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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Younan Zhang (zyn0217)

<details>
<summary>Changes</summary>

Since the implementation of DR458 (d1446017), 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:

```cpp
namespace NS {
  int CC;
  template &lt;typename&gt; struct C;
}

template &lt;typename CC&gt;
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`)
```
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)
```

Fixes https://github.com/llvm/llvm-project/issues/64082

---
Full diff: https://github.com/llvm/llvm-project/pull/101020.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaTemplate.cpp (+24) 
- (modified) clang/test/CXX/temp/temp.res/temp.local/p8.cpp (+22) 


``````````diff
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index 87b1f98bbe5ac..a4ab9f9926992 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -1876,6 +1876,27 @@ 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();
@@ -8085,6 +8106,9 @@ DeclResult Sema::ActOnClassTemplateSpecialization(
     return true;
   }
 
+  if (S->isTemplateParamScope())
+    EnterTemplatedContext(S, ClassTemplate->getTemplatedDecl());
+
   bool isMemberSpecialization = false;
   bool isPartialSpecialization = false;
 
diff --git a/clang/test/CXX/temp/temp.res/temp.local/p8.cpp 
b/clang/test/CXX/temp/temp.res/temp.local/p8.cpp
index 6b2071eb12ce0..7404c14118c76 100644
--- a/clang/test/CXX/temp/temp.res/temp.local/p8.cpp
+++ b/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 +152,25 @@ 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;
+#if __cplusplus >= 202003L
+  template <typename> struct D;
+  template <class> concept True = true;
+#endif
 }
+
+template <typename CC>
+struct SearchClassBetweenTemplateParameterLists::C {
+  void foo(CC) {} // This should find the template type parameter.
+};
+
+#if __cplusplus >= 202003L
+
+template <True CC>
+struct SearchClassBetweenTemplateParameterLists::D<CC> {
+  void foo(CC);
+};
+
+#endif

``````````

</details>


https://github.com/llvm/llvm-project/pull/101020
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