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
@@ -420,7 +420,10 @@
 }
 
 bool isInsideMainFile(SourceLocation Loc, const SourceManager &SM) {
-  return Loc.isValid() && SM.isWrittenInMainFile(SM.getExpansionLoc(Loc));
+  if (!Loc.isValid())
+    return false;
+  FileID FID = SM.getFileID(SM.getExpansionLoc(Loc));
+  return FID == SM.getMainFileID() || FID == SM.getPreambleFileID();
 }
 
 llvm::Optional<SourceRange> toHalfOpenFileRange(const SourceManager &SM,
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
@@ -461,10 +461,12 @@
     class Header {};
   )cpp";
   TU.Code = R"cpp(
+    #define DEFINE_MAIN4 class Main4{};
     class Main1 {};
     DEFINE_CLASS(Main2)
     DEFINE_YY
     class Main {};
+    DEFINE_MAIN4
   )cpp";
   TU.ExtraArgs.push_back("-DHeader=Header3");
   TU.ExtraArgs.push_back("-DMain=Main3");
@@ -474,10 +476,13 @@
     return findDecl(AST, Name).getLocation();
   };
   for (const auto *HeaderDecl : {"Header1", "Header2", "Header3"})
-    EXPECT_FALSE(isInsideMainFile(DeclLoc(HeaderDecl), SM));
+    EXPECT_FALSE(isInsideMainFile(DeclLoc(HeaderDecl), SM)) << HeaderDecl;
 
-  for (const auto *MainDecl : {"Main1", "Main2", "Main3", "YY"})
-    EXPECT_TRUE(isInsideMainFile(DeclLoc(MainDecl), SM));
+  for (const auto *MainDecl : {"Main1", "Main2", "Main3", "Main4", "YY"})
+    EXPECT_TRUE(isInsideMainFile(DeclLoc(MainDecl), SM)) << MainDecl;
+
+  // Main4 is *spelled* in the preamble, but in the main-file part of it.
+  EXPECT_TRUE(isInsideMainFile(SM.getSpellingLoc(DeclLoc("Main4")), SM));
 }
 
 // Test for functions toHalfOpenFileRange and getHalfOpenFileRange