[clangd] Fix locateMacroAt() for macro definition outside preamble

Fixes clangd/clangd#577

Differential Revision: https://reviews.llvm.org/D91025

clang-tools-extra/clangd/SourceCode.cpp

@@ -975,17 +975,30 @@ llvm::Optional<DefinedMacro> locateMacroAt(const syntax::Token &SpelledTok,
   if (!IdentifierInfo || !IdentifierInfo->hadMacroDefinition())
     return None;
 
-  // Get the definition just before the searched location so that a macro
-  // referenced in a '#undef MACRO' can still be found. Note that we only do
-  // that if Loc is not pointing at start of file.
-  if (SM.getLocForStartOfFile(SM.getFileID(Loc)) != Loc)
-    Loc = Loc.getLocWithOffset(-1);
-  MacroDefinition MacroDef = PP.getMacroDefinitionAtLoc(IdentifierInfo, Loc);
-  if (auto *MI = MacroDef.getMacroInfo())
-    return DefinedMacro{
-        IdentifierInfo->getName(), MI,
-        translatePreamblePatchLocation(MI->getDefinitionLoc(), SM)};
-  return None;
+  // We need to take special case to handle #define and #undef.
+  // Preprocessor::getMacroDefinitionAtLoc() only considers a macro
+  // definition to be in scope *after* the location of the macro name in a
+  // #define that introduces it, and *before* the location of the macro name
+  // in an #undef that undefines it. To handle these cases, we check for
+  // the macro being in scope either just after or just before the location
+  // of the token. In getting the location before, we also take care to check
+  // for start-of-file.
+  FileID FID = SM.getFileID(Loc);
+  assert(Loc != SM.getLocForEndOfFile(FID));
+  SourceLocation JustAfterToken = Loc.getLocWithOffset(1);
+  auto *MacroInfo =
+      PP.getMacroDefinitionAtLoc(IdentifierInfo, JustAfterToken).getMacroInfo();
+  if (!MacroInfo && SM.getLocForStartOfFile(FID) != Loc) {
+    SourceLocation JustBeforeToken = Loc.getLocWithOffset(-1);
+    MacroInfo = PP.getMacroDefinitionAtLoc(IdentifierInfo, JustBeforeToken)
+                    .getMacroInfo();
+  }
+  if (!MacroInfo) {
+    return None;
+  }
+  return DefinedMacro{
+      IdentifierInfo->getName(), MacroInfo,
+      translatePreamblePatchLocation(MacroInfo->getDefinitionLoc(), SM)};
 }
 
 llvm::Expected<std::string> Edit::apply() const {

clang-tools-extra/clangd/unittests/XRefsTests.cpp

@@ -1624,6 +1624,14 @@ TEST(FindReferences, WithinAST) {
         }
       )cpp",
 
+      R"cpp(// Macro outside preamble
+        int breakPreamble;
+        #define [[MA^CRO]](X) (X+1)
+        void test() {
+          int x = [[MACRO]]([[MACRO]](1));
+        }
+      )cpp",
+
       R"cpp(
         int [[v^ar]] = 0;
         void foo(int s = [[var]]);