[clang] [Clang][Sema] Diagnose friend declarations with enum elaborated-type-specifier in all language modes (PR #80171)

[Aaron Ballman via cfe-commits]

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@@ -1637,10 +1637,8 @@ def err_inline_namespace_std : Error<
 def err_unexpected_friend : Error<
   "friends can only be classes or functions">;
 def ext_enum_friend : ExtWarn<
-  "befriending enumeration type %0 is a C++11 extension">, InGroup<CXX11>;
-def warn_cxx98_compat_enum_friend : Warning<
-  "befriending enumeration type %0 is incompatible with C++98">,
-  InGroup<CXX98Compat>, DefaultIgnore;
+  "elaborated enumeration type cannot be a friend">,
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AaronBallman wrote:

I'd go with: 

`friend declaration of %0 with an elaborated specifier is not allowed in ISO C++`

and I agree that a fix-it to remove the `enum` keyword would be great if we can do it (not all users are going to know what an "elaborated specifier" is, so the fix-it gives a bit of context to the diagnostic).

https://github.com/llvm/llvm-project/pull/80171