Index: clang-tools-extra/trunk/clangd/Diagnostics.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/Diagnostics.cpp
+++ clang-tools-extra/trunk/clangd/Diagnostics.cpp
@@ -122,8 +122,12 @@
     return SM.getIncludeLoc(SM.getFileID(SLoc));
   };
   for (auto IncludeLocation = GetIncludeLoc(DiagLoc); IncludeLocation.isValid();
-       IncludeLocation = GetIncludeLoc(IncludeLocation))
-    IncludeInMainFile = IncludeLocation;
+       IncludeLocation = GetIncludeLoc(IncludeLocation)) {
+    if (clangd::isInsideMainFile(IncludeLocation, SM)) {
+      IncludeInMainFile = IncludeLocation;
+      break;
+    }
+  }
   if (IncludeInMainFile.isInvalid())
     return false;
 
Index: clang-tools-extra/trunk/clangd/unittests/DiagnosticsTests.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/unittests/DiagnosticsTests.cpp
+++ clang-tools-extra/trunk/clangd/unittests/DiagnosticsTests.cpp
@@ -948,6 +948,15 @@
   EXPECT_THAT(TU.build().getDiagnostics(), UnorderedElementsAre());
 }
 
+TEST(IgnoreDiags, FromNonWrittenInclude) {
+  TestTU TU = TestTU::withCode("");
+  TU.ExtraArgs.push_back("--include=a.h");
+  TU.AdditionalFiles = {{"a.h", "void main();"}};
+  // The diagnostic "main must return int" is from the header, we don't attempt
+  // to render it in the main file as there is no written location there.
+  EXPECT_THAT(TU.build().getDiagnostics(), UnorderedElementsAre());
+}
+
 } // namespace
 
 } // namespace clangd