[clangd] Support UnresolvedUsingTypeLoc AST node in FindTarget.

[hokein]

Summary

to make features like hover, go-to-def work when the cursor is on the
UnresolvedUsingTypeLoc.

[kadircet]

thanks!

[kadircet]

clang-tools-extra/clangd/FindTarget.cpp

384

nit: formatting around *

clang-tools-extra/clangd/FindTarget.cpp

 //  - normalization may require these traversals again (e.g. unwrapping a
 //    typedef reveals a decltype which must be traversed)
 //  - it doesn't simplify that much, e.g. the first stage must still be able
 //    to yield multiple decls to handle OverloadExpr
 //  - there are cases where it's required for correctness. e.g:
 //      template<class X> using pvec = vector<x*>; pvec<int> x;
 //    There's no Decl `pvec<int>`, we must choose `pvec<X>` or `vector<int*>`
 //    and both are lossy. We must know upfront what the caller ultimately wants.
-//
-// FIXME: improve common dependent scope using name lookup in primary templates.
-// We currently handle several dependent constructs, but some others remain to
-// be handled:
-//  - UnresolvedUsingTypenameDecl
 struct TargetFinder {
   using RelSet = DeclRelationSet;
   using Rel = DeclRelation;
 
 private:
   const HeuristicResolver *Resolver;
   llvm::SmallDenseMap<const NamedDecl *,
                       std::pair<RelSet, /*InsertionOrder*/ size_t>>
void add(const Decl *Dcl, RelSet Flags) {
     } else if (const UnresolvedUsingValueDecl *UUVD =
                    dyn_cast<UnresolvedUsingValueDecl>(D)) {
       if (Resolver) {
         for (const NamedDecl *Target : Resolver->resolveUsingValueDecl(UUVD)) {
           add(Target, Flags); // no Underlying as this is a non-renaming alias
         }
       }
       Flags |= Rel::Alias; // continue with the alias
+    } else if (isa<UnresolvedUsingTypenameDecl>(D)) {
+      // FIXME: improve common dependent scope using name lookup in primary
+      // templates.
+      Flags |= Rel::Alias;
     } else if (const UsingShadowDecl *USD = dyn_cast<UsingShadowDecl>(D)) {
       // Include the Introducing decl, but don't traverse it. This may end up
       // including *all* shadows, which we don't want.
       report(USD->getIntroducer(), Flags | Rel::Alias);
       // Shadow decls are synthetic and not themselves interesting.
       // Record the underlying decl instead, if allowed.
       D = USD->getTargetDecl();
     } else if (const auto *DG = dyn_cast<CXXDeductionGuideDecl>(D)) {
struct Visitor : public TypeVisitor<Visitor> {
       void VisitDecltypeType(const DecltypeType *DTT) {
         Outer.add(DTT->getUnderlyingType(), Flags | Rel::Underlying);
       }
       void VisitDeducedType(const DeducedType *DT) {
         // FIXME: In practice this doesn't work: the AutoType you find inside
         // TypeLoc never has a deduced type. https://llvm.org/PR42914
         Outer.add(DT->getDeducedType(), Flags);
       }
+      void VisitUnresolvedUsingType(const UnresolvedUsingType * UUT) {
+        Outer.add(UUT->getDecl(), Flags);
+      }
       void VisitDeducedTemplateSpecializationType(
           const DeducedTemplateSpecializationType *DTST) {
         if (const auto *USD = DTST->getTemplateName().getAsUsingShadowDecl())
           Outer.add(USD, Flags);
 
         // FIXME: This is a workaround for https://llvm.org/PR42914,
         // which is causing DTST->getDeducedType() to be empty. We
         // fall back to the template pattern and miss the instantiation

clang-tools-extra/clangd/unittests/FindTargetTests.cpp

Code = R"cpp(
 
     using ns::S;
     [[S]] s(123);
   )cpp";
   Flags.push_back("-std=c++17"); // For CTAD feature.
   EXPECT_DECLS("DeducedTemplateSpecializationTypeLoc",
                {"using ns::S", Rel::Alias}, {"template <typename T> class S"},
                {"class S", Rel::TemplatePattern});
+
+  Code = R"cpp(
+    template<typename T>
+    class Foo { public: class foo {}; };
+    template <class T> class A : public Foo<T> {
+      using typename Foo<T>::foo;
+      [[foo]] abc;
+    };
+  )cpp";
+  EXPECT_DECLS("UnresolvedUsingTypeLoc",
+               {"using typename Foo<T>::foo", Rel::Alias});
 }
 
 TEST_F(TargetDeclTest, BaseSpecifier) {
   Code = R"cpp(
     struct X {};
     struct Y : [[private]] X {};
   )cpp";
   EXPECT_DECLS("CXXBaseSpecifier", "struct X");