[clang] [Clang][Sema] Diagnose variable template explicit specializations with storage-class-specifiers (PR #93873)

[Alexander Kornienko via cfe-commits]

alexfh wrote:

Thanks for proposing a fix! I've been hinted though that Clang may be incorrect in accepting explicit specializations of a templated static class data member in the class scope. It looks like with all the restrictions in the standard it may only be allowed to place explicit specializations of a templated static class data member outside of the class. Though IANALL and I'm not sure what the _intention_ of the standard is in this area.

In any case, introducing any restrictions should be done with a clear migration path, so #97425 is likely helpful anyway.

https://github.com/llvm/llvm-project/pull/93873