[Sema] Don't mark plain MS enums as fixed

Summary:
This fixes a flaw in our AST: PR27098

MSVC always gives plain enums the underlying type 'int'. Clang does this
as well, but we claim the enum is "fixed", as if the user actually wrote
': int'. It means we end up emitting spurious -Wsign-compare warnings on
code like this:

  enum Vals { E1, E2, E3 };
  bool f(unsigned v1, Vals v2) {
    return v1 == v2;
  }

We think 'v2' can take on negative values because we think 'Vals' is
fixed. This fixes that.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D43110

llvm-svn: 324913

1 files changed, 1 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 15b53ca7612..1ccd0c84c42 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -1041,8 +1041,7 @@ Decl *TemplateDeclInstantiator::VisitEnumDecl(EnumDecl *D) {
         SemaRef.SubstType(TI->getType(), TemplateArgs,
                           UnderlyingLoc, DeclarationName());
       SemaRef.CheckEnumRedeclaration(Def->getLocation(), Def->isScoped(),
-                                     DefnUnderlying,
-                                     /*EnumUnderlyingIsImplicit=*/false, Enum);
+                                     DefnUnderlying, /*IsFixed=*/true, Enum);
     }
   }