[MSVC Compat] Try to treat an implicit, fixed enum as an unfixed enum

consider the following:
enum E *p;
enum E { e };

The above snippet is not ANSI C because 'enum E' has not bee defined
when we are processing the declaration of 'p'; however, it is a popular
extension to make the above work.  This would fail using the Microsoft
enum semantics because the definition of 'E' would implicitly have a
fixed underlying type of 'int' which would trigger diagnostic messages
about a mismatch between the declaration and the definition.

Instead, treat fixed underlying types as not fixed for the purposes of
the diagnostic.

llvm-svn: 249674

1 files changed, 11 insertions, 4 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index fcc40955836..a12ff3a02e9 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -11454,9 +11454,9 @@ bool Sema::CheckEnumUnderlyingType(TypeSourceInfo *TI) {
 
 /// Check whether this is a valid redeclaration of a previous enumeration.
 /// \return true if the redeclaration was invalid.
-bool Sema::CheckEnumRedeclaration(SourceLocation EnumLoc, bool IsScoped,
-                                  QualType EnumUnderlyingTy,
-                                  const EnumDecl *Prev) {
+bool Sema::CheckEnumRedeclaration(
+    SourceLocation EnumLoc, bool IsScoped, QualType EnumUnderlyingTy,
+    bool EnumUnderlyingIsImplicit, const EnumDecl *Prev) {
   bool IsFixed = !EnumUnderlyingTy.isNull();
 
   if (IsScoped != Prev->isScoped()) {
@@ -11478,6 +11478,10 @@ bool Sema::CheckEnumRedeclaration(SourceLocation EnumLoc, bool IsScoped,
           << Prev->getIntegerTypeRange();
       return true;
     }
+  } else if (IsFixed && !Prev->isFixed() && EnumUnderlyingIsImplicit) {
+    ;
+  } else if (!IsFixed && Prev->isFixed() && !Prev->getIntegerTypeSourceInfo()) {
+    ;
   } else if (IsFixed != Prev->isFixed()) {
     Diag(EnumLoc, diag::err_enum_redeclare_fixed_mismatch)
       << Prev->isFixed();
@@ -11747,6 +11751,7 @@ Decl *Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK,
   // this early, because it's needed to detect if this is an incompatible
   // redeclaration.
   llvm::PointerUnion<const Type*, TypeSourceInfo*> EnumUnderlying;
+  bool EnumUnderlyingIsImplicit = false;
 
   if (Kind == TTK_Enum) {
     if (UnderlyingType.isInvalid() || (!UnderlyingType.get() && ScopedEnum))
@@ -11772,6 +11777,7 @@ Decl *Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK,
       if (getLangOpts().MSVCCompat || TUK == TUK_Definition) {
         // Microsoft enums are always of int type.
         EnumUnderlying = Context.IntTy.getTypePtr();
+        EnumUnderlyingIsImplicit = true;
       }
     }
   }
@@ -12119,7 +12125,8 @@ Decl *Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK,
           // returning the previous declaration, unless this is a definition,
           // in which case we want the caller to bail out.
           if (CheckEnumRedeclaration(NameLoc.isValid() ? NameLoc : KWLoc,
-                                     ScopedEnum, EnumUnderlyingTy, PrevEnum))
+                                     ScopedEnum, EnumUnderlyingTy,
+                                     EnumUnderlyingIsImplicit, PrevEnum))
             return TUK == TUK_Declaration ? PrevTagDecl : nullptr;
         }