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

[frederic-tingaud-sonarsource]

Summary

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.

[rnk]

clang/lib/Sema/SemaDeclCXX.cpp

4317

Is this cast always safe? Is there a simpler way to check if the base type is a template specialization type?

[frederic-tingaud-sonarsource]

Use getAs to see through ElaboratedType as recommended by https://reviews.llvm.org/D112374

[frederic-tingaud-sonarsource]

@rnk , does this change answer your points?

[rnk]

Yes, looks good to me, thanks!

clang/lib/Sema/SemaDeclCXX.cpp

if (!TyD) {
           }
 
           R.clear();
           R.setLookupName(MemberOrBase);
         }
       }
 
       if (getLangOpts().MSVCCompat && !getLangOpts().CPlusPlus20) {
-        auto UnqualifiedBase = R.getAsSingle<ClassTemplateDecl>();
-        if (UnqualifiedBase) {
+        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 = UnqualifiedBase->getInjectedClassNameSpecialization();
+              BaseType = Base.getType();
+              break;
+            }
+          }
         }
       }
 
       // If no results were found, try to correct typos.
       TypoCorrection Corr;
       MemInitializerValidatorCCC CCC(ClassDecl);
       if (R.empty() && BaseType.isNull() &&
           (Corr = CorrectTypo(R.getLookupNameInfo(), R.getLookupKind(), S, &SS,

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

int main() {
 
   IntegerDerived<4> ID;
 
   ConformingDerived<int> CD{2};
   I = CD.f();
 
   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>;