[clang-format] Avoid adding space after the name of a function-like macro when the name is a keyword.

Fixes https://github.com/llvm/llvm-project/issues/31086.

Before the code:
```
#define if(x)
```

was erroneously formatted to:
```
#define if (x)
```

Reviewed By: HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D118844

2 files changed, 18 insertions, 0 deletions

diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
index c43c8da6f398..37fad4d9f49d 100644
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -1007,6 +1007,12 @@ void UnwrappedLineParser::parsePPDefine() {
     }
   }
 
+  // In the context of a define, even keywords should be treated as normal
+  // identifiers. Setting the kind to identifier is not enough, because we need
+  // to treat additional keywords like __except as well, which are already
+  // identifiers.
+  FormatTok->Tok.setKind(tok::identifier);
+  FormatTok->Tok.setIdentifierInfo(nullptr);
   nextToken();
   if (FormatTok->Tok.getKind() == tok::l_paren &&
       !FormatTok->hasWhitespaceBefore())
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index 720c6e9b6b2f..3d9e38616450 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -1795,6 +1795,18 @@ TEST_F(FormatTest, FormatShortBracedStatements) {
                AllowSimpleBracedStatements);
 }
 
+TEST_F(FormatTest, UnderstandsMacros) {
+  verifyFormat("#define A (parentheses)");
+  verifyFormat("#define true ((int)1)");
+  verifyFormat("#define and(x)");
+  verifyFormat("#define if(x) x");
+  verifyFormat("#define return(x) (x)");
+  verifyFormat("#define while(x) for (; x;)");
+  verifyFormat("#define xor(x) (^(x))");
+  verifyFormat("#define __except(x)");
+  verifyFormat("#define __try(x)");
+}
+
 TEST_F(FormatTest, ShortBlocksInMacrosDontMergeWithCodeAfterMacro) {
   FormatStyle Style = getLLVMStyleWithColumns(60);
   Style.AllowShortBlocksOnASingleLine = FormatStyle::SBS_Always;