ignore<BoolNegation>
+            )
+
+        // Should report as duplicate argument
+        exc.Message |> shouldContainText "supplied multiple times"
+
+    [<Test>]
+    let ``CombinedFeatures: verbose with default value works with negation`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = CombinedFeatures.parse' getEnvVar [ "--debug" ; "--normal-bool" ]
+
+        result.Verbose |> shouldEqual (Choice2Of2 false)
+        result.Debug |> shouldEqual true
+        result.NormalBool |> shouldEqual true
+
+    [<Test>]
+    let ``CombinedFeatures: can override default with --verbose`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result =
+            CombinedFeatures.parse' getEnvVar [ "--verbose" ; "--debug" ; "--normal-bool" ]
+
+        result.Verbose |> shouldEqual (Choice1Of2 true)
+        result.Debug |> shouldEqual true
+        result.NormalBool |> shouldEqual true
+
+    [<Test>]
+    let ``CombinedFeatures: can override default with --no-verbose`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result =
+            CombinedFeatures.parse' getEnvVar [ "--no-verbose" ; "--debug" ; "--normal-bool" ]
+
+        result.Verbose |> shouldEqual (Choice1Of2 false)
+        result.Debug |> shouldEqual true
+        result.NormalBool |> shouldEqual true
+
+    [<Test>]
+    let ``CombinedFeatures: --no-debug sets debug to false`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = CombinedFeatures.parse' getEnvVar [ "--no-debug" ; "--normal-bool" ]
+
+        result.Debug |> shouldEqual false
+
+    [<Test>]
+    let ``CombinedFeatures: help text shows negation for fields with attribute`` () =
+        let getEnvVar (_ : string) = failwith "do not call"
+
+        let exc =
+            Assert.Throws<exn> (fun () -> CombinedFeatures.parse' getEnvVar [ "--help" ] |> ignore<CombinedFeatures>)
+
+        // Verbose and Debug should have --no- forms, NormalBool should not
+        exc.Message |> shouldContainText "--verbose / --no-verbose"
+        exc.Message |> shouldContainText "--debug / --no-debug"
+        exc.Message |> shouldContainText "--normal-bool  bool"
+        exc.Message |> shouldNotContainText "--no-normal-bool"
+
+    [<Test>]
+    let ``Case insensitivity: --NO-ENABLE-FEATURE works`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = BoolNegation.parse' getEnvVar [ "--NO-ENABLE-FEATURE" ]
+
+        result.EnableFeature |> shouldEqual false
+
+    [<Test>]
+    let ``Case insensitivity: --No-Enable-Feature works`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = BoolNegation.parse' getEnvVar [ "--No-Enable-Feature" ]
+
+        result.EnableFeature |> shouldEqual false
+
+    [<Test>]
+    let ``Case insensitivity: --NO-DRY-RUN works`` () =
+        let getEnvVar (_ : string) = failwith "should not call"
+
+        let result = FlagNegation.parse' getEnvVar [ "--NO-DRY-RUN" ]
+
+        result.DryRun |> shouldEqual TestDryRunMode.Wet

WoofWare.Myriad.Plugins.Test/WoofWare.Myriad.Plugins.Test.fsproj

@@ -38,6 +38,7 @@
     <Compile Include="TestCataGenerator\TestMyList.fs" />
     <Compile Include="TestCataGenerator\TestMyList2.fs" />
     <Compile Include="TestArgParser\TestArgParser.fs" />
+    <Compile Include="TestArgParser\TestArgParserNegation.fs" />
     <Compile Include="TestSwagger\TestSwaggerParse.fs" />
     <Compile Include="TestSwagger\TestOpenApi3Parse.fs" />
     <EmbeddedResource Include="TestSwagger\api-with-examples.json" />
@@ -55,13 +56,13 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="ApiSurface" Version="5.0.2" />
-    <PackageReference Include="FsCheck" Version="3.3.1" />
+    <PackageReference Include="ApiSurface" Version="5.0.3" />
+    <PackageReference Include="FsCheck" Version="3.3.2" />
     <PackageReference Include="FsUnit" Version="7.1.1" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.0.1" />
     <PackageReference Include="NUnit" Version="4.3.2" />
     <PackageReference Include="NUnit3TestAdapter" Version="5.2.0" />
-    <PackageReference Include="WoofWare.Expect" Version="0.8.3" />
+    <PackageReference Include="WoofWare.Expect" Version="0.8.4" />
   </ItemGroup>
 
   <ItemGroup>

WoofWare.Myriad.Plugins/ArgParserGenerator.fs

@@ -72,16 +72,38 @@ type private ParseFunction<'acc> =
         /// and choices and so on.
         TargetType : SynType
         Accumulation : 'acc
+        /// If true, this boolean/flag field accepts --no- prefix for negation (has [<ArgumentNegateWithPrefix>])
+        AcceptsNegation : bool
     }
 
