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

[Aaron Ballman via cfe-commits]

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@@ -17563,6 +17573,16 @@ Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
                                           UPPC_FixedUnderlyingType))
         EnumUnderlying = Context.IntTy.getTypePtr();
 
+      // If the underlying type is atomic, we need to adjust the type before
+      // continuing. This only happens in the case we stored a TypeSourceInfo
+      // into EnumUnderlying because the other cases are error recovery up to
+      // this point. But because it's not possible to gin up a TypeSourceInfo
+      // for a non-atomic type from an atomic one, we'll store into the Type
+      // field instead.
+      if (TypeSourceInfo *TI = dyn_cast<TypeSourceInfo *>(EnumUnderlying);
+          TI && TI->getType()->isAtomicType())
+        EnumUnderlying = TI->getType().getAtomicUnqualifiedType().getTypePtr();
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AaronBallman wrote:

Ah! Because we want to keep the `TypeSourceInfo *` whenever possible (it carries more information than a `Type *`). But ginning up a `TypeSourceInfo *` for a part of a different type is nontrivial; we may want to do it someday, but it seemed like overkill to solve this issue. So in the atomic case we degrade the information a little bit by storing a `Type *` instead of a `TypeSourceInfo *`, but in usual cases we still store the `TypeSourceInfo *` we got.

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