Index: clang-tools-extra/trunk/clangd/index/SymbolCollector.h
===================================================================
--- clang-tools-extra/trunk/clangd/index/SymbolCollector.h
+++ clang-tools-extra/trunk/clangd/index/SymbolCollector.h
@@ -110,6 +110,7 @@
 
   SymbolSlab takeSymbols() { return std::move(Symbols).build(); }
   RefSlab takeRefs() { return std::move(Refs).build(); }
+  RelationSlab takeRelations() { return std::move(Relations).build(); }
 
   void finish() override;
 
@@ -117,6 +118,8 @@
   const Symbol *addDeclaration(const NamedDecl &, SymbolID,
                                bool IsMainFileSymbol);
   void addDefinition(const NamedDecl &, const Symbol &DeclSymbol);
+  void processRelations(const NamedDecl &ND, const SymbolID &ID,
+                        ArrayRef<index::SymbolRelation> Relations);
 
   llvm::Optional<std::string> getIncludeHeader(llvm::StringRef QName, FileID);
   bool isSelfContainedHeader(FileID);
@@ -135,6 +138,8 @@
   // Only symbols declared in preamble (from #include) and referenced from the
   // main file will be included.
   RefSlab::Builder Refs;
+  // All relations collected from the AST.
+  RelationSlab::Builder Relations;
   ASTContext *ASTCtx;
   std::shared_ptr<Preprocessor> PP;
   std::shared_ptr<GlobalCodeCompletionAllocator> CompletionAllocator;
Index: clang-tools-extra/trunk/clangd/index/SymbolCollector.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/index/SymbolCollector.cpp
+++ clang-tools-extra/trunk/clangd/index/SymbolCollector.cpp
@@ -193,6 +193,11 @@
   return static_cast<RefKind>(static_cast<unsigned>(RefKind::All) & Roles);
 }
 
+bool shouldIndexRelation(const index::SymbolRelation &R) {
+  // We currently only index BaseOf relations, for type hierarchy subtypes.
+  return R.Roles & static_cast<unsigned>(index::SymbolRole::RelationBaseOf);
+}
+
 } // namespace
 
 SymbolCollector::SymbolCollector(Options Opts) : Opts(std::move(Opts)) {}
@@ -291,6 +296,16 @@
       SM.getFileID(SpellingLoc) == SM.getMainFileID())
     ReferencedDecls.insert(ND);
 
+  auto ID = getSymbolID(ND);
+  if (!ID)
+    return true;
+
+  // Note: we need to process relations for all decl occurrences, including
+  // refs, because the indexing code only populates relations for specific
+  // occurrences. For example, RelationBaseOf is only populated for the
+  // occurrence inside the base-specifier.
+  processRelations(*ND, *ID, Relations);
+
   bool CollectRef = static_cast<unsigned>(Opts.RefFilter) & Roles;
   bool IsOnlyRef =
       !(Roles & (static_cast<unsigned>(index::SymbolRole::Declaration) |
@@ -315,10 +330,6 @@
   if (IsOnlyRef)
     return true;
 
-  auto ID = getSymbolID(ND);
-  if (!ID)
-    return true;
-
   // FIXME: ObjCPropertyDecl are not properly indexed here:
   // - ObjCPropertyDecl may have an OrigD of ObjCPropertyImplDecl, which is
   // not a NamedDecl.
@@ -338,6 +349,7 @@
 
   if (Roles & static_cast<unsigned>(index::SymbolRole::Definition))
     addDefinition(*OriginalDecl, *BasicSymbol);
+
   return true;
 }
 