     /// A SynExpr of type `string` which we can display to the user at generated-program runtime to display all
     /// the ways they can refer to this arg.
     member arg.HumanReadableArgForm : SynExpr =
-        let formatString = List.replicate arg.ArgForm.Length "--%s" |> String.concat " / "
+        if arg.AcceptsNegation then
+            // Include both standard and --no- variants
+            // E.g., "--foo / --bar / --no-foo / --no-bar"
+            let standardFormatString =
+                List.replicate arg.ArgForm.Length "--%s" |> String.concat " / "
 
-        (SynExpr.applyFunction (SynExpr.createIdent "sprintf") (SynExpr.CreateConst formatString), arg.ArgForm)
-        ||> List.fold SynExpr.applyFunction
-        |> SynExpr.paren
+            let negatedFormatString =
+                List.replicate arg.ArgForm.Length "--no-%s" |> String.concat " / "
+
+            let combinedFormatString = standardFormatString + " / " + negatedFormatString
+
+            // Apply all arg forms twice (once for standard, once for negated)
+            let allArgForms = arg.ArgForm @ arg.ArgForm
+
+            (SynExpr.applyFunction (SynExpr.createIdent "sprintf") (SynExpr.CreateConst combinedFormatString),
+             allArgForms)
+            ||> List.fold SynExpr.applyFunction
+            |> SynExpr.paren
+        else
+            // Standard behavior: just --foo / --bar
+            let formatString = List.replicate arg.ArgForm.Length "--%s" |> String.concat " / "
+
+            (SynExpr.applyFunction (SynExpr.createIdent "sprintf") (SynExpr.CreateConst formatString), arg.ArgForm)
+            ||> List.fold SynExpr.applyFunction
+            |> SynExpr.paren
 
 [<RequireQualifiedAccess>]
 type private ChoicePositional =
@@ -222,8 +244,8 @@ module private ParseTree =
             nonPos @ nonPos2, Some pos
 
         |> fun (nonPos, pos) ->
-            let duplicateArgs =
-                // This is best-effort. We can't necessarily detect all SynExprs here, but usually it'll be strings.
+            // Extract all arg form strings for validation
+            let allArgForms =
                 Option.toList (pos |> Option.map _.ArgForm) @ (nonPos |> List.map _.ArgForm)
                 |> Seq.concat
                 |> Seq.choose (fun expr ->
@@ -232,14 +254,57 @@ module private ParseTree =
                     | _ -> None
                 )
                 |> List.ofSeq
+
+            // Check for direct duplicates
+            let duplicateArgs =
+                allArgForms
                 |> List.groupBy id
                 |> List.choose (fun (key, v) -> if v.Length > 1 then Some key else None)
 
             match duplicateArgs with
-            | [] -> nonPos, pos
-            | dups ->
+            | dups when not dups.IsEmpty ->
                 let dups = dups |> String.concat " "
                 failwith $"Duplicate args detected! %s{dups}"
+            | _ ->
+
+            // Check for --no- prefix conflicts
+            // Build a map of arg names that have AcceptsNegation=true
+            let negatedForms =
+                nonPos
+                |> List.filter _.AcceptsNegation
+                |> List.collect (fun pf ->
+                    pf.ArgForm
+                    |> List.choose (fun expr ->
+                        match expr |> SynExpr.stripOptionalParen with
+                        | SynExpr.Const (SynConst.String (s, _, _), _) -> Some (pf.FieldName.idText, s)
+                        | _ -> None
+                    )
+                )
+                |> List.map (fun (fieldName, argForm) -> $"no-%s{argForm}", fieldName)
+                |> Map.ofList
+
+            // Check if any existing arg form conflicts with a --no- variant
+            let conflicts =
+                allArgForms
+                |> List.choose (fun argForm ->
+                    match negatedForms.TryFind argForm with
+                    | Some fieldWithNegation -> Some (argForm, fieldWithNegation)
+                    | None -> None
+                )
+
+            match conflicts with
+            | [] -> ()
+            | conflicts ->
+                let conflictMessages =
+                    conflicts
+                    |> List.map (fun (argForm, fieldWithNegation) ->
+                        $"Argument name conflict: '--%s{argForm}' collides with the --no- variant of field '%s{fieldWithNegation}' (which has [<ArgumentNegateWithPrefix>])"
+                    )
+                    |> String.concat "\n"
+
+                failwith $"Conflicting argument names detected:\n%s{conflictMessages}"
+
+            nonPos, pos
 
