\@ifnextchar consumes spaces (#2118)

* \@ifnextchar consumes spaces

Fix #2117 by fixing our implementation of \@ifnextchar to match LaTeX's:
consuming spaces before checking the next character.  See #2117 for details.

* Update MacroContextInterface

src/macros.js

@@ -36,6 +36,11 @@ export interface MacroContextInterface {
      */
     popToken(): Token;
 
+    /**
+     * Consume all following space tokens, without expansion.
+     */
+    consumeSpaces(): void;
+
     /**
      * Expand the next token only once (if possible), and return the resulting
      * top token on the stack (without removing anything from the stack).
@@ -108,10 +113,12 @@ defineMacro("\\@secondoftwo", function(context) {
 });
 
 // LaTeX's \@ifnextchar{#1}{#2}{#3} looks ahead to the next (unexpanded)
-// symbol.  If it matches #1, then the macro expands to #2; otherwise, #3.
-// Note, however, that it does not consume the next symbol in either case.
+// symbol that isn't a space, consuming any spaces but not consuming the
+// first nonspace character.  If that nonspace character matches #1, then
+// the macro expands to #2; otherwise, it expands to #3.
 defineMacro("\\@ifnextchar", function(context) {
     const args = context.consumeArgs(3);  // symbol, if, else
+    context.consumeSpaces();
     const nextToken = context.future();
     if (args[0].length === 1 && args[0][0].text === nextToken.text) {
         return {tokens: args[1], numArgs: 0};

test/katex-spec.js

@@ -3008,11 +3008,16 @@ describe("A macro expander", function() {
         expect`\@ifstar{yes}{no}?!`.toParseLike`no?!`;
     });
 
-    it("\\@ifnextchar should not consume anything", function() {
+    it("\\@ifnextchar should not consume nonspaces", function() {
         expect`\@ifnextchar!{yes}{no}!!`.toParseLike`yes!!`;
         expect`\@ifnextchar!{yes}{no}?!`.toParseLike`no?!`;
     });
 
+    it("\\@ifnextchar should consume spaces", function() {
+        expect`\def\x#1{\@ifnextchar x{yes}{no}}\x{}x\x{} x`
+            .toParseLike`yesxyesx`;
+    });
+
     it("\\@ifstar should consume star but nothing else", function() {
         expect`\@ifstar{yes}{no}*!`.toParseLike`yes!`;
         expect`\@ifstar{yes}{no}?!`.toParseLike`no?!`;