[clang] [Clang][Sema] Diagnose friend function specialization definitions (PR #72863)

[Erich Keane via cfe-commits]

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@@ -4,7 +4,8 @@ void f(T);
 
 template<typename T>
 struct A {
-  // expected-error at +1{{cannot declare an explicit specialization in a friend}}
+  // expected-error at +2{{cannot declare an explicit specialization in a friend}}
+  // expected-error at +1{{friend function specialization cannot be defined}}
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erichkeane wrote:

We probably shouldn't be double-erroring on all of these.  In this case, the existing error seems preferential.

https://github.com/llvm/llvm-project/pull/72863