     /// Build the return value.
     let rec instantiate<'a> (tree : ParseTree<'a>) : SynExpr =
@@ -615,6 +680,7 @@ module internal ArgParserGenerator =
                             ArgForm = longForms
                             Help = helpText
                             BoolCases = isBoolLike
+                            AcceptsNegation = false
                         }
                         |> fun t -> ParseTree.PositionalLeaf (t, Teq.refl)
                     | Accumulation.List Accumulation.Required ->
@@ -627,6 +693,7 @@ module internal ArgParserGenerator =
                             ArgForm = longForms
                             Help = helpText
                             BoolCases = isBoolLike
+                            AcceptsNegation = false
                         }
                         |> fun t -> ParseTree.PositionalLeaf (t, Teq.refl)
                     | Accumulation.Choice _
@@ -651,6 +718,27 @@ module internal ArgParserGenerator =
                             Some (Choice2Of2 ())
                         | parseTy -> identifyAsFlag flagDus parseTy |> Option.map Choice1Of2
 
+                    let hasNegateAttr =
+                        attrs
+                        |> List.exists (fun attr ->
+                            match attr.TypeName with
+                            | SynLongIdent.SynLongIdent (ident, _, _) ->
+                                match (List.last ident).idText with
+                                | "ArgumentNegateWithPrefixAttribute"
+                                | "ArgumentNegateWithPrefix" -> true
+                                | _ -> false
+                        )
+
+                    let acceptsNegation =
+                        if hasNegateAttr then
+                            match isBoolLike with
+                            | Some _ -> true
+                            | None ->
+                                failwith
+                                    $"[<ArgumentNegateWithPrefix>] can only be applied to boolean or flag DU fields, but was applied to field %s{ident.idText} of type %O{fieldType}"
+                        else
+                            false
+
                     {
                         FieldName = ident
                         Parser = parser
@@ -660,6 +748,7 @@ module internal ArgParserGenerator =
                         ArgForm = longForms
                         Help = helpText
                         BoolCases = isBoolLike
+                        AcceptsNegation = acceptsNegation
                     }
                     |> fun t -> ParseTree.NonPositionalLeaf (t, Teq.refl)
                     |> ParseTreeCrate.make
@@ -773,6 +862,37 @@ module internal ArgParserGenerator =
         )
         |> SynBinding.basic [ Ident.create "helpText" ] [ SynPat.unit ]
 
+    /// Helper to create a negated parser for boolean/flag fields.
+    /// Returns a SynExpr that represents: string -> (negated bool or negated flag DU)
+    /// For booleans: `fun x -> not (Boolean.Parse x)`
+    /// For flag DUs: `fun x -> FlagDu.FromBoolean flagDu (not (Boolean.Parse x))`
+    let private createNegatedParser (arg : ParseFunction<'acc>) : SynExpr =
+        match arg.BoolCases with
+        | None -> failwith $"LOGIC ERROR: createNegatedParser called on non-boolean field %s{arg.FieldName.idText}"
+        | Some (Choice2Of2 ()) ->
+            // Boolean: parse and negate
+            // fun x -> not (System.Boolean.Parse x)
+            let parseExpr =
+                SynExpr.createIdent "x"
+                |> SynExpr.applyFunction (SynExpr.createLongIdent [ "System" ; "Boolean" ; "Parse" ])
+                |> SynExpr.paren
+
+            parseExpr
+            |> SynExpr.applyFunction (SynExpr.createIdent "not")
+            |> SynExpr.createLambda "x"
+        | Some (Choice1Of2 flagDu) ->
+            // Flag DU: parse as bool, negate, then convert to flag DU
+            // fun x -> x |> System.Boolean.Parse |> not |> FlagDu.FromBoolean flagDu
+            let parseExpr =
+                SynExpr.createIdent "x"
+                |> SynExpr.applyFunction (SynExpr.createLongIdent [ "System" ; "Boolean" ; "Parse" ])
+                |> SynExpr.paren
+
+            parseExpr
+            |> SynExpr.applyFunction (SynExpr.createIdent "not")
+            |> FlagDu.FromBoolean flagDu
+            |> SynExpr.createLambda "x"
+
     /// `let processKeyValue (key : string) (value : string) : Result<unit, string option> = ...`
     /// Returns a possible error.
     /// A parse failure might not be fatal (e.g. maybe the input was optionally of arity 0, and we failed to do
@@ -786,109 +906,209 @@ module internal ArgParserGenerator =
         let args =
             args
             |> List.map (fun arg ->
-                match arg.Accumulation with
-                | Accumulation.Required
-                | Accumulation.Choice _
-                | Accumulation.Optional ->
-                    let multipleErrorMessage =
-                        SynExpr.createIdent "sprintf"
-                        |> SynExpr.applyTo (SynExpr.CreateConst "Argument '%s' was supplied multiple times: %s and %s")
-                        |> SynExpr.applyTo arg.HumanReadableArgForm
-                        |> SynExpr.applyTo (SynExpr.createIdent "x" |> SynExpr.callMethod "ToString" |> SynExpr.paren)
-                        |> SynExpr.applyTo (
-                            SynExpr.createIdent "value" |> SynExpr.callMethod "ToString" |> SynExpr.paren
-                        )
+                let assignmentExpr =
+                    match arg.Accumulation with
+                    | Accumulation.Required
+                    | Accumulation.Choice _
+                    | Accumulation.Optional ->
+                        let multipleErrorMessage =
+                            SynExpr.createIdent "sprintf"
+                            |> SynExpr.applyTo (
+                                SynExpr.CreateConst "Argument '%s' was supplied multiple times: %s and %s"
+                            )
+                            |> SynExpr.applyTo arg.HumanReadableArgForm
+                            |> SynExpr.applyTo (
+                                SynExpr.createIdent "x" |> SynExpr.callMethod "ToString" |> SynExpr.paren
+                            )
+                            |> SynExpr.applyTo (
+                                SynExpr.createIdent "value" |> SynExpr.callMethod "ToString" |> SynExpr.paren
+                            )
 
