diff --git a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
--- a/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp
@@ -54,6 +54,9 @@
   return Field(&Diag::Fixes, UnorderedElementsAre(FixMatcher1, FixMatcher2));
 }
 
+::testing::Matcher<const Diag &> withID(unsigned ID) {
+  return Field(&Diag::ID, ID);
+}
 ::testing::Matcher<const Diag &>
 withNote(::testing::Matcher<Note> NoteMatcher) {
   return Field(&Diag::Notes, ElementsAre(NoteMatcher));
@@ -1869,6 +1872,20 @@
                       "'int' to 'int *' for 1st argument; take the address of "
                       "the argument with &")))));
 }
+
+TEST(DiagnosticsTest, UnusedInHeader) {
+  // Clang diagnoses unused static inline functions outside headers.
+  auto TU = TestTU::withCode("static inline void foo(void) {}");
+  TU.ExtraArgs.push_back("-Wunused-function");
+  TU.Filename = "test.c";
+  EXPECT_THAT(*TU.build().getDiagnostics(),
+              ElementsAre(withID(diag::warn_unused_function)));
+  // Sema should recognize a *.h file open in clangd as a header.
+  // https://github.com/clangd/vscode-clangd/issues/360
+  TU.Filename = "test.h";
+  EXPECT_THAT(*TU.build().getDiagnostics(), IsEmpty());
+}
+
 } // namespace
 } // namespace clangd
 } // namespace clang
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -1773,7 +1773,7 @@
 // FIXME: This needs to be refactored; some other isInMainFile users want
 // these semantics.
 static bool isMainFileLoc(const Sema &S, SourceLocation Loc) {
-  if (S.TUKind != TU_Complete)
+  if (S.TUKind != TU_Complete || S.getLangOpts().IsHeaderFile)
     return false;
   return S.SourceMgr.isInMainFile(Loc);
 }