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

[Vlad Serebrennikov via cfe-commits]

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@@ -252,4 +252,14 @@ namespace dr2397 { // dr2397: 17
     auto (*c)[5] = &a;
   }
 } // namespace dr2397
+
+// CWG2363 was closed as NAD, but its resolution does affirm that
+// a friend declaration cannot have an opaque-enumm-specifier.
+namespace dr2363 { // dr2363: yes
+struct A {
+  friend enum class E; // since-cxx11-error {{reference to enumeration must use 'enum' not 'enum class'}}
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Endilll wrote:

Can you follow the `// expected-error at -1` style of expected directives, like the rest of the file?

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