-                    let performAssignment =
+                        let performAssignment =
+                            [
+                                SynExpr.createIdent "value"
+                                |> SynExpr.pipeThroughFunction arg.Parser
+                                |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Some")
+                                |> SynExpr.assign (SynLongIdent.createI arg.TargetVariable)
+
+                                SynExpr.applyFunction (SynExpr.createIdent "Ok") (SynExpr.CreateConst ())
+                            ]
+                            |> SynExpr.sequential
+
+                        [
+                            SynMatchClause.create
+                                (SynPat.nameWithArgs "Some" [ SynPat.named "x" ])
+                                (SynExpr.sequential
+                                    [
+                                        multipleErrorMessage
+                                        |> SynExpr.pipeThroughFunction (
+                                            SynExpr.dotGet "Add" (SynExpr.createIdent' argParseErrors)
+                                        )
+                                        SynExpr.applyFunction (SynExpr.createIdent "Ok") (SynExpr.CreateConst ())
+                                    ])
+                            SynMatchClause.create
+                                (SynPat.named "None")
+                                (SynExpr.pipeThroughTryWith
+                                    SynPat.anon
+                                    (SynExpr.createLongIdent [ "exc" ; "Message" ]
+                                     |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Some")
+                                     |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Error"))
+                                    performAssignment)
+                        ]
+                        |> SynExpr.createMatch (SynExpr.createIdent' arg.TargetVariable)
+                    | Accumulation.List (Accumulation.List _)
+                    | Accumulation.List Accumulation.Optional
+                    | Accumulation.List (Accumulation.Choice _) ->
+                        failwith
+                            "WoofWare.Myriad invariant violated: expected a list to contain only a Required accumulation. Non-positional lists cannot be optional or Choice, nor can they themselves contain lists."
+                    | Accumulation.List Accumulation.Required ->
                         [
                             SynExpr.createIdent "value"
                             |> SynExpr.pipeThroughFunction arg.Parser
-                            |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Some")
-                            |> SynExpr.assign (SynLongIdent.createI arg.TargetVariable)
-
-                            SynExpr.applyFunction (SynExpr.createIdent "Ok") (SynExpr.CreateConst ())
+                            |> SynExpr.pipeThroughFunction (
+                                SynExpr.createLongIdent' [ arg.TargetVariable ; Ident.create "Add" ]
+                            )
+                            SynExpr.CreateConst () |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Ok")
                         ]
                         |> SynExpr.sequential
 
-                    [
-                        SynMatchClause.create
-                            (SynPat.nameWithArgs "Some" [ SynPat.named "x" ])
-                            (SynExpr.sequential
-                                [
-                                    multipleErrorMessage
-                                    |> SynExpr.pipeThroughFunction (
-                                        SynExpr.dotGet "Add" (SynExpr.createIdent' argParseErrors)
-                                    )
-                                    SynExpr.applyFunction (SynExpr.createIdent "Ok") (SynExpr.CreateConst ())
-                                ])
-                        SynMatchClause.create
-                            (SynPat.named "None")
-                            (SynExpr.pipeThroughTryWith
-                                SynPat.anon
-                                (SynExpr.createLongIdent [ "exc" ; "Message" ]
-                                 |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Some")
-                                 |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Error"))
-                                performAssignment)
-                    ]
-                    |> SynExpr.createMatch (SynExpr.createIdent' arg.TargetVariable)
-                | Accumulation.List (Accumulation.List _)
-                | Accumulation.List Accumulation.Optional
-                | Accumulation.List (Accumulation.Choice _) ->
-                    failwith
-                        "WoofWare.Myriad invariant violated: expected a list to contain only a Required accumulation. Non-positional lists cannot be optional or Choice, nor can they themselves contain lists."
-                | Accumulation.List Accumulation.Required ->
-                    [
-                        SynExpr.createIdent "value"
-                        |> SynExpr.pipeThroughFunction arg.Parser
-                        |> SynExpr.pipeThroughFunction (
-                            SynExpr.createLongIdent' [ arg.TargetVariable ; Ident.create "Add" ]
-                        )
-                        SynExpr.CreateConst () |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Ok")
-                    ]
-                    |> SynExpr.sequential
-                |> fun expr -> arg.ArgForm, expr
+                // Return (argForms, assignmentExpr), argMetadata
+                (arg.ArgForm, assignmentExpr), Some arg
             )
 
