Permit duplicate explicit class instantiations if MSVCCompat is enabled

llvm-svn: 208402

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3396,6 +3396,9 @@ def err_nested_name_spec_non_tag : Error<
 // C++ Explicit Instantiation
 def err_explicit_instantiation_duplicate : Error<
     "duplicate explicit instantiation of %0">;
+def warn_explicit_instantiation_duplicate : ExtWarn<
+    "duplicate explicit instantiation of %0 ignored as a Microsoft extension">,
+    InGroup<Microsoft>;
 def note_previous_explicit_instantiation : Note<
     "previous explicit instantiation is here">;
 def ext_explicit_instantiation_after_specialization : Extension<

clang/lib/Sema/SemaTemplate.cpp

@@ -6526,8 +6526,12 @@ Sema::CheckSpecializationInstantiationRedecl(SourceLocation NewLoc,
       //   For a given template and a given set of template-arguments,
       //     - an explicit instantiation definition shall appear at most once
       //       in a program,
-      Diag(NewLoc, diag::err_explicit_instantiation_duplicate)
-        << PrevDecl;
+
+      // MSVCCompat: MSVC silently ignores duplicate explicit instantiations.
+      Diag(NewLoc, (getLangOpts().MSVCCompat)
+                       ? diag::warn_explicit_instantiation_duplicate
+                       : diag::err_explicit_instantiation_duplicate)
+          << PrevDecl;
       Diag(DiagLocForExplicitInstantiation(PrevDecl, PrevPointOfInstantiation),
            diag::note_previous_explicit_instantiation);
       HasNoEffect = true;

clang/test/SemaTemplate/ms-class-specialization-duplicate.cpp

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -fms-compatibility -fdelayed-template-parsing -fsyntax-only -verify %s
+
+template <typename T>
+class A {
+};
+typedef int TInt;
+
+template class A<int>;  // expected-note {{previous explicit instantiation is here}}
+template class A<TInt>; // expected-warning {{duplicate explicit instantiation of 'A<int>' ignored as a Microsoft extension}}