[clangd] Support UnresolvedUsingTypeLoc AST node in FindTarget.

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

Differential Revision: https://reviews.llvm.org/D125684

clang-tools-extra/clangd/FindTarget.cpp

@@ -127,11 +127,6 @@ bool shouldSkipTypedef(const TypedefNameDecl *TD) {
 //      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;
@@ -207,6 +202,10 @@ public:
         }
       }
       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.
@@ -382,6 +381,9 @@ public:
         // 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())

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

@@ -268,6 +268,17 @@ TEST_F(TargetDeclTest, UsingDecl) {
   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) {