         let posArg =
             match pos with
             | None -> []
             | Some pos ->
-                [
-                    SynExpr.createIdent "value"
-                    |> SynExpr.pipeThroughFunction pos.Parser
-                    |> fun p ->
-                        match pos.Accumulation with
-                        | ChoicePositional.Choice _ ->
-                            p |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Choice1Of2")
-                        | ChoicePositional.Normal _ -> p
-                    |> SynExpr.pipeThroughFunction (
-                        SynExpr.createLongIdent' [ pos.TargetVariable ; Ident.create "Add" ]
-                    )
-                    SynExpr.CreateConst () |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Ok")
-                ]
-                |> SynExpr.sequential
-                |> fun expr -> pos.ArgForm, expr
-                |> List.singleton
+                let posExpr =
+                    [
+                        SynExpr.createIdent "value"
+                        |> SynExpr.pipeThroughFunction pos.Parser
+                        |> fun p ->
+                            match pos.Accumulation with
+                            | ChoicePositional.Choice _ ->
+                                p |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Choice1Of2")
+                            | ChoicePositional.Normal _ -> p
+                        |> SynExpr.pipeThroughFunction (
+                            SynExpr.createLongIdent' [ pos.TargetVariable ; Ident.create "Add" ]
+                        )
+                        SynExpr.CreateConst () |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Ok")
+                    ]
+                    |> SynExpr.sequential
+
+                // Positional args don't support negation, so metadata is None
+                [ (pos.ArgForm, posExpr), None ]
 
