[Clang] Fix name lookup for dependent bases (#114978)

Currently the following example is a compilation failure: 
```cpp
template<typename T> struct A {
    typedef int M;
    struct B {
      typedef void M;
      struct C;
    };
};

template<typename T> struct A<T>::B::C : A<T> {
    M m; // void or int ?
};
```

According to the point 13.8.3.2

```
A dependent base class is a base class that is a dependent type and is not the current instantiation.
Note 2 : A base class can be the current instantiation in the case of a nested class naming an enclosing class as a base.
```

The base class `A` is the current instantiation, because `C` is a nested
class for an enclosing class `A<T>`, it's is the not-dependent base
class and we need to search the names through its scope.

This patch makes this example compile

1 files changed, 12 insertions, 6 deletions

diff --git a/clang/lib/AST/CXXInheritance.cpp b/clang/lib/AST/CXXInheritance.cpp
index aefc06e..10b8d52 100644
--- a/clang/lib/AST/CXXInheritance.cpp
+++ b/clang/lib/AST/CXXInheritance.cpp
@@ -134,7 +134,7 @@ bool CXXRecordDecl::forallBases(ForallBasesCallback BaseMatches) const {
         return false;
 
       CXXRecordDecl *Base =
-            cast_or_null<CXXRecordDecl>(Ty->getDecl()->getDefinition());
+          cast_if_present<CXXRecordDecl>(Ty->getDecl()->getDefinition());
       if (!Base ||
           (Base->isDependentContext() &&
            !Base->isCurrentInstantiation(Record))) {
@@ -169,13 +169,21 @@ bool CXXBasePaths::lookupInBases(ASTContext &Context,
     QualType BaseType =
         Context.getCanonicalType(BaseSpec.getType()).getUnqualifiedType();
 
+    bool isCurrentInstantiation = isa<InjectedClassNameType>(BaseType);
+    if (!isCurrentInstantiation) {
+      if (auto *BaseRecord = cast_if_present<CXXRecordDecl>(
+              BaseSpec.getType()->getAsRecordDecl()))
+        isCurrentInstantiation = BaseRecord->isDependentContext() &&
+                                 BaseRecord->isCurrentInstantiation(Record);
+    }
     // C++ [temp.dep]p3:
     //   In the definition of a class template or a member of a class template,
     //   if a base class of the class template depends on a template-parameter,
     //   the base class scope is not examined during unqualified name lookup
     //   either at the point of definition of the class template or member or
     //   during an instantiation of the class tem- plate or member.
-    if (!LookupInDependent && BaseType->isDependentType())
+    if (!LookupInDependent &&
+        (BaseType->isDependentType() && !isCurrentInstantiation))
       continue;
 
     // Determine whether we need to visit this base class at all,
@@ -243,9 +251,8 @@ bool CXXBasePaths::lookupInBases(ASTContext &Context,
         return FoundPath;
       }
     } else if (VisitBase) {
-      CXXRecordDecl *BaseRecord;
+      CXXRecordDecl *BaseRecord = nullptr;
       if (LookupInDependent) {
-        BaseRecord = nullptr;
         const TemplateSpecializationType *TST =
             BaseSpec.getType()->getAs<TemplateSpecializationType>();
         if (!TST) {
@@ -264,8 +271,7 @@ bool CXXBasePaths::lookupInBases(ASTContext &Context,
             BaseRecord = nullptr;
         }
       } else {
-        BaseRecord = cast<CXXRecordDecl>(
-            BaseSpec.getType()->castAs<RecordType>()->getDecl());
+        BaseRecord = cast<CXXRecordDecl>(BaseSpec.getType()->getAsRecordDecl());
       }
       if (BaseRecord &&
           lookupInBases(Context, BaseRecord, BaseMatches, LookupInDependent)) {