[clang] Fix a bug with qualified name lookup into current instantiation (#73018)

Due to d0d2ee0e4b clang doesn't perform
qualified name lookup into the current instantiation when it has
dependent bases, because of that `getTypeName` call always returns null
for unknown specialization case. When there is a `typename` keyword,
`DependentNameType` is constructed instead of simply returning null.
This change attempts to do the same in case of `typename` absence.

Fixes https://github.com/llvm/llvm-project/issues/13826

clang/docs/ReleaseNotes.rst

@@ -645,6 +645,9 @@ Bug Fixes in This Version
 - Fix crash when the object used as a ``static_assert`` message has ``size`` or ``data`` members
   which are not member functions.
 - Support UDLs in ``static_assert`` message.
+- Fixed false positive error emitted by clang when performing qualified name
+  lookup and the current class instantiation has dependent bases.
+  Fixes (`#13826 <https://github.com/llvm/llvm-project/issues/13826>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaDecl.cpp

@@ -442,7 +442,6 @@ ParsedType Sema::getTypeName(const IdentifierInfo &II, SourceLocation NameLoc,
   UsingShadowDecl *FoundUsingShadow = nullptr;
   switch (Result.getResultKind()) {
   case LookupResult::NotFound:
-  case LookupResult::NotFoundInCurrentInstantiation:
     if (CorrectedII) {
       TypeNameValidatorCCC CCC(/*AllowInvalid=*/true, isClassName,
                                AllowDeducedTemplate);
@@ -482,7 +481,18 @@ ParsedType Sema::getTypeName(const IdentifierInfo &II, SourceLocation NameLoc,
         }
       }
     }
-    // If typo correction failed or was not performed, fall through
+    Result.suppressDiagnostics();
+    return nullptr;
+  case LookupResult::NotFoundInCurrentInstantiation:
+    if (AllowImplicitTypename == ImplicitTypenameContext::Yes) {
+      QualType T = Context.getDependentNameType(ElaboratedTypeKeyword::None,
+                                                SS->getScopeRep(), &II);
+      TypeLocBuilder TLB;
+      DependentNameTypeLoc TL = TLB.push<DependentNameTypeLoc>(T);
+      TL.setQualifierLoc(SS->getWithLocInContext(Context));
+      TL.setNameLoc(NameLoc);
+      return CreateParsedType(T, TLB.getTypeSourceInfo(Context, T));
+    }
     [[fallthrough]];
   case LookupResult::FoundOverloaded:
   case LookupResult::FoundUnresolvedValue:

clang/test/SemaTemplate/dependent-base-classes.cpp

@@ -130,3 +130,17 @@ namespace PR5812 {
 
   Derived<int> di;
 }
+
+namespace GH13826 {
+template <typename T> struct A {
+  typedef int type;
+  struct B;
+};
+
+template <typename T> struct A<T>::B : A<T> {
+  B::type t;
+};
+
+A<int> a;
+A<int>::B b;
+}