         (SynExpr.applyFunction (SynExpr.createIdent "Error") (SynExpr.createIdent "None"), posArg @ args)
-        ||> List.fold (fun finalBranch (argForm, arg) ->
+        ||> List.fold (fun finalBranch ((argForm, arg), argMetadata) ->
             (finalBranch, argForm)
             ||> List.fold (fun finalBranch argForm ->
-                arg
-                |> SynExpr.ifThenElse
-                    (SynExpr.applyFunction
-                        (SynExpr.createLongIdent [ "System" ; "String" ; "Equals" ])
-                        (SynExpr.tuple
+                // Standard match: --argForm
+                let standardMatch =
+                    arg
+                    |> SynExpr.ifThenElse
+                        (SynExpr.applyFunction
+                            (SynExpr.createLongIdent [ "System" ; "String" ; "Equals" ])
+                            (SynExpr.tuple
+                                [
+                                    SynExpr.createIdent "key"
+                                    SynExpr.applyFunction
+                                        (SynExpr.applyFunction
+                                            (SynExpr.createIdent "sprintf")
+                                            (SynExpr.CreateConst "--%s"))
+                                        argForm
+                                    SynExpr.createLongIdent [ "System" ; "StringComparison" ; "OrdinalIgnoreCase" ]
+                                ]))
+                        finalBranch
+
+                // If this arg accepts negation, also match --no-argForm
+                match argMetadata with
+                | None -> standardMatch
+                | Some (parseFn : ParseFunctionNonPositional) when parseFn.AcceptsNegation ->
+                    // Create negated assignment (same structure as `arg` but with negated parser)
+                    let negatedParser = createNegatedParser parseFn
+
+                    let negatedArg =
+                        match parseFn.Accumulation with
+                        | Accumulation.Required
+                        | Accumulation.Choice _
+                        | Accumulation.Optional ->
+                            let multipleErrorMessage =
+                                SynExpr.createIdent "sprintf"
+                                |> SynExpr.applyTo (
+                                    SynExpr.CreateConst "Argument '%s' was supplied multiple times: %s and %s"
+                                )
+                                |> SynExpr.applyTo parseFn.HumanReadableArgForm
+                                |> SynExpr.applyTo (
+                                    SynExpr.createIdent "x" |> SynExpr.callMethod "ToString" |> SynExpr.paren
+                                )
+                                |> SynExpr.applyTo (
+                                    SynExpr.createIdent "value" |> SynExpr.callMethod "ToString" |> SynExpr.paren
+                                )
+
+                            let performNegatedAssignment =
+                                [
+                                    SynExpr.createIdent "value"
+                                    |> SynExpr.pipeThroughFunction negatedParser
+                                    |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Some")
+                                    |> SynExpr.assign (SynLongIdent.createI parseFn.TargetVariable)
+
+                                    SynExpr.applyFunction (SynExpr.createIdent "Ok") (SynExpr.CreateConst ())
+                                ]
+                                |> SynExpr.sequential
+
                             [
-                                SynExpr.createIdent "key"
-                                SynExpr.applyFunction
-                                    (SynExpr.applyFunction
-                                        (SynExpr.createIdent "sprintf")
-                                        (SynExpr.CreateConst "--%s"))
-                                    argForm
-                                SynExpr.createLongIdent [ "System" ; "StringComparison" ; "OrdinalIgnoreCase" ]
-                            ]))
-                    finalBranch
+                                SynMatchClause.create
+                                    (SynPat.nameWithArgs "Some" [ SynPat.named "x" ])
+                                    (SynExpr.sequential
+                                        [
+                                            multipleErrorMessage
+                                            |> SynExpr.pipeThroughFunction (
+                                                SynExpr.dotGet "Add" (SynExpr.createIdent' argParseErrors)
+                                            )
+                                            SynExpr.applyFunction (SynExpr.createIdent "Ok") (SynExpr.CreateConst ())
+                                        ])
+                                SynMatchClause.create
+                                    (SynPat.named "None")
+                                    (SynExpr.pipeThroughTryWith
+                                        SynPat.anon
+                                        (SynExpr.createLongIdent [ "exc" ; "Message" ]
+                                         |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Some")
+                                         |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Error"))
+                                        performNegatedAssignment)
+                            ]
+                            |> SynExpr.createMatch (SynExpr.createIdent' parseFn.TargetVariable)
+                        | Accumulation.List Accumulation.Required ->
+                            [
+                                SynExpr.createIdent "value"
+                                |> SynExpr.pipeThroughFunction negatedParser
+                                |> SynExpr.pipeThroughFunction (
+                                    SynExpr.createLongIdent' [ parseFn.TargetVariable ; Ident.create "Add" ]
+                                )
+                                SynExpr.CreateConst () |> SynExpr.pipeThroughFunction (SynExpr.createIdent "Ok")
+                            ]
+                            |> SynExpr.sequential
+                        | _ ->
+                            failwith
+                                "WoofWare.Myriad invariant violated: unexpected accumulation type for negated arg"
+
+                    // Match --no-argForm
+                    negatedArg
+                    |> SynExpr.ifThenElse
+                        (SynExpr.applyFunction
+                            (SynExpr.createLongIdent [ "System" ; "String" ; "Equals" ])
+                            (SynExpr.tuple
+                                [
+                                    SynExpr.createIdent "key"
+                                    SynExpr.applyFunction
+                                        (SynExpr.applyFunction
+                                            (SynExpr.createIdent "sprintf")
+                                            (SynExpr.CreateConst "--no-%s"))
+                                        argForm
+                                    SynExpr.createLongIdent [ "System" ; "StringComparison" ; "OrdinalIgnoreCase" ]
+                                ]))
+                        standardMatch
+                | Some _ -> standardMatch
             )
         )
         |> SynBinding.basic
@@ -909,6 +1129,34 @@ module internal ArgParserGenerator =
             |> PreXmlDoc.create'
         )
 
+    /// Try to extract a constant bool value from a SynExpr.
+    /// Returns Some true/false if the expr is a constant bool, None otherwise.
+    let private tryGetConstBool (expr : SynExpr) : bool option =
+        match expr |> SynExpr.stripOptionalParen with
+        | SynExpr.Const (SynConst.Bool v, _) -> Some v
+        | _ -> None
+
+    /// Helper to get the "false" case for a boolean/flag field.
+    /// For booleans: `false`
+    /// For flag DUs: the case marked with [<ArgumentFlag false>]
+    let private getFalseCase (flag : ParseFunction<'a>) : SynExpr =
+        match flag.BoolCases with
+        | None -> failwith $"LOGIC ERROR: getFalseCase called on non-boolean field %s{flag.FieldName.idText}"
+        | Some (Choice2Of2 ()) ->
+            // Boolean: return false
+            SynExpr.CreateConst false
+        | Some (Choice1Of2 flagDu) ->
+            // Flag DU: return the case associated with false
+            // Check which case has false as its arg
+            match tryGetConstBool flagDu.Case1Arg, tryGetConstBool flagDu.Case2Arg with
+            | Some false, _ -> SynExpr.createLongIdent' [ flagDu.Name ; flagDu.Case1Name ]
+            | _, Some false -> SynExpr.createLongIdent' [ flagDu.Name ; flagDu.Case2Name ]
+            | Some true, _ -> SynExpr.createLongIdent' [ flagDu.Name ; flagDu.Case2Name ]
+            | _, Some true -> SynExpr.createLongIdent' [ flagDu.Name ; flagDu.Case1Name ]
+            | None, None ->
+                // Can't determine at compile time, use FlagDu.FromBoolean with false
+                FlagDu.FromBoolean flagDu (SynExpr.CreateConst false)
+
     /// `let setFlagValue (key : string) : bool = ...`
     /// The second member of the `flags` list tuple is the constant "true" with which we will interpret the
     /// arity-0 `--foo`. So in the case of a boolean-typed field, this is `true`; in the case of a Flag-typed field,
@@ -949,21 +1197,72 @@ module internal ArgParserGenerator =
 
