[clang] [C23] Accept an _Atomic underlying type (PR #147802)

[Erich Keane via cfe-commits]

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@@ -17152,6 +17152,13 @@ bool Sema::CheckEnumUnderlyingType(TypeSourceInfo *TI) {
   SourceLocation UnderlyingLoc = TI->getTypeLoc().getBeginLoc();
   QualType T = TI->getType();
 
+  // C++0x 7.2p2: The type-specifier-seq of an enum-base shall name an
+  // integral type; any cv-qualification is ignored.
+  // C23 6.7.3.3p5: The underlying type of the enumeration is the unqualified,
+  // non-atomic version of the type specified by the type specifiers in the
+  // specifier qualifier list.
+  T = T.getAtomicUnqualifiedType();
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erichkeane wrote:

Since we're now stripping qualifications, we should probably make sure that all of the callers of this 'treat' that like underlying.

https://github.com/llvm/llvm-project/pull/147802