[clangd] Fix a crash for accessing a null template decl returned by findExplicitReferences.

Summary: Fixes clangd/clangd#347.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78626

Author: hokein

clang-tools-extra/clangd/FindTarget.cpp

@@ -860,15 +860,17 @@ class ExplicitReferenceCollector
   // TemplateArgumentLoc is the only way to get locations for references to
   // template template parameters.
   bool TraverseTemplateArgumentLoc(TemplateArgumentLoc A) {
+    llvm::SmallVector<const NamedDecl *, 1> Targets;
     switch (A.getArgument().getKind()) {
     case TemplateArgument::Template:
     case TemplateArgument::TemplateExpansion:
+      if (const auto *D = A.getArgument()
+                              .getAsTemplateOrTemplatePattern()
+                              .getAsTemplateDecl())
+        Targets.push_back(D);
       reportReference(ReferenceLoc{A.getTemplateQualifierLoc(),
                                    A.getTemplateNameLoc(),
-                                   /*IsDecl=*/false,
-                                   {A.getArgument()
-                                        .getAsTemplateOrTemplatePattern()
-                                        .getAsTemplateDecl()}},
+                                   /*IsDecl=*/false, Targets},
                       DynTypedNode::create(A.getArgument()));
       break;
     case TemplateArgument::Declaration:

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

@@ -1286,6 +1286,20 @@ TEST_F(FindExplicitReferencesTest, All) {
         "1: targets = {}\n"
         "2: targets = {T}\n"
       },
+      // unknown template name should not crash.
+      {R"cpp(
+        template <template <typename> typename T>
+        struct Base {};
+        namespace foo {
+        template <typename $0^T>
+        struct $1^Derive : $2^Base<$3^T::template $4^Unknown> {};
+        }
+      )cpp",
+      "0: targets = {foo::Derive::T}, decl\n"
+      "1: targets = {foo::Derive}, decl\n"
+      "2: targets = {Base}\n"
+      "3: targets = {foo::Derive::T}\n"
+      "4: targets = {}, qualifier = 'T::'\n"},
     };
 
   for (const auto &C : Cases) {