             (finalExpr, flag.ArgForm)
             ||> List.fold (fun finalExpr argForm ->
-                SynExpr.ifThenElse
-                    (SynExpr.applyFunction
-                        (SynExpr.createLongIdent [ "System" ; "String" ; "Equals" ])
-                        (SynExpr.tuple
-                            [
-                                SynExpr.createIdent "key"
-                                SynExpr.applyFunction
-                                    (SynExpr.applyFunction
-                                        (SynExpr.createIdent "sprintf")
-                                        (SynExpr.CreateConst "--%s"))
-                                    argForm
-                                SynExpr.createLongIdent [ "System" ; "StringComparison" ; "OrdinalIgnoreCase" ]
-                            ]))
-                    finalExpr
-                    matchFlag
+                // Standard match: --argForm sets to trueCase
+                let standardMatch =
+                    SynExpr.ifThenElse
+                        (SynExpr.applyFunction
+                            (SynExpr.createLongIdent [ "System" ; "String" ; "Equals" ])
+                            (SynExpr.tuple
+                                [
+                                    SynExpr.createIdent "key"
+                                    SynExpr.applyFunction
+                                        (SynExpr.applyFunction
+                                            (SynExpr.createIdent "sprintf")
+                                            (SynExpr.CreateConst "--%s"))
+                                        argForm
+                                    SynExpr.createLongIdent [ "System" ; "StringComparison" ; "OrdinalIgnoreCase" ]
+                                ]))
+                        finalExpr
+                        matchFlag
+
+                // If this flag accepts negation, also match --no-argForm sets to falseCase
+                if flag.AcceptsNegation then
+                    let falseCase = getFalseCase flag
+
+                    let matchNegatedFlag =
+                        [
+                            SynMatchClause.create
+                                (SynPat.nameWithArgs "Some" [ SynPat.named "x" ])
+                                // This is an error, but it's one we can gracefully report at the end.
+                                (SynExpr.sequential
+                                    [
+                                        multipleErrorMessage
+                                        |> SynExpr.pipeThroughFunction (
+                                            SynExpr.dotGet "Add" (SynExpr.createIdent' argParseErrors)
+                                        )
+                                        SynExpr.CreateConst true
+                                    ])
+
+                            SynMatchClause.create
+                                (SynPat.named "None")
+                                ([
+                                    SynExpr.assign
+                                        (SynLongIdent.createI flag.TargetVariable)
+                                        (SynExpr.pipeThroughFunction (SynExpr.createIdent "Some") falseCase)
+                                    SynExpr.CreateConst true
+                                 ]
+                                 |> SynExpr.sequential)
+                        ]
+                        |> SynExpr.createMatch (SynExpr.createIdent' flag.TargetVariable)
+
+                    // Match --no-argForm
+                    SynExpr.ifThenElse
+                        (SynExpr.applyFunction
+                            (SynExpr.createLongIdent [ "System" ; "String" ; "Equals" ])
+                            (SynExpr.tuple
+                                [
+                                    SynExpr.createIdent "key"
+                                    SynExpr.applyFunction
+                                        (SynExpr.applyFunction
+                                            (SynExpr.createIdent "sprintf")
+                                            (SynExpr.CreateConst "--no-%s"))
+                                        argForm
+                                    SynExpr.createLongIdent [ "System" ; "StringComparison" ; "OrdinalIgnoreCase" ]
+                                ]))
+                        standardMatch
+                        matchNegatedFlag
+                else
+                    standardMatch
             )
         )
         |> SynBinding.basic [ Ident.create "setFlagValue" ] [ SynPat.annotateType SynType.string (SynPat.named "key") ]

WoofWare.Myriad.Plugins/version.json

@@ -1,5 +1,5 @@
 {
-  "version": "9.0",
+  "version": "9.1",
   "publicReleaseRefSpec": [
     "^refs/heads/main$"
   ],

analyzers/analyzers.fsproj

@@ -10,8 +10,8 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageDownload Include="WoofWare.FSharpAnalyzers" Version="[0.2.1]" />
-    <PackageDownload Include="G-Research.FSharp.Analyzers" Version="[0.19.0]" />
+    <PackageDownload Include="WoofWare.FSharpAnalyzers" Version="[0.2.2]" />
+    <PackageDownload Include="G-Research.FSharp.Analyzers" Version="[0.20.0]" />
   </ItemGroup>
 
