[clangd] Avoid type hierarchy crash on incomplete type

[nridge]

Summary

Fixes https://github.com/clangd/clangd/issues/597

[kadircet]

Thanks, LGTM!

clang-tools-extra/clangd/XRefs.cpp

std::vector<const CXXRecordDecl *> typeParents(const CXXRecordDecl *CXXRD) {
 
   // If this is an invalid instantiation, instantiation of the bases
   // may not have succeeded, so fall back to the template pattern.
   if (auto *CTSD = dyn_cast<ClassTemplateSpecializationDecl>(CXXRD)) {
     if (CTSD->isInvalidDecl())
       CXXRD = CTSD->getSpecializedTemplate()->getTemplatedDecl();
   }
 
+  // Can't query bases without a definition.
+  if (!CXXRD->hasDefinition())
+    return Result;
+
   for (auto Base : CXXRD->bases()) {
     const CXXRecordDecl *ParentDecl = nullptr;
 
     const Type *Type = Base.getType().getTypePtr();
     if (const RecordType *RT = Type->getAs<RecordType>()) {
       ParentDecl = RT->getAsCXXRecordDecl();
     }
 

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

 
 namespace clang {
 namespace clangd {
 namespace {
 
 using ::testing::AllOf;
 using ::testing::ElementsAre;
 using ::testing::Field;
+using ::testing::IsEmpty;
 using ::testing::Matcher;
 using ::testing::UnorderedElementsAre;
 
 // GMock helpers for matching TypeHierarchyItem.
 MATCHER_P(WithName, N, "") { return arg.name == N; }
 MATCHER_P(WithKind, Kind, "") { return arg.kind == Kind; }
 MATCHER_P(SelectionRangeIs, R, "") { return arg.selectionRange == R; }
 template <class... ParentMatchers>
)cpp");
   // For "Parent<T>", use the primary template as a best-effort guess.
   EXPECT_THAT(typeParents(Child1), ElementsAre(Parent));
   // For "Parent<T>::Type", there is nothing we can do.
   EXPECT_THAT(typeParents(Child2), ElementsAre());
   // Likewise for "T".
   EXPECT_THAT(typeParents(Child3), ElementsAre());
 }
 
+TEST(TypeParents, IncompleteClass) {
+  Annotations Source(R"cpp(
+    class Incomplete;
+  )cpp");
+  TestTU TU = TestTU::withCode(Source.code());
+  auto AST = TU.build();
+
+  const CXXRecordDecl *Incomplete =
+      dyn_cast<CXXRecordDecl>(&findDecl(AST, "Incomplete"));
+  EXPECT_THAT(typeParents(Incomplete), IsEmpty());
+}
+
 // Parts of getTypeHierarchy() are tested in more detail by the
 // FindRecordTypeAt.* and TypeParents.* tests above. This test exercises the
 // entire operation.
 TEST(TypeHierarchy, Parents) {
   Annotations Source(R"cpp(
 struct $Parent1Def[[Parent1]] {
   int a;
 };