[clang] [Clang] Ensure default arguments in friend declarations are only allowed in defining declarations to prevent multiple reachable declarations (PR #113777)

[Matheus Izvekov via cfe-commits]

================
@@ -185,3 +185,27 @@ template<typename T> struct S {
   friend void X::f(T::type);
 };
 }
+
+namespace GH113324 {
+template <typename = int> struct ct {
+  friend void f1(ct, int = 0);               // expected-error {{friend declaration specifying a default argument must be a definition}}
+  friend void f2(ct a, ct = decltype(a){ }); // expected-error {{friend declaration specifying a default argument must be a definition}}
+};
+
+template<typename T>
+class C {
+public:
+  friend void foo(T a = 1); // expected-error {{friend declaration specifying a default argument must be a definition}}
+};
+
+template<typename T>
+void foo(T a) { } // expected-note {{candidate function template not viable: requires single argument 'a', but no arguments were provided}}
+
+void test() {
+  f1(ct<>{});
+  f2(ct<>{});
+
+  C<int> c;
+  foo(); // expected-error {{no matching function for call to 'foo'}}
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mizvekov wrote:

I also meant that, whichever way you choose, please add the above test case. In case of keeping the current approach, add a FIXME noting that we miss diagnosing the instantiation failure.

https://github.com/llvm/llvm-project/pull/113777