 </Project>

flake.lock

@@ -20,11 +20,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1760596604,
-        "narHash": "sha256-J/i5K6AAz/y5dBePHQOuzC7MbhyTOKsd/GLezSbEFiM=",
+        "lastModified": 1763191728,
+        "narHash": "sha256-esRhOS0APE6k40Hs/jjReXg+rx+J5LkWw7cuWFKlwYA=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "3cbe716e2346710d6e1f7c559363d14e11c32a43",
+        "rev": "1d4c88323ac36805d09657d13a5273aea1b34f0c",
         "type": "github"
       },
       "original": {

nix/deps.json

@@ -1,8 +1,8 @@
 [
   {
     "pname": "ApiSurface",
-    "version": "5.0.2",
-    "hash": "sha256-zcq1H1ccQzsZQf4kolzoOBSbyz07skihgPAvQ9Jri+E="
+    "version": "5.0.3",
+    "hash": "sha256-uU5mqLL6zMt17oPYMzhB57ryYC6O6FzSjmdTFg7LvNo="
   },
   {
     "pname": "fantomas",
@@ -26,13 +26,13 @@
   },
   {
     "pname": "FsCheck",
-    "version": "3.3.1",
-    "hash": "sha256-k65ksdOSOGz+meRUUND+yuqJtm5ChaKuaxmRIdKzx2Y="
+    "version": "3.3.2",
+    "hash": "sha256-3ydyTGpqySynjbcWbmFVeCBnT3KDH3miPSJYJlyxrGs="
   },
   {
     "pname": "fsharp-analyzers",
-    "version": "0.33.1",
-    "hash": "sha256-vYXvqnf3en487svFv3CmNl24SolwMYzu6zKKGXNxSu8="
+    "version": "0.34.1",
+    "hash": "sha256-Y6PzfVGob2EgX29ZhZIde5EhiZ28Y1+U2pJ6ybIsHV0="
   },
   {
     "pname": "FSharp.Core",
@@ -91,13 +91,13 @@
   },
   {
     "pname": "Microsoft.CodeCoverage",
-    "version": "18.0.0",
-    "hash": "sha256-1RNxheCYASMusDC48BXtaO3MhnInw15JVfjfLM1VMGA="
+    "version": "18.0.1",
+    "hash": "sha256-G6y5iyHZ3R2shlLCW/uTusio/UqcnWT79X+UAbxvDQY="
   },
   {
     "pname": "Microsoft.NET.Test.Sdk",
-    "version": "18.0.0",
-    "hash": "sha256-9iW+9mvMeZgDXwSoR08bnvRNsN4jT8OVWcjq3lcE+cs="
+    "version": "18.0.1",
+    "hash": "sha256-0c3/rp9di0w7E5UmfRh6Prrm3Aeyi8NOj5bm2i6jAh0="
   },
   {
     "pname": "Microsoft.NETCore.App.Host.linux-arm64",
@@ -201,13 +201,13 @@
   },
   {
     "pname": "Microsoft.TestPlatform.ObjectModel",
-    "version": "18.0.0",
-    "hash": "sha256-O/ivHdoIO+q1n0byJ9OZO4quOqACOD3K3Qm00wfhuZk="
+    "version": "18.0.1",
+    "hash": "sha256-oJbS7SZ46RzyOQ+gCysW7qJRy7V8RlQVa5d8uajb91M="
   },
   {
     "pname": "Microsoft.TestPlatform.TestHost",
-    "version": "18.0.0",
-    "hash": "sha256-qAIX2Rqxrnh1xaYRjCbkkvvMm407oyKihqyVjURX5wY="
+    "version": "18.0.1",
+    "hash": "sha256-OXYf5vg4piDr10ve0bZ2ZSb+nb3yOiHayJV3cu5sMV4="
   },
   {
     "pname": "Myriad.Core",
@@ -221,8 +221,8 @@
   },
   {
     "pname": "Nerdbank.GitVersioning",
-    "version": "3.8.118",
-    "hash": "sha256-Hmyy0ZKOmwN4zIhI4+MqoN8geZNc1sd033aZJ6APrO8="
+    "version": "3.9.50",
+    "hash": "sha256-BiBfXwr8ob2HTaFk2L5TwAgtvd/EPoqudSI9nhAjQPI="
   },
   {
     "pname": "NETStandard.Library",
@@ -386,8 +386,8 @@
   },
   {
     "pname": "WoofWare.Expect",
-    "version": "0.8.3",
-    "hash": "sha256-Fc4xpqXjD2hMnaR9kxYiXi/Kxhy6T1QA5Xe2XjTM/t8="
+    "version": "0.8.4",
+    "hash": "sha256-UI7f2nt4g4Gg1Ke/IChrA4fpVOYAChXpvR6zkKfkmzE="
   },
   {
     "pname": "WoofWare.Whippet.Fantomas",