In MSVC compatibility mode, handle unqualified templated base class initialization

Before C++20, MSVC was supporting not mentioning the template argument of the base class when initializing a class inheriting a templated base class.
So the following code compiled correctly:
```
template <class T>
class Base {
};

template <class T>
class Derived : public Base<T> {
public:
  Derived() : Base() {}
};

void test() {
    Derived<int> d;
}
```
See https://godbolt.org/z/Pxxe7nccx for a conformance view.

This patch adds support for such construct when in MSVC compatibility mode.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D124666

Author: frederic-tingaud-sonarsource

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5509,6 +5509,9 @@ def err_found_later_in_class : Error<"member %0 used before its declaration">;
 def ext_found_later_in_class : ExtWarn<
   "use of member %0 before its declaration is a Microsoft extension">,
   InGroup<MicrosoftTemplate>;
+def ext_unqualified_base_class : ExtWarn<
+  "unqualified base initializer of class templates is a Microsoft extension">,
+  InGroup<MicrosoftTemplate>;
 def note_dependent_member_use : Note<
   "must qualify identifier to find this declaration in dependent base class">;
 def err_not_found_by_two_phase_lookup : Error<"call to function %0 that is neither "

clang/lib/Sema/SemaDeclCXX.cpp

@@ -4307,6 +4307,15 @@ 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 no results were found, try to correct typos.
       TypoCorrection Corr;
       MemInitializerValidatorCCC CCC(ClassDecl);

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

@@ -0,0 +1,85 @@
+// RUN: %clang_cc1 -std=c++17 -fms-compatibility -fsyntax-only -verify=before,expected %s
+// RUN: %clang_cc1 -std=c++17 -fms-compatibility -fdelayed-template-parsing -fsyntax-only -verify=before,expected %s
+// RUN: %clang_cc1 -std=c++20 -fms-compatibility -fsyntax-only -verify=after,expected %s
+// RUN: %clang_cc1 -std=c++20 -fms-compatibility -fdelayed-template-parsing -fsyntax-only -verify=after,expected %s
+
+template <class T>
+class Base {
+};
+
+template <class T>
+class Based {}; // Trying to trick the typo detection
+
+template <class T>
+class Derived : public Base<T> {
+public:
+  // after-error@+1 {{member initializer 'Base' does not name a non-static data member or base class}}
+  Derived() : Base() {} // before-warning {{unqualified base initializer of class templates is a Microsoft extension}}
+private:
+  int Baze; // Trying to trick the typo detection
+};
+
+template <class T> struct AggregateBase {
+  T i;
+};
+
+template <class T>
+struct AggregateDerived : public AggregateBase<T> {
+  int i;
+
+  // after-error@+1 {{member initializer 'AggregateBase' does not name a non-static data member or base class}}
+  AggregateDerived(T j) : AggregateBase{4}, i{j} {} // before-warning {{unqualified base initializer of class templates is a Microsoft extension}}
+  int f() {
+    return i + AggregateBase::i; // expected-warning {{use of undeclared identifier 'AggregateBase'; unqualified lookup into dependent bases of class template 'AggregateDerived' is a Microsoft extension}}
+  }
+};
+
+template <class T, typename U> struct MultiTypesBase {
+};
+
+template <class T, class U>
+struct MultiTypesDerived : public MultiTypesBase<T, U> {
+  // after-error@+1 {{member initializer 'MultiTypesBase' does not name a non-static data member or base class}}
+  MultiTypesDerived() : MultiTypesBase{} {} // before-warning {{unqualified base initializer of class templates is a Microsoft extension}}
+};
+
+template <int I> struct IntegerBase {
+};
+
+template <int I>
+struct IntegerDerived : public IntegerBase<I> {
+  // after-error@+1 {{member initializer 'IntegerBase' does not name a non-static data member or base class}}
+  IntegerDerived() : IntegerBase{} {} // before-warning {{unqualified base initializer of class templates is a Microsoft extension}}
+};
+
+template <class T> struct ConformingBase {
+  T i;
+};
+
+template <class T>
+struct ConformingDerived : public ConformingBase<T> {
+  int i;
+
+  ConformingDerived(T j) : ConformingBase<T>{4}, i{j} {}
+  int f() {
+    return i + ConformingBase<T>::i;
+  }
+};
+
+int main() {
+  int I;
+  Derived<int> t;
+
+  AggregateDerived<int> AD{2};
+  AD.AggregateBase::i = 3;
+  I = AD.f();
+
+  MultiTypesDerived<int, double> MTD;
+
+  IntegerDerived<4> ID;
+
+  ConformingDerived<int> CD{2};
+  I = CD.f();
+
+  return I;
+}