[clang] Diagnose shadowing of lambda's template parameter by a capture

expr.prim.lambda.capture p5 says:
If an identifier in a capture appears as the declarator-id of a parameter of
the lambda-declarator's parameter-declaration-clause or as the name of a
template parameter of the lambda-expression's template-parameter-list,
the program is ill-formed.
and also has the following example:
```
auto h = [y = 0]<typename y>(y) { return 0; };
```
which now results in
```
error: declaration of 'y' shadows template parameter
  auto l1 = [y = 0]<typename y>(y) { return 0; };
             ^
note: template parameter is declared here
  auto l1 = [y = 0]<typename y>(y) { return 0; };
                             ^
```

Fixes https://github.com/llvm/llvm-project/issues/61105

Reviewed By: shafik, cor3ntin

Differential Revision: https://reviews.llvm.org/D148712

1 files changed, 20 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index 7f9bec2..730f70e 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -1365,6 +1365,26 @@ void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
     PushOnScopeChains(P, CurScope);
   }
 
+  // C++23 [expr.prim.lambda.capture]p5:
+  // If an identifier in a capture appears as the declarator-id of a parameter
+  // of the lambda-declarator's parameter-declaration-clause or as the name of a
+  // template parameter of the lambda-expression's template-parameter-list, the
+  // program is ill-formed.
+  TemplateParameterList *TemplateParams =
+      getGenericLambdaTemplateParameterList(LSI, *this);
+  if (TemplateParams) {
+    for (const auto *TP : TemplateParams->asArray()) {
+      if (!TP->getIdentifier())
+        continue;
+      for (const auto &Capture : Intro.Captures) {
+        if (Capture.Id == TP->getIdentifier()) {
+          Diag(Capture.Loc, diag::err_template_param_shadow) << Capture.Id;
+          Diag(TP->getLocation(), diag::note_template_param_here);
+        }
+      }
+    }
+  }
+
   // C++20: dcl.decl.general p4:
   // The optional requires-clause ([temp.pre]) in an init-declarator or
   // member-declarator shall be present only if the declarator declares a