[clang] [clang] Add tests for Core issues about friend templates (PR #94288)

[Shafik Yaghmour via cfe-commits]

================
@@ -373,6 +373,98 @@ namespace cwg1837 { // cwg1837: 3.3
 #endif
 }
 
+namespace cwg1862 { // cwg1862: no
+template<class T>
+struct A {
+  struct B {
+    void e();
+  };
+  
+  void f();
+  
+  struct D {
+    void g();
+  };
+  
+  T h();
+
+  template<T U>
+  T i();
+};
+
+template<>
+struct A<int> {
+  struct B {
+    void e();
+  };
+  
+  int f();
+  
+  struct D {
+    void g();
+  };
+  
+  template<int U>
+  int i();
+};
+
+template<>
+struct A<float*> {
+  int* h();
+};
+
+class C {
+  int private_int;
+
+  template<class T>
+  friend struct A<T>::B;
+  // expected-warning at -1 {{dependent nested name specifier 'A<T>::' for friend class declaration is not supported; turning off access control for 'C'}}
+
+  template<class T>
+  friend void A<T>::f();
+  // expected-warning at -1 {{dependent nested name specifier 'A<T>::' for friend class declaration is not supported; turning off access control for 'C'}}
+
+  // FIXME: this is ill-formed, because A<T>​::​D does not end with a simple-template-id
+  template<class T>
+  friend void A<T>::D::g();
+  // expected-warning at -1 {{dependent nested name specifier 'A<T>::D::' for friend class declaration is not supported; turning off access control for 'C'}}
+  
+  template<class T>
+  friend int *A<T*>::h();
+  // expected-warning at -1 {{dependent nested name specifier 'A<T *>::' for friend class declaration is not supported; turning off access control for 'C'}}
+  
+  template<class T>
+  template<T U>
+  friend T A<T>::i();
+  // expected-warning at -1 {{dependent nested name specifier 'A<T>::' for friend class declaration is not supported; turning off access control for 'C'}}
+};
+
+C c;
+
+template<class T>
+void A<T>::B::e() { (void)c.private_int; }
----------------
shafik wrote:

I guess these work b/c as the diagnostic above said "we turn off access control"?

https://github.com/llvm/llvm-project/pull/94288