[clangd] Have template template arguments target their referenced template decl

Fixes https://github.com/clangd/clangd/issues/473

Differential Revision: https://reviews.llvm.org/D85503

clang-tools-extra/clangd/FindTarget.cpp

@@ -596,6 +596,19 @@ public:
       add(CCI->getAnyMember(), Flags);
     // Constructor calls contain a TypeLoc node, so we don't handle them here.
   }
+
+  void add(const TemplateArgument &Arg, RelSet Flags) {
+    // Only used for template template arguments.
+    // For type and non-type template arguments, SelectionTree
+    // will hit a more specific node (e.g. a TypeLoc or a
+    // DeclRefExpr).
+    if (Arg.getKind() == TemplateArgument::Template ||
+        Arg.getKind() == TemplateArgument::TemplateExpansion) {
+      if (TemplateDecl *TD = Arg.getAsTemplate().getAsTemplateDecl()) {
+        report(TD, Flags);
+      }
+    }
+  }
 };
 
 } // namespace
@@ -619,6 +632,8 @@ allTargetDecls(const ast_type_traits::DynTypedNode &N) {
     Finder.add(*QT, Flags);
   else if (const CXXCtorInitializer *CCI = N.get<CXXCtorInitializer>())
     Finder.add(CCI, Flags);
+  else if (const TemplateArgumentLoc *TAL = N.get<TemplateArgumentLoc>())
+    Finder.add(TAL->getArgument(), Flags);
 
   return Finder.takeDecls();
 }

clang-tools-extra/clangd/Selection.cpp

@@ -479,6 +479,10 @@ public:
   bool TraverseTypeLoc(TypeLoc X) {
     return traverseNode(&X, [&] { return Base::TraverseTypeLoc(X); });
   }
+  bool TraverseTemplateArgumentLoc(const TemplateArgumentLoc &X) {
+    return traverseNode(&X,
+                        [&] { return Base::TraverseTemplateArgumentLoc(X); });
+  }
   bool TraverseNestedNameSpecifierLoc(NestedNameSpecifierLoc X) {
     return traverseNode(
         &X, [&] { return Base::TraverseNestedNameSpecifierLoc(X); });

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

@@ -376,6 +376,15 @@ TEST_F(TargetDeclTest, ClassTemplate) {
                {"template<> class Foo<int *>", Rel::TemplateInstantiation},
                {"template <typename T> class Foo<T *>", Rel::TemplatePattern});
 
+  Code = R"cpp(
+    // Template template argument.
+    template<typename T> struct Vector {};
+    template <template <typename> class Container>
+    struct A {};
+    A<[[Vector]]> a;
+  )cpp";
+  EXPECT_DECLS("TemplateArgumentLoc", {"template <typename T> struct Vector"});
+
   Flags.push_back("-std=c++17"); // for CTAD tests
 
   Code = R"cpp(

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

@@ -407,8 +407,16 @@ TEST(SelectionTest, CommonAncestor) {
           s2[[-^>]]f();
         }
       )cpp",
-       "DeclRefExpr"} // DeclRefExpr to the "operator->" method.
-  };
+       "DeclRefExpr"}, // DeclRefExpr to the "operator->" method.
+
+      // Template template argument.
+      {R"cpp(
+        template <typename> class Vector {};
+        template <template <typename> class Container> class A {};
+        A<[[V^ector]]> a;
+      )cpp",
+       "TemplateArgumentLoc"}};
+
   for (const Case &C : Cases) {
     trace::TestTracer Tracer;
     Annotations Test(C.Code);