[clang][modules] Make the type_info builtin declaration a singleton.

This solves the "reference to `type_info` is ambiguous" error you get when attempting to reference `type_info` while depending on a module with `-fms-compatibility` enabled.

Fixes #38400

Author: matts1

clang/docs/ReleaseNotes.rst

@@ -137,6 +137,8 @@ Bug Fixes in This Version
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+- Fix an ambiguous reference to the builtin `type_info` (available when using
+  `-fms-compatibility`) with modules. (#GH38400)
 
 Bug Fixes to Attribute Support
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/AST/ASTContext.h

@@ -1290,6 +1290,9 @@ class ASTContext : public RefCountedBase<ASTContext> {
   // Implicitly-declared type 'struct _GUID'.
   mutable TagDecl *MSGuidTagDecl = nullptr;
 
+  // Implicitly-declared type 'struct type_info'.
+  mutable TagDecl *MSTypeInfoTagDecl = nullptr;
+
   /// Keep track of CUDA/HIP device-side variables ODR-used by host code.
   /// This does not include extern shared variables used by device host
   /// functions as addresses of shared variables are per warp, therefore
@@ -2381,6 +2384,15 @@ class ASTContext : public RefCountedBase<ASTContext> {
     return getTagDeclType(MSGuidTagDecl);
   }
 
+  /// Retrieve the implicitly-predeclared 'struct type_info' declaration.
+  TagDecl *getMSTypeInfoTagDecl() const {
+    // Lazily create this type on demand - it's only needed for MS builds.
+    if (!MSTypeInfoTagDecl) {
+      MSTypeInfoTagDecl = buildImplicitRecord("type_info");
+    }
+    return MSTypeInfoTagDecl;
+  }
+
   /// Return whether a declaration to a builtin is allowed to be
   /// overloaded/redeclared.
   bool canBuiltinBeRedeclared(const FunctionDecl *) const;

clang/include/clang/AST/DeclID.h

@@ -77,6 +77,9 @@ enum PredefinedDeclIDs {
   /// The internal '__NSConstantString' tag type.
   PREDEF_DECL_CF_CONSTANT_STRING_TAG_ID,
 
+  /// The predeclared 'type_info' struct.
+  PREDEF_DECL_BUILTIN_MS_TYPE_INFO_TAG_ID,
+
 #define BuiltinTemplate(BTName) PREDEF_DECL##BTName##_ID,
 #include "clang/Basic/BuiltinTemplates.inc"
 

clang/lib/Sema/Sema.cpp

@@ -443,9 +443,7 @@ void Sema::Initialize() {
   if (getLangOpts().MSVCCompat) {
     if (getLangOpts().CPlusPlus &&
         IdResolver.begin(&Context.Idents.get("type_info")) == IdResolver.end())
-      PushOnScopeChains(
-          Context.buildImplicitRecord("type_info", TagTypeKind::Class),
-          TUScope);
+      PushOnScopeChains(Context.getMSTypeInfoTagDecl(), TUScope);
 
     addImplicitTypedef("size_t", Context.getSizeType());
   }

clang/lib/Serialization/ASTReader.cpp

@@ -8318,6 +8318,9 @@ Decl *ASTReader::getPredefinedDecl(PredefinedDeclIDs ID) {
     NewLoaded = Context.getCFConstantStringTagDecl();
     break;
 
+  case PREDEF_DECL_BUILTIN_MS_TYPE_INFO_TAG_ID:
+    return Context.getMSTypeInfoTagDecl();
+
 #define BuiltinTemplate(BTName)                                                \
   case PREDEF_DECL##BTName##_ID:                                               \
     if (Context.Decl##BTName)                                                  \

clang/lib/Serialization/ASTWriter.cpp

@@ -5618,6 +5618,8 @@ void ASTWriter::PrepareWritingSpecialDecls(Sema &SemaRef) {
                      PREDEF_DECL_BUILTIN_MS_VA_LIST_ID);
   RegisterPredefDecl(Context.MSGuidTagDecl,
                      PREDEF_DECL_BUILTIN_MS_GUID_ID);
+  RegisterPredefDecl(Context.MSTypeInfoTagDecl,
+                     PREDEF_DECL_BUILTIN_MS_TYPE_INFO_TAG_ID);
   RegisterPredefDecl(Context.ExternCContext, PREDEF_DECL_EXTERN_C_CONTEXT_ID);
   RegisterPredefDecl(Context.CFConstantStringTypeDecl,
                      PREDEF_DECL_CF_CONSTANT_STRING_ID);

clang/test/Modules/pr151277.cpp

@@ -0,0 +1,15 @@
+// RUN: split-file %s %t
+
+// RUN: %clang_cc1 -I%t -emit-module -o %t/a.pcm -fmodules %t/module.modulemap -fno-implicit-modules -fmodule-name=a -x c++-header -fms-compatibility
+// RUN: %clang_cc1 -I%t -emit-module -o %t/b.pcm -fmodules %t/module.modulemap -fno-implicit-modules -fmodule-name=b -x c++-header -fms-compatibility -fmodule-file=%t/a.pcm
+
+//--- module.modulemap
+module a { header "a.h" }
+module b { header "b.h" }
+
+//--- a.h
+type_info* foo;
+
+//--- b.h
+type_info* bar;
+