[clang] [clang] Allow class with anonymous union member to be const-default-constructible even if a union member has a default member initializer (#95854) (PR #96301)

[via cfe-commits]

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@@ -1395,7 +1395,8 @@ class CXXRecordDecl : public RecordDecl {
   bool allowConstDefaultInit() const {
     return !data().HasUninitializedFields ||
            !(data().HasDefaultedDefaultConstructor ||
-             needsImplicitDefaultConstructor());
+             needsImplicitDefaultConstructor()) ||
+           (isUnion() && isEmpty());
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Sirraide wrote:

Er, why do we special-case empty unions here? Shouldn’t that just work automatically because an empty union has no fields at all and therefore no uninitialised fields either?

https://github.com/llvm/llvm-project/pull/96301