[Clang][Parser] Fix name lookup of template parameters for out-of-line specializations (#101020)

Since the implementation of DR458 (d144601), we have had an algorithm
that template parameters would take precedence over its parent scopes at
the name lookup. However, we failed to handle the following case where
the member function declaration is not yet deferral parsed (This is
where the patch of DR458 applies):

```cpp
namespace NS {
  int CC;
  template <typename> struct C;
}

template <typename CC>
struct NS::C {
  void foo(CC);
};
```

When parsing the parameter of the function declaration `void foo(CC)`,
we used to perform a name lookup following such a Scope chain:
```
FunctionScope foo (failed)
RecordScope C (failed)
NamespaceScope NS (found `int CC`)
(If failed)
TemplateParameterScope of C
```
This doesn't seem right because according to `[temp.local]`, a template
parameter scope should be searched before its parent scope to which the
parameter appertains. This patch corrects the search scopes by setting a
lookup Entity for template parameter Scopes so that we can bail out in
CppNameLookup() when reaching the RecordScope. Afterward, the search
chain would be like:
```
FunctionScope foo (failed)
RecordScope C (failed)
TemplateParameterScope of C (found CC)
(If failed)
NamespaceScope NS
```

Fixes #64082

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -336,6 +336,8 @@ Bug Fixes to C++ Support
 - Mangle placeholders for deduced types as a template-prefix, such that mangling
   of template template parameters uses the correct production. (#GH106182)
 - Fixed an assertion failure when converting vectors to int/float with invalid expressions. (#GH105486)
+- Template parameter names are considered in the name lookup of out-of-line class template
+  specialization right before its declaration context. (#GH64082)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplate.cpp

@@ -1873,6 +1873,27 @@ DeclResult Sema::CheckClassTemplate(
   if (Previous.isAmbiguous())
     return true;
 
+  // Let the template parameter scope enter the lookup chain of the current
+  // class template. For example, given
+  //
+  //  namespace ns {
+  //    template <class> bool Param = false;
+  //    template <class T> struct N;
+  //  }
+  //
+  //  template <class Param> struct ns::N { void foo(Param); };
+  //
+  // When we reference Param inside the function parameter list, our name lookup
+  // chain for it should be like:
+  //  FunctionScope foo
+  //  -> RecordScope N
+  //  -> TemplateParamScope (where we will find Param)
+  //  -> NamespaceScope ns
+  //
+  // See also CppLookupName().
+  if (S->isTemplateParamScope())
+    EnterTemplatedContext(S, SemanticContext);
+
   NamedDecl *PrevDecl = nullptr;
   if (Previous.begin() != Previous.end())
     PrevDecl = (*Previous.begin())->getUnderlyingDecl();
@@ -8089,6 +8110,9 @@ DeclResult Sema::ActOnClassTemplateSpecialization(
     return true;
   }
 
+  if (S->isTemplateParamScope())
+    EnterTemplatedContext(S, ClassTemplate->getTemplatedDecl());
+
   DeclContext *DC = ClassTemplate->getDeclContext();
 
   bool isMemberSpecialization = false;

clang/test/CXX/temp/temp.res/temp.local/p8.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
 
 namespace N { 
   enum { C };
@@ -151,4 +152,38 @@ namespace SearchClassBetweenTemplateParameterLists {
   void A<T>::B<BB>::k(V) { // expected-error {{does not match}}
     BB bb; // expected-error {{incomplete type}}
   }
+
+  int CC;
+  template <typename> struct C;
+  template <template<typename> typename> struct D;
+#if __cplusplus >= 202002L
+  template <bool CC> requires (CC) struct E;
+  template <typename> struct F;
+  template <typename> concept True = true;
+#endif
 }
+
+template <typename CC>
+struct SearchClassBetweenTemplateParameterLists::C {
+  void foo(CC); // This should find the template type parameter.
+};
+
+template <template<typename> typename CC>
+struct SearchClassBetweenTemplateParameterLists::D {
+  template <typename AA>
+  CC<AA> foo(CC<AA>);
+};
+
+#if __cplusplus >= 202002L
+
+template <bool CC> requires (CC)
+struct SearchClassBetweenTemplateParameterLists::E {
+  void foo() requires (CC);
+};
+
+template <SearchClassBetweenTemplateParameterLists::True CC>
+struct SearchClassBetweenTemplateParameterLists::F<CC> {
+  void foo(CC);
+};
+
+#endif