Index: clang-tools-extra/trunk/clangd/CodeComplete.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/CodeComplete.cpp
+++ clang-tools-extra/trunk/clangd/CodeComplete.cpp
@@ -235,6 +235,7 @@
                        llvm::StringRef SemaDocComment) const {
     assert(bool(SemaResult) == bool(SemaCCS));
     CompletionItem I;
+    bool ShouldInsertInclude = true;
     if (SemaResult) {
       I.kind = toCompletionItemKind(SemaResult->Kind, SemaResult->CursorKind);
       getLabelAndInsertText(*SemaCCS, &I.label, &I.insertText,
@@ -242,6 +243,20 @@
       I.filterText = getFilterText(*SemaCCS);
       I.documentation = formatDocumentation(*SemaCCS, SemaDocComment);
       I.detail = getDetail(*SemaCCS);
+      // Avoid inserting new #include if the declaration is found in the current
+      // file e.g. the symbol is forward declared.
+      if (SemaResult->Kind == CodeCompletionResult::RK_Declaration) {
+        if (const auto *D = SemaResult->getDeclaration()) {
+          const auto &SM = D->getASTContext().getSourceManager();
+          ShouldInsertInclude =
+              ShouldInsertInclude &&
+              std::none_of(D->redecls_begin(), D->redecls_end(),
+                           [&SM](const Decl *RD) {
+                             return SM.isInMainFile(
+                                 SM.getExpansionLoc(RD->getLocStart()));
+                           });
+        }
+      }
     }
     if (IndexResult) {
       if (I.kind == CompletionItemKind::Missing)
@@ -263,7 +278,7 @@
           I.documentation = D->Documentation;
         if (I.detail.empty())
           I.detail = D->CompletionDetail;
-        if (Includes && !D->IncludeHeader.empty()) {
+        if (ShouldInsertInclude && Includes && !D->IncludeHeader.empty()) {
           auto Edit = [&]() -> Expected<Optional<TextEdit>> {
             auto ResolvedDeclaring = toHeaderFile(
                 IndexResult->CanonicalDeclaration.FileURI, FileName);
Index: clang-tools-extra/trunk/unittests/clangd/CodeCompleteTests.cpp
===================================================================
--- clang-tools-extra/trunk/unittests/clangd/CodeCompleteTests.cpp
+++ clang-tools-extra/trunk/unittests/clangd/CodeCompleteTests.cpp
@@ -159,7 +159,7 @@
   return sym(Name, index::SymbolKind::Function, "@F@\\0#"); // no args
 }
 Symbol cls(StringRef Name) {
-  return sym(Name, index::SymbolKind::Class, "@S@\\0@S@\\0");
+  return sym(Name, index::SymbolKind::Class, "@S@\\0");
 }
 Symbol var(StringRef Name) {
   return sym(Name, index::SymbolKind::Variable, "@\\0");
@@ -425,10 +425,9 @@
   auto Results = completions(
       R"cpp(
           class Adapter {
-            void method();
           };
 
-          void Adapter::method() {
+          void f() {
             Adapter^
           }
       )cpp",
@@ -559,6 +558,37 @@
               ElementsAre(AllOf(Named("X"), Not(HasAdditionalEdits()))));
 }
 
+TEST(CompletionTest, NoIncludeInsertionWhenDeclFoundInFile) {
+  MockFSProvider FS;
+  MockCompilationDatabase CDB;
+
+  IgnoreDiagnostics DiagConsumer;
+  ClangdServer Server(CDB, FS, DiagConsumer, ClangdServer::optsForTest());
+  Symbol::Details Scratch;
+  Symbol SymX = cls("ns::X");
+  Symbol SymY = cls("ns::Y");
+  std::string BarHeader = testPath("bar.h");
+  auto BarURI = URI::createFile(BarHeader).toString();
+  SymX.CanonicalDeclaration.FileURI = BarURI;
+  SymY.CanonicalDeclaration.FileURI = BarURI;
+  Scratch.IncludeHeader = "<bar>";
+  SymX.Detail = &Scratch;
+  SymY.Detail = &Scratch;
+  // Shoten include path based on search dirctory and insert.
+  auto Results = completions(Server,
+                             R"cpp(
+          namespace ns {
+            class X;
+            class Y {}
+          }
+          int main() { ns::^ }
+      )cpp",
+                             {SymX, SymY});
+  EXPECT_THAT(Results.items,
+              ElementsAre(AllOf(Named("X"), Not(HasAdditionalEdits())),
+                          AllOf(Named("Y"), Not(HasAdditionalEdits()))));
+}
+
 TEST(CompletionTest, IndexSuppressesPreambleCompletions) {
   MockFSProvider FS;
   MockCompilationDatabase CDB;