@@ -416,6 +428,37 @@
   return true;
 }
 
+void SymbolCollector::processRelations(
+    const NamedDecl &ND, const SymbolID &ID,
+    ArrayRef<index::SymbolRelation> Relations) {
+  // Store subtype relations.
+  if (!dyn_cast<TagDecl>(&ND))
+    return;
+
+  for (const auto &R : Relations) {
+    if (!shouldIndexRelation(R))
+      continue;
+
+    const Decl *Object = R.RelatedSymbol;
+
+    auto ObjectID = getSymbolID(Object);
+    if (!ObjectID)
+      continue;
+
+    // Record the relation.
+    // TODO: There may be cases where the object decl is not indexed for some
+    // reason. Those cases should probably be removed in due course, but for
+    // now there are two possible ways to handle it:
+    //   (A) Avoid storing the relation in such cases.
+    //   (B) Store it anyways. Clients will likely lookup() the SymbolID
+    //       in the index and find nothing, but that's a situation they
+    //       probably need to handle for other reasons anyways.
+    // We currently do (B) because it's simpler.
+    this->Relations.insert(
+        Relation{ID, index::SymbolRole::RelationBaseOf, *ObjectID});
+  }
+}
+
 void SymbolCollector::setIncludeLocation(const Symbol &S, SourceLocation Loc) {
   if (Opts.CollectIncludePath)
     if (shouldCollectIncludePath(S.SymInfo.Kind))
Index: clang-tools-extra/trunk/clangd/unittests/SymbolCollectorTests.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/unittests/SymbolCollectorTests.cpp
+++ clang-tools-extra/trunk/clangd/unittests/SymbolCollectorTests.cpp
@@ -123,8 +123,9 @@
     assert(AST.hasValue());
     const NamedDecl &ND =
         Qualified ? findDecl(*AST, Name) : findUnqualifiedDecl(*AST, Name);
-    const SourceManager& SM = AST->getSourceManager();
-    bool MainFile = SM.isWrittenInMainFile(SM.getExpansionLoc(ND.getBeginLoc()));
+    const SourceManager &SM = AST->getSourceManager();
+    bool MainFile =
+        SM.isWrittenInMainFile(SM.getExpansionLoc(ND.getBeginLoc()));
     return SymbolCollector::shouldCollectSymbol(
         ND, AST->getASTContext(), SymbolCollector::Options(), MainFile);
   }
@@ -272,13 +273,14 @@
         Args, Factory->create(), Files.get(),
         std::make_shared<PCHContainerOperations>());
 
-    InMemoryFileSystem->addFile(
-        TestHeaderName, 0, llvm::MemoryBuffer::getMemBuffer(HeaderCode));
+    InMemoryFileSystem->addFile(TestHeaderName, 0,
+                                llvm::MemoryBuffer::getMemBuffer(HeaderCode));
     InMemoryFileSystem->addFile(TestFileName, 0,
                                 llvm::MemoryBuffer::getMemBuffer(MainCode));
     Invocation.run();
     Symbols = Factory->Collector->takeSymbols();
     Refs = Factory->Collector->takeRefs();
+    Relations = Factory->Collector->takeRelations();
     return true;
   }
 
@@ -290,6 +292,7 @@
   std::string TestFileURI;
   SymbolSlab Symbols;
   RefSlab Refs;
+  RelationSlab Relations;
   SymbolCollector::Options CollectorOpts;
   std::unique_ptr<CommentHandler> PragmaHandler;
 };
@@ -634,6 +637,19 @@
                                   HaveRanges(Header.ranges()))));
 }
 
+TEST_F(SymbolCollectorTest, Relations) {
+  std::string Header = R"(
+  class Base {};
+  class Derived : public Base {};
+  )";
+  runSymbolCollector(Header, /*Main=*/"");
+  const Symbol &Base = findSymbol(Symbols, "Base");
+  const Symbol &Derived = findSymbol(Symbols, "Derived");
+  EXPECT_THAT(Relations,
+              Contains(Relation{Base.ID, index::SymbolRole::RelationBaseOf,
+                                Derived.ID}));
+}
+
 TEST_F(SymbolCollectorTest, References) {
   const std::string Header = R"(
     class W;
@@ -783,10 +799,9 @@
     void f1() {}
   )";
   runSymbolCollector(/*Header=*/"", Main);
-  EXPECT_THAT(Symbols,
-              UnorderedElementsAre(QName("Foo"), QName("f1"), QName("f2"),
-                                   QName("ff"), QName("foo"), QName("foo::Bar"),
-                                   QName("main_f")));
+  EXPECT_THAT(Symbols, UnorderedElementsAre(
+                           QName("Foo"), QName("f1"), QName("f2"), QName("ff"),
+                           QName("foo"), QName("foo::Bar"), QName("main_f")));
 }
 
 TEST_F(SymbolCollectorTest, Documentation) {