[Sema] Avoid -Wshadow warning when a "redefinition of " error is presented

This commit ensures that clang avoids the redundant -Wshadow warning for
variables that already get a "redefinition of " error.

rdar://29067894

Differential Revision: https://reviews.llvm.org/D28350

llvm-svn: 291564

clang/include/clang/Sema/Sema.h

@@ -1709,7 +1709,8 @@ public:
 
   static bool adjustContextForLocalExternDecl(DeclContext *&DC);
   void DiagnoseFunctionSpecifiers(const DeclSpec &DS);
-  void CheckShadow(Scope *S, VarDecl *D, const LookupResult& R);
+  NamedDecl *getShadowedDeclaration(const VarDecl *D, const LookupResult &R);
+  void CheckShadow(VarDecl *D, NamedDecl *ShadowedDecl, const LookupResult &R);
   void CheckShadow(Scope *S, VarDecl *D);
 
   /// Warn if 'E', which is an expression that is about to be modified, refers

clang/lib/Sema/SemaDecl.cpp

@@ -6426,9 +6426,10 @@ NamedDecl *Sema::ActOnVariableDeclarator(
     }
   }
 
-  // Diagnose shadowed variables before filtering for scope.
-  if (D.getCXXScopeSpec().isEmpty())
-    CheckShadow(S, NewVD, Previous);
+  // Find the shadowed declaration before filtering for scope.
+  NamedDecl *ShadowedDecl = D.getCXXScopeSpec().isEmpty()
+                                ? getShadowedDeclaration(NewVD, Previous)
+                                : nullptr;
 
   // Don't consider existing declarations that are in a different
   // scope and are out-of-semantic-context declarations (if the new
@@ -6523,6 +6524,10 @@ NamedDecl *Sema::ActOnVariableDeclarator(
     }
   }
 
+  // Diagnose shadowed variables iff this isn't a redeclaration.
+  if (ShadowedDecl && !D.isRedeclaration())
+    CheckShadow(NewVD, ShadowedDecl, Previous);
+
   ProcessPragmaWeak(S, NewVD);
 
   // If this is the first declaration of an extern C variable, update
@@ -6596,34 +6601,41 @@ static SourceLocation getCaptureLocation(const LambdaScopeInfo *LSI,
   return SourceLocation();
 }
 
+/// \brief Return the declaration shadowed by the given variable \p D, or null
+/// if it doesn't shadow any declaration or shadowing warnings are disabled.
+NamedDecl *Sema::getShadowedDeclaration(const VarDecl *D,
+                                        const LookupResult &R) {
+  // Return if warning is ignored.
+  if (Diags.isIgnored(diag::warn_decl_shadow, R.getNameLoc()))
+    return nullptr;
+
+  // Don't diagnose declarations at file scope.
+  if (D->hasGlobalStorage())
+    return nullptr;
+
+  // Only diagnose if we're shadowing an unambiguous field or variable.
+  if (R.getResultKind() != LookupResult::Found)
+    return nullptr;
+
+  NamedDecl *ShadowedDecl = R.getFoundDecl();
+  return isa<VarDecl>(ShadowedDecl) || isa<FieldDecl>(ShadowedDecl)
+             ? ShadowedDecl
+             : nullptr;
+}
+
 /// \brief Diagnose variable or built-in function shadowing.  Implements
 /// -Wshadow.
 ///
 /// This method is called whenever a VarDecl is added to a "useful"
 /// scope.
 ///
-/// \param S the scope in which the shadowing name is being declared
+/// \param ShadowedDecl the declaration that is shadowed by the given variable
 /// \param R the lookup of the name
 ///
-void Sema::CheckShadow(Scope *S, VarDecl *D, const LookupResult& R) {
-  // Return if warning is ignored.
-  if (Diags.isIgnored(diag::warn_decl_shadow, R.getNameLoc()))
-    return;
-
-  // Don't diagnose declarations at file scope.
-  if (D->hasGlobalStorage())
-    return;
-
+void Sema::CheckShadow(VarDecl *D, NamedDecl *ShadowedDecl,
+                       const LookupResult &R) {
   DeclContext *NewDC = D->getDeclContext();
 
-  // Only diagnose if we're shadowing an unambiguous field or variable.
-  if (R.getResultKind() != LookupResult::Found)
-    return;
-
-  NamedDecl* ShadowedDecl = R.getFoundDecl();
-  if (!isa<VarDecl>(ShadowedDecl) && !isa<FieldDecl>(ShadowedDecl))
-    return;
-
   if (FieldDecl *FD = dyn_cast<FieldDecl>(ShadowedDecl)) {
     // Fields are not shadowed by variables in C++ static methods.
     if (CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(NewDC))
@@ -6733,7 +6745,8 @@ void Sema::CheckShadow(Scope *S, VarDecl *D) {
   LookupResult R(*this, D->getDeclName(), D->getLocation(),
                  Sema::LookupOrdinaryName, Sema::ForRedeclaration);
   LookupName(R, S);
-  CheckShadow(S, D, R);
+  if (NamedDecl *ShadowedDecl = getShadowedDeclaration(D, R))
+    CheckShadow(D, ShadowedDecl, R);
 }
 
 /// Check if 'E', which is an expression that is about to be modified, refers

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

@@ -137,3 +137,11 @@ void foo(int param) { // expected-note 1+ {{previous declaration is here}}
   auto g3 = [param] // expected-note {{variable 'param' is explicitly captured here}}
    (auto param) { ; }; // expected-warning {{declaration shadows a local variable}}
 }
+
+void avoidWarningWhenRedefining() {
+  int a = 1;
+  auto l = [b = a] { // expected-note {{previous definition is here}}
+    // Don't warn on redefinitions.
+    int b = 0; // expected-error {{redefinition of 'b'}}
+  };
+}

clang/test/SemaCXX/warn-shadow.cpp

@@ -97,3 +97,13 @@ void rdar8883302() {
 void test8() {
   int bob; // expected-warning {{declaration shadows a variable in the global namespace}}
 }
+
+namespace rdar29067894 {
+
+void avoidWarningWhenRedefining(int b) { // expected-note {{previous definition is here}}
+  int a = 0; // expected-note {{previous definition is here}}
+  int a = 1; // expected-error {{redefinition of 'a'}}
+  int b = 2; // expected-error {{redefinition of 'b'}}
+}
+
+}