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
@@ -134,9 +134,10 @@
   void setIncludeLocation(const Symbol &S, SourceLocation);
   // Indexed macros, to be erased if they turned out to be include guards.
   llvm::DenseSet<const IdentifierInfo *> IndexedMacros;
-  // All refs collected from the AST.
-  // Only symbols declared in preamble (from #include) and referenced from the
-  // main file will be included.
+  // All refs collected from the AST. It includes:
+  //   1) symbols declared in the preamble and referenced from the main file (
+  //     which is not a header), or
+  //   2) symbols declared and referenced from the main file (which is a header)
   RefSlab::Builder Refs;
   // All relations collected from the AST.
   RelationSlab::Builder Relations;
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
@@ -314,10 +314,12 @@
   if (IsOnlyRef && !CollectRef)
     return true;
 
-  // ND is the canonical (i.e. first) declaration. If it's in the main file,
-  // then no public declaration was visible, so assume it's main-file only.
+  // ND is the canonical (i.e. first) declaration. If it's in the main file
+  // (which is not a header), then no public declaration was visible, so assume
+  // it's main-file only.
   bool IsMainFileOnly =
-      SM.isWrittenInMainFile(SM.getExpansionLoc(ND->getBeginLoc()));
+      SM.isWrittenInMainFile(SM.getExpansionLoc(ND->getBeginLoc())) &&
+      !ASTCtx->getLangOpts().IsHeaderFile;
   // In C, printf is a redecl of an implicit builtin! So check OrigD instead.
   if (ASTNode.OrigD->isImplicit() ||
       !shouldCollectSymbol(*ND, *ASTCtx, Opts, IsMainFileOnly))
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
@@ -626,6 +626,33 @@
   EXPECT_THAT(Refs, Not(Contains(Pair(findSymbol(MainSymbols, "c").ID, _))));
 }
 
+
+TEST_F(SymbolCollectorTest, HeaderAsMainFile) {
+  CollectorOpts.RefFilter = RefKind::All;
+  Annotations Header(R"(
+  class $Foo[[Foo]] {};
+
+  void $Func[[Func]]() {
+    $Foo[[Foo]] fo;
+  }
+  )");
+  // The main file is normal .cpp file, we shouldn't collect any refs of symbols
+  // which are not declared in the preamble.
+  TestFileName = testPath("foo.cpp");
+  runSymbolCollector("", Header.code());
+  EXPECT_THAT(Refs, UnorderedElementsAre());
+
+  // Run the .h file as main file, we should collect the refs.
+  TestFileName = testPath("foo.h");
+  runSymbolCollector("", Header.code(),
+                     /*ExtraArgs=*/{"-xobjective-c++-header"});
+  EXPECT_THAT(Symbols, UnorderedElementsAre(QName("Foo"), QName("Func")));
+  EXPECT_THAT(Refs, UnorderedElementsAre(Pair(findSymbol(Symbols, "Foo").ID,
+                                  HaveRanges(Header.ranges("Foo"))),
+                             Pair(findSymbol(Symbols, "Func").ID,
+                                  HaveRanges(Header.ranges("Func")))));
+}
+
 TEST_F(SymbolCollectorTest, RefsInHeaders) {
   CollectorOpts.RefFilter = RefKind::All;
   CollectorOpts.RefsInHeaders = true;