Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -4308,11 +4308,21 @@
       }
 
       if (getLangOpts().MSVCCompat && !getLangOpts().CPlusPlus20) {
-        auto UnqualifiedBase = R.getAsSingle<ClassTemplateDecl>();
-        if (UnqualifiedBase) {
-          Diag(IdLoc, diag::ext_unqualified_base_class)
-              << SourceRange(IdLoc, Init->getSourceRange().getEnd());
-          BaseType = UnqualifiedBase->getInjectedClassNameSpecialization();
+        if (auto UnqualifiedBase = R.getAsSingle<ClassTemplateDecl>()) {
+          auto *TempSpec = cast<TemplateSpecializationType>(
+              UnqualifiedBase->getInjectedClassNameSpecialization());
+          TemplateName TN = TempSpec->getTemplateName();
+          for (auto const &Base : ClassDecl->bases()) {
+            QualType BT = cast<ElaboratedType>(Base.getType())->getNamedType();
+            const auto *BaseTemplate = dyn_cast<TemplateSpecializationType>(BT);
+            if (BaseTemplate && Context.hasSameTemplateName(
+                                    BaseTemplate->getTemplateName(), TN)) {
+              Diag(IdLoc, diag::ext_unqualified_base_class)
+                  << SourceRange(IdLoc, Init->getSourceRange().getEnd());
+              BaseType = BT;
+              break;
+            }
+          }
         }
       }
 
Index: clang/test/SemaTemplate/ms-unqualified-base-class.cpp
===================================================================
--- clang/test/SemaTemplate/ms-unqualified-base-class.cpp
+++ clang/test/SemaTemplate/ms-unqualified-base-class.cpp
@@ -83,3 +83,37 @@
 
   return I;
 }
+
+template <typename Type, int TSize> class Vec {}; // expected-note {{template is declared here}}
+
+template <int TDim> class Index : public Vec<int, TDim> {
+  // after-error@+1 {{member initializer 'Vec' does not name a non-static data member or base class}}
+  Index() : Vec() {} // before-warning {{unqualified base initializer of class templates is a Microsoft extension}}
+};
+
+template class Index<0>;
+
+template <typename T> class Array : public Vec<T, 4> {
+  // after-error@+1 {{member initializer 'Vec' does not name a non-static data member or base class}}
+  Array() : Vec() {} // before-warning {{unqualified base initializer of class templates is a Microsoft extension}}
+};
+
+template class Array<double>;
+
+template <typename T> class Wrong : public Vec<T, 4> {
+  Wrong() : NonExistent() {} // expected-error {{member initializer 'NonExistent' does not name a non-static data member or base class}}
+};
+
+template class Wrong<double>;
+
+template <typename T> class Wrong2 : public Vec<T, 4> {
+  Wrong2() : Vec<T>() {} // expected-error {{too few template arguments for class template 'Vec'}}
+};
+
+template class Wrong2<double>;
+
+template <typename T> class Wrong3 : public Vec<T, 4> {
+  Wrong3() : Base() {} // expected-error {{member initializer 'Base' does not name a non-static data member or base class}}
+};
+
+template class Wrong3<double>;