[clang] Reland [Clang] skip default argument instantiation for non-defining friend declarations to meet [dcl.fct.default] p4 (PR #115487)

[Younan Zhang via cfe-commits]

================
@@ -185,3 +185,46 @@ template<typename T> struct S {
   friend void X::f(T::type);
 };
 }
+
+namespace GH113324 {
+template <typename = int> struct S1 {
+  friend void f1(S1, int = 0); // expected-error {{friend declaration specifying a default argument must be a definition}}
+  friend void f2(S1 a, S1 = decltype(a){}); // expected-error {{friend declaration specifying a default argument must be a definition}}
+};
+
+template <class T> using alias = int;
+template <typename T> struct S2 {
+  // FIXME: We miss diagnosing the default argument instantiation failure
+  // (forming reference to void)
+  friend void f3(S2, int a = alias<T &>(1)); // expected-error {{friend declaration specifying a default argument must be a definition}}
+};
+
+struct S3 {
+  friend void f4(S3, int = 42) { }
+};
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zyn0217 wrote:

Does the regression fail within `fsyntax-only` mode previously? I remembered those reports were related to codegen parts, so you might need to move the case to e.g. `test/CodeGenCXX/default-arguments.cpp`.

https://github.com/llvm/llvm-project/pull/115487