[Sema] Suppress a warning about a forward-declared fixed enum in C mode

[erik.pilkington]

Summary

As of r343360, we support fixed-enums in C. This lead to some warnings in project headers where a fixed enum is forward declared then later defined. In C++, this is fine, the forward declaration is treated as a complete type even though the definition isn't present. We use this rule in C too, but still warn about the forward declaration anyways. This patch suppresses the warning.

rdar://problem/47356469

Thanks for taking a look!
Erik

[aaron.ballman]

clang/test/Sema/fixed-enum.c

4–5

Can you add a RUN line with -pedantic to ensure that we still warn on the forward declare of the enum even when it's fixed?

[erik.pilkington]

Add a -pedantic RUN line.

[aaron.ballman]

LGTM!

clang/test/Sema/fixed-enum.c

4–5

This was meant to say "don't warn" but I see you figured that out. :-)

clang/lib/Sema/SemaDecl.cpp

                            ScopedEnumUsesClassTag, IsFixed);
 
     if (isStdAlignValT && (!StdAlignValT || getStdAlignValT()->isImplicit()))
       StdAlignValT = cast<EnumDecl>(New);
 
     // If this is an undefined enum, warn.
     if (TUK != TUK_Definition && !Invalid) {
       TagDecl *Def;
-      if (IsFixed && (getLangOpts().CPlusPlus11 || getLangOpts().ObjC) &&
-          cast<EnumDecl>(New)->isFixed()) {
+      if (IsFixed && cast<EnumDecl>(New)->isFixed()) {
         // C++0x: 7.2p2: opaque-enum-declaration.
         // Conflicts are diagnosed above. Do nothing.
       }
       else if (PrevDecl && (Def = cast<EnumDecl>(PrevDecl)->getDefinition())) {
         Diag(Loc, diag::ext_forward_ref_enum_def)
           << New;
         Diag(Def->getLocation(), diag::note_previous_definition);
       } else {

clang/test/Sema/fixed-enum.c

 // RUN: %clang_cc1 -Weverything -xc++ -std=c++11 -DCXX11 -verify %s
 // RUN: %clang_cc1 -Weverything -xc++ -std=c++03 -DCXX03 -verify %s
 // RUN: %clang_cc1 -Weverything -xobjective-c -DOBJC -verify %s
 // RUN: %clang_cc1 -Weverything -std=c11 -xc -DC11 -verify %s
+// RUN: %clang_cc1 -pedantic    -std=c11 -xc -DC11 -verify %s
 // RUN: %clang_cc1 -Weverything -std=c11 -xc -fms-extensions -DMS -verify %s
 
 enum X : int {e};
 #if defined(CXX11)
 // expected-warning@-2{{enumeration types with a fixed underlying type are incompatible with C++98}}
 #elif defined(CXX03)
 // expected-warning@-4{{enumeration types with a fixed underlying type are a C++11 extension}}
 #elif defined(OBJC)
-// expected-no-diagnostics
+// No diagnostic
 #elif defined(C11)
 // expected-warning@-8{{enumeration types with a fixed underlying type are a Clang extension}}
 #elif defined(MS)
 // expected-warning@-10{{enumeration types with a fixed underlying type are a Microsoft extension}}
 #endif
+
+// Don't warn about the forward declaration in any language mode.
+enum Fwd : int;
+enum Fwd : int { e2 };
+#ifndef OBJC
+// expected-warning@-3 {{enumeration types with a fixed underlying type}}
+// expected-warning@-3 {{enumeration types with a fixed underlying type}}
+#endif
+
+// Always error on the incompatible redeclaration.
+enum BadFwd : int;
+#ifndef OBJC
+// expected-warning@-2 {{enumeration types with a fixed underlying type}}
+#endif
+// expected-note@-4 {{previous declaration is here}}
+enum BadFwd : char { e3 };
+#ifndef OBJC
+// expected-warning@-2 {{enumeration types with a fixed underlying type}}
+#endif
+// expected-error@-4 {{enumeration redeclared with different underlying type 'char' (was 'int')}}