[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

Author: vbe-sc

clang/docs/ReleaseNotes.rst

@@ -279,6 +279,9 @@ Resolutions to C++ Defect Reports
   by default.
   (`CWG2521: User-defined literals and reserved identifiers <https://cplusplus.github.io/CWG/issues/2521.html>`_).
 
+- Fix name lookup for a dependent base class that is the current instantiation.  
+  (`CWG591: When a dependent base class is the current instantiation <https://cplusplus.github.io/CWG/issues/591.html>`_).
+
 C Language Changes
 ------------------
 

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)) {

clang/test/CXX/drs/cwg5xx.cpp

@@ -1178,17 +1178,61 @@ namespace cwg590 { // cwg590: yes
   template<typename T> typename A<T>::B::C A<T>::B::C::f(A<T>::B::C) {}
 }
 
-namespace cwg591 { // cwg591: no
+namespace cwg591 { // cwg591: yes
   template<typename T> struct A {
     typedef int M;
     struct B {
       typedef void M;
       struct C;
+      struct D;
+    };
+  };
+
+  template<typename T> struct G {
+    struct B {
+      typedef int M;
+      struct C {
+        typedef void M;
+        struct D;
+      };
+    };
+  };
+
+  template<typename T> struct H {
+    template<typename U> struct B {
+      typedef int M;
+      template<typename F> struct C {
+        typedef void M;
+        struct D;
+        struct P;
+      };
     };
   };
 
   template<typename T> struct A<T>::B::C : A<T> {
-    // FIXME: Should find member of non-dependent base class A<T>.
+    M m;
+  };
+
+  template<typename T> struct G<T>::B::C::D : B {
+    M m;
+  };
+
+  template<typename T>
+  template<typename U>
+  template<typename F>
+  struct H<T>::B<U>::C<F>::D : B<U> {
+    M m;
+  };
+
+  template<typename T> struct A<T>::B::D : A<T*> {
+    M m;
+    // expected-error@-1 {{field has incomplete type 'M' (aka 'void'}}
+  };
+
+  template<typename T>
+  template<typename U>
+  template<typename F>
+  struct H<T>::B<U>::C<F>::P : B<F> {
     M m;
     // expected-error@-1 {{field has incomplete type 'M' (aka 'void'}}
   };

clang/www/cxx_dr_status.html

@@ -3599,7 +3599,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/591.html">591</a></td>
     <td>CD4</td>
     <td>When a dependent base class is the current instantiation</td>
-    <td class="none" align="center">No</td>
+    <td class="none" align="center">Yes</td>
   </tr>
   <tr id="592">
     <td><a href="https://cplusplus.github.io/CWG/issues/592.html">592</a></td>