[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@345308 91177308-0d34-0410-b5e6-96231b3b80d8

include/clang/Basic/DiagnosticSemaKinds.td

@@ -6630,6 +6630,10 @@ let CategoryName = "Lambda Issue" in {
   def ext_star_this_lambda_capture_cxx17 : ExtWarn<
     "capture of '*this' by copy is a C++17 extension">, InGroup<CXX17>;
 
+  // C++17 parameter shadows capture
+  def err_parameter_shadow_capture : Error<
+    "a lambda parameter cannot shadow an explicitly captured entity">;
+
   // C++2a [=, this] captures.
   def warn_cxx17_compat_equals_this_lambda_capture : Warning<
     "explicit capture of 'this' with a capture default of '=' is incompatible "

include/clang/Sema/Sema.h

@@ -5580,7 +5580,9 @@ class Sema {
   void finishLambdaExplicitCaptures(sema::LambdaScopeInfo *LSI);
 
   /// Introduce the lambda parameters into scope.
-  void addLambdaParameters(CXXMethodDecl *CallOperator, Scope *CurScope);
+  void addLambdaParameters(
+      ArrayRef<LambdaIntroducer::LambdaCapture> Captures,
+      CXXMethodDecl *CallOperator, Scope *CurScope);
 
   /// Deduce a block or lambda's return type based on the return
   /// statements present in the body.

lib/Sema/SemaLambda.cpp

@@ -493,15 +493,29 @@ 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) {
     ParmVarDecl *Param = CallOperator->getParamDecl(p);
 
     // 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.

test/CXX/drs/dr22xx.cpp

@@ -15,3 +15,14 @@ struct AnonBitfieldQualifiers {
   const volatile unsigned i3 : 1;
 };
 }
+
+#if __cplusplus >= 201103L
+namespace dr2211 { // dr2211: 8
+void f() {
+  int a;
+  auto f = [a](int a) { (void)a; }; // expected-error {{a lambda parameter cannot shadow an explicitly captured entity}}
+  // expected-note@-1{{variable 'a' is explicitly captured here}}
+  auto g = [=](int a) { (void)a; };
+}
+}
+#endif

test/SemaCXX/warn-shadow-in-lambdas.cpp

@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -std=c++14 -verify -fsyntax-only -Wshadow -D AVOID %s
 // RUN: %clang_cc1 -std=c++14 -verify -fsyntax-only -Wshadow -Wshadow-uncaptured-local %s
 // RUN: %clang_cc1 -std=c++14 -verify -fsyntax-only -Wshadow-all %s
+// RUN: %clang_cc1 -std=c++17 -verify -fsyntax-only -Wshadow-all %s
 
 void foo(int param) { // expected-note 1+ {{previous declaration is here}}
   int var = 0; // expected-note 1+ {{previous declaration is here}}
@@ -79,7 +80,7 @@ void foo(int param) { // expected-note 1+ {{previous declaration is here}}
       int var = 1; // expected-warning {{declaration shadows a local variable}}
     };
     auto f2 = [param]   // expected-note {{variable 'param' is explicitly captured here}}
-     (int param) { ; }; // expected-warning {{declaration shadows a local variable}}
+     (int param) { ; }; // expected-error {{a lambda parameter cannot shadow an explicitly captured entity}}
   }
 
   // Warn for variables defined in the capture list.
@@ -135,7 +136,7 @@ void foo(int param) { // expected-note 1+ {{previous declaration is here}}
   auto g2 = [=](auto param) { ; }; // expected-warning {{declaration shadows a local variable}}
 #endif
   auto g3 = [param] // expected-note {{variable 'param' is explicitly captured here}}
-   (auto param) { ; }; // expected-warning {{declaration shadows a local variable}}
+   (auto param) { ; }; // expected-error {{a lambda parameter cannot shadow an explicitly captured entity}}
 }
 
 void avoidWarningWhenRedefining() {

www/cxx_dr_status.html

@@ -13081,7 +13081,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#2211">2211</a></td>
     <td>C++17</td>
     <td>Hiding by lambda captures and parameters</td>
-    <td class="none" align="center">Unknown</td>
+    <td class="svn" align="center">SVN</td>
   </tr>
   <tr class="open" id="2212">
     <td><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#2212">2212</a></td>