diff --git a/clang-tools-extra/clangd/SourceCode.h b/clang-tools-extra/clangd/SourceCode.h
--- a/clang-tools-extra/clangd/SourceCode.h
+++ b/clang-tools-extra/clangd/SourceCode.h
@@ -292,10 +292,15 @@
   llvm::StringRef Name;
   const MacroInfo *Info;
 };
-// Gets the macro at a specified \p Loc.
+/// Gets the macro at a specified \p Loc.
 llvm::Optional<DefinedMacro> locateMacroAt(SourceLocation Loc,
                                            Preprocessor &PP);
 
+/// Infers whether this is a header from the FileName and LangOpts (if
+/// presents).
+bool isHeaderFile(llvm::StringRef FileName,
+                  llvm::Optional<LangOptions> LangOpts = llvm::None);
+
 } // namespace clangd
 } // namespace clang
 #endif
diff --git a/clang-tools-extra/clangd/SourceCode.cpp b/clang-tools-extra/clangd/SourceCode.cpp
--- a/clang-tools-extra/clangd/SourceCode.cpp
+++ b/clang-tools-extra/clangd/SourceCode.cpp
@@ -17,6 +17,7 @@
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Basic/TokenKinds.h"
+#include "clang/Driver/Types.h"
 #include "clang/Format/Format.h"
 #include "clang/Lex/Lexer.h"
 #include "clang/Lex/Preprocessor.h"
@@ -1122,5 +1123,17 @@
   return ER;
 }
 
+bool isHeaderFile(llvm::StringRef FileName,
+                  llvm::Optional<LangOptions> LangOpts) {
+  // Respect the langOpts, for non-file-extension cases, e.g. standard library
+  // files.
+  if (LangOpts && LangOpts->IsHeaderFile)
+    return true;
+  namespace types = clang::driver::types;
+  auto Lang = types::lookupTypeForExtension(
+      llvm::sys::path::extension(FileName).substr(1));
+  return Lang != types::TY_INVALID && types::onlyPrecompileType(Lang);
+}
+
 } // namespace clangd
 } // namespace clang
diff --git a/clang-tools-extra/clangd/index/Background.cpp b/clang-tools-extra/clangd/index/Background.cpp
--- a/clang-tools-extra/clangd/index/Background.cpp
+++ b/clang-tools-extra/clangd/index/Background.cpp
@@ -205,11 +205,7 @@
 }
 
 void BackgroundIndex::boostRelated(llvm::StringRef Path) {
-  namespace types = clang::driver::types;
-  auto Type =
-      types::lookupTypeForExtension(llvm::sys::path::extension(Path).substr(1));
-  // is this a header?
-  if (Type != types::TY_INVALID && types::onlyPrecompileType(Type))
+  if (isHeaderFile(Path))
     Queue.boost(filenameWithoutExtension(Path), IndexBoostedFile);
 }
 
diff --git a/clang-tools-extra/clangd/unittests/SourceCodeTests.cpp b/clang-tools-extra/clangd/unittests/SourceCodeTests.cpp
--- a/clang-tools-extra/clangd/unittests/SourceCodeTests.cpp
+++ b/clang-tools-extra/clangd/unittests/SourceCodeTests.cpp
@@ -701,6 +701,30 @@
             collectIdentifierRanges("Foo", Code.code(), LangOpts));
 }
 
+TEST(SourceCodeTests, isHeaderFile) {
+  // Without lang options.
+  EXPECT_TRUE(isHeaderFile("foo.h"));
+  EXPECT_TRUE(isHeaderFile("foo.hh"));
+  EXPECT_TRUE(isHeaderFile("foo.hpp"));
+
+  EXPECT_FALSE(isHeaderFile("foo.cpp"));
+  EXPECT_FALSE(isHeaderFile("foo.c++"));
+  EXPECT_FALSE(isHeaderFile("foo.cxx"));
+  EXPECT_FALSE(isHeaderFile("foo.cc"));
+  EXPECT_FALSE(isHeaderFile("foo.c"));
+  EXPECT_FALSE(isHeaderFile("foo.mm"));
+  EXPECT_FALSE(isHeaderFile("foo.m"));
+
+  // With lang options
+  LangOptions LangOpts;
+  LangOpts.IsHeaderFile = true;
+  EXPECT_TRUE(isHeaderFile("string", LangOpts));
+  // Emulate cases where there is no "-x header" flag for a .h file, we still
+  // want to treat it as a header.
+  LangOpts.IsHeaderFile = false;
+  EXPECT_TRUE(isHeaderFile("header.h", LangOpts));
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang