Suppress Wsign-conversion for enums with matching underlying type

As reported here: https://bugs.llvm.org/show_bug.cgi?id=34692

A non-defined enum with a backing type was always defaulting to
being treated as a signed type. IN the case where it IS defined,
the signed-ness of the actual items is used.

This patch uses the underlying type's signed-ness in the non-defined
case to test signed-comparision.

Differential Revision: https://reviews.llvm.org/D38145

llvm-svn: 313907

clang/lib/Sema/SemaChecking.cpp

@@ -8171,8 +8171,11 @@ struct IntRange {
     // For enum types, use the known bit width of the enumerators.
     if (const EnumType *ET = dyn_cast<EnumType>(T)) {
       EnumDecl *Enum = ET->getDecl();
+      // In C++11, enums without definitions can have an explicitly specified
+      // underlying type.  Use this type to compute the range.
       if (!Enum->isCompleteDefinition())
-        return IntRange(C.getIntWidth(QualType(T, 0)), false);
+        return IntRange(C.getIntWidth(QualType(T, 0)),
+                        !ET->isSignedIntegerOrEnumerationType());
 
       unsigned NumPositive = Enum->getNumPositiveBits();
       unsigned NumNegative = Enum->getNumNegativeBits();