MSVC compatibility mode: fix error on unqualified templated base class initialization in case of partial specialization

I introduced a patch to handle unqualified templated base class
initialization in MSVC compatibility mode:
https://reviews.llvm.org/rGc894e85fc64dd8d83b460de81080fff93c5ca334
We identified a problem with this patch in the case where the base class
is partially specialized, which can lead to triggering an assertion in
the case of a mix between types and values.
The minimal test case is:

  template <typename Type, int TSize> class Vec {};
  template <int TDim> class Index : public Vec<int, TDim> {
    Index() : Vec() {}
  };
  template class Index<0>;

The detailed problem is that I was using the
`InjectedClassNameSpecialization`, to which the class template arguments
were then applied in order. But in the process, we were losing all the
partial specializations of the base class and creating an index mismatch
between the expected and passed arguments.

Patch By: frederic-tingaud-sonarsource

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D130709

clang/lib/Sema/SemaDeclCXX.cpp

@@ -4309,11 +4309,21 @@ Sema::BuildMemInitializer(Decl *ConstructorD,
       }
 
       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()) {
+            auto BaseTemplate =
+                Base.getType()->getAs<TemplateSpecializationType>();
+            if (BaseTemplate && Context.hasSameTemplateName(
+                                    BaseTemplate->getTemplateName(), TN)) {
+              Diag(IdLoc, diag::ext_unqualified_base_class)
+                  << SourceRange(IdLoc, Init->getSourceRange().getEnd());
+              BaseType = Base.getType();
+              break;
+            }
+          }
         }
       }
 

clang/test/SemaTemplate/ms-unqualified-base-class.cpp

@@ -83,3 +83,37 @@ int main() {
 
   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>;