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

[hokein]

Summary

Fixes https://github.com/clangd/clangd/issues/347.

[kadircet]

thanks, lgtm!

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

1293

is namespace required?

[hokein]

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

1293

yes, the test expects to look for foo namespace or foo function.

clang-tools-extra/clangd/FindTarget.cpp

bool TraversePseudoObjectExpr(PseudoObjectExpr *POE) {
     // (The semantic form also contains lots of duplication)
     return RecursiveASTVisitor::TraverseStmt(POE->getSyntacticForm());
   }
 
   // We re-define Traverse*, since there's no corresponding Visit*.
   // 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:
       break; // FIXME: can this actually happen in TemplateArgumentLoc?
     case TemplateArgument::Integral:
     case TemplateArgument::Null:
     case TemplateArgument::NullPtr:
       break; // no references.

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

#endif
            void foo() {
              $0^crash({.$1^x = $2^T()});
            }
         )cpp",
         "0: targets = {crash}\n"
         "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) {
     llvm::StringRef ExpectedCode = C.first;
     llvm::StringRef ExpectedRefs = C.second;
 
     auto Actual =
         annotateReferencesInFoo(llvm::Annotations(ExpectedCode).code());
     EXPECT_EQ(ExpectedCode, Actual.AnnotatedCode);
     EXPECT_EQ(ExpectedRefs, Actual.DumpedReferences) << ExpectedCode;
   }
 }
 
 } // namespace
 } // namespace clangd
 } // namespace clang