[clang] [clang] Distinguish unresolved templates in UnresolvedLookupExpr (PR #89019)

[Younan Zhang via cfe-commits]

================
@@ -186,3 +186,74 @@ class E {
 #endif
 template<typename T> using D = int; // expected-note {{declared here}} 
 E<D> ed; // expected-note {{instantiation of}}
+
+namespace non_functions {
+
+#if __cplusplus >= 201103L
+namespace PR88832 {
+template <typename T> struct O {
+  static const T v = 0;
+};
+
+struct P {
+  template <typename T> using I = typename O<T>::v; // #TypeAlias
+};
+
+struct Q {
+  template <typename T> int foo() {
+    return T::template I<int>; // expected-error {{'P::I' is expected to be a non-type template, but instantiated to a type alias template}}
+    // expected-note@#TypeAlias {{type alias template declared here}}
+  }
+};
+
+int bar() {
+  return Q().foo<P>(); // expected-note-re {{function template specialization {{.*}} requested here}}
+}
+
+} // namespace PR88832
+#endif
+
+namespace PR63243 {
+
+namespace std {
+template <class T> struct add_pointer { // #add_pointer
+};
+} // namespace std
+
+class A {};
+
+int main() {
+  std::__add_pointer<A>::type ptr; // #ill-formed-decl
+  // expected-error@#ill-formed-decl {{no template named '__add_pointer'}}
+  // expected-note@#add_pointer {{'add_pointer' declared here}}
+  // expected-error@#ill-formed-decl {{expected ';' after expression}}
+  // expected-error@#ill-formed-decl {{no type named 'type' in the global namespace}}
+  // expected-error@#ill-formed-decl {{'std::add_pointer' is expected to be a non-type template, but instantiated to a class template}}
----------------
zyn0217 wrote:

RFC: this diagnostic is probably not optimal (and maybe confusing), since `CheckPlaceholderExpr` is called almost everywhere apart from a full expression. I was considering moving the diagnostic out of that function, but that requires much work I think. (and I'm a bit lazy :)

https://github.com/llvm/llvm-project/pull/89019