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

[Krystian Stasiowski via cfe-commits]

================
@@ -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">,
----------------
sdkrystian wrote:


> What if we skip complicated standardese altogether?
> 
> > 'enum' keyword is not allowed after 'friend' keyword

This isn't quite correct either because `enum` _can_ follow the `friend` keyword 😛, e.g. 
```cpp
enum class E;
struct A
{
    friend enum E f(); // ok
};
```

> Possibly followed by
> 
> > Omit 'enum' keyword if you intend to befriend an enumeration

I don't think a note would be particularly helpful since the friend declaration would be ignored anyways...

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