[MS] Warn when shadowing template parameters under -fms-compatibility

Summary:
C++ does not allow shadowing template parameters, but previously we
allowed it under -fms-extensions. Now this behavior is controlled by
-fms-compatibility, and we emit a -Wmicrosoft-template warning when it
happens.

Fixes PR43265

Reviewers: thakis, hans

Subscribers: amccarth, rsmith, STL_MSFT, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67463

llvm-svn: 371753

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -4000,6 +4000,8 @@ def err_ovl_no_viable_literal_operator : Error<
 // C++ Template Declarations
 def err_template_param_shadow : Error<
   "declaration of %0 shadows template parameter">;
+def ext_template_param_shadow : ExtWarn<
+  err_template_param_shadow.Text>, InGroup<MicrosoftTemplate>;
 def note_template_param_here : Note<"template parameter is declared here">;
 def warn_template_export_unsupported : Warning<
   "exported templates are unsupported">;

clang/lib/Sema/SemaTemplate.cpp

@@ -849,15 +849,14 @@ bool Sema::DiagnoseUninstantiableTemplate(SourceLocation PointOfInstantiation,
 void Sema::DiagnoseTemplateParameterShadow(SourceLocation Loc, Decl *PrevDecl) {
   assert(PrevDecl->isTemplateParameter() && "Not a template parameter");
 
-  // Microsoft Visual C++ permits template parameters to be shadowed.
-  if (getLangOpts().MicrosoftExt)
-    return;
-
   // C++ [temp.local]p4:
   //   A template-parameter shall not be redeclared within its
   //   scope (including nested scopes).
-  Diag(Loc, diag::err_template_param_shadow)
-    << cast<NamedDecl>(PrevDecl)->getDeclName();
+  //
+  // Make this a warning when MSVC compatibility is requested.
+  unsigned DiagId = getLangOpts().MSVCCompat ? diag::ext_template_param_shadow
+                                             : diag::err_template_param_shadow;
+  Diag(Loc, DiagId) << cast<NamedDecl>(PrevDecl)->getDeclName();
   Diag(PrevDecl->getLocation(), diag::note_template_param_here);
 }
 

clang/test/Parser/DelayedTemplateParsing.cpp

@@ -48,22 +48,6 @@ template <class T> void foo5() {} // expected-error {{redefinition of 'foo5'}}
 
               
 
-namespace Inner_Outer_same_template_param_name {              
-
-template <class T>
-class Outmost {
-public:
-    template <class T>
-    class Inner {
-    public:
-        void f() {
-            T* var;
-        }
-   };
-};
-
-}
-
 
 namespace PR11931 {
 

clang/test/SemaCXX/MicrosoftCompatibility.cpp

@@ -366,3 +366,21 @@ struct S {
 int S::fn() { return 0; } // expected-warning {{is missing exception specification}}
 }
 
+namespace PR43265 {
+template <int N> // expected-note {{template parameter is declared here}}
+struct Foo {
+  static const int N = 42; // expected-warning {{declaration of 'N' shadows template parameter}}
+};
+}
+
+namespace Inner_Outer_same_template_param_name {
+template <typename T> // expected-note {{template parameter is declared here}}
+struct Outmost {
+  template <typename T> // expected-warning {{declaration of 'T' shadows template parameter}}
+  struct Inner {
+    void f() {
+      T *var;
+    }
+  };
+};
+}