[C++17] Reject shadowing of capture by parameter in lambda

Summary:
This change rejects the shadowing of a capture by a parameter in lambdas in C++17.

```
int main() {
  int a;
  auto f = [a](int a) { return a; };
}
```

results in:

```
main.cpp:3:20: error: a lambda parameter cannot shadow an explicitly captured entity
  auto f = [a](int a) { return a; };
                   ^
main.cpp:3:13: note: variable a is explicitly captured here
  auto f = [a](int a) { return a; };
            ^
```

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: lebedev.ri, erik.pilkington, cfe-commits

Differential Revision: https://reviews.llvm.org/D53595

llvm-svn: 345308

1 files changed, 17 insertions, 3 deletions

diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index 803b253e458..aa60ce113dc 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -493,7 +493,9 @@ void Sema::finishLambdaExplicitCaptures(LambdaScopeInfo *LSI) {
   LSI->finishedExplicitCaptures();
 }
 
-void Sema::addLambdaParameters(CXXMethodDecl *CallOperator, Scope *CurScope) {
+void Sema::addLambdaParameters(
+    ArrayRef<LambdaIntroducer::LambdaCapture> Captures,
+    CXXMethodDecl *CallOperator, Scope *CurScope) {
   // Introduce our parameters into the function scope
   for (unsigned p = 0, NumParams = CallOperator->getNumParams();
        p < NumParams; ++p) {
@@ -501,7 +503,19 @@ void Sema::addLambdaParameters(CXXMethodDecl *CallOperator, Scope *CurScope) {
 
     // If this has an identifier, add it to the scope stack.
     if (CurScope && Param->getIdentifier()) {
-      CheckShadow(CurScope, Param);
+      bool Error = false;
+      // Resolution of CWG 2211 in C++17 renders shadowing ill-formed, but we
+      // retroactively apply it.
+      for (const auto &Capture : Captures) {
+        if (Capture.Id == Param->getIdentifier()) {
+          Error = true;
+          Diag(Param->getLocation(), diag::err_parameter_shadow_capture);
+          Diag(Capture.Loc, diag::note_var_explicitly_captured_here)
+              << Capture.Id << true;
+        }
+      }
+      if (!Error)
+        CheckShadow(CurScope, Param);
 
       PushOnScopeChains(Param, CurScope);
     }
@@ -1142,7 +1156,7 @@ void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
   LSI->ContainsUnexpandedParameterPack = ContainsUnexpandedParameterPack;
 
   // Add lambda parameters into scope.
-  addLambdaParameters(Method, CurScope);
+  addLambdaParameters(Intro.Captures, Method, CurScope);
 
   // Enter a new evaluation context to insulate the lambda from any
   // cleanups from the enclosing full-expression.