Index: clang-tools-extra/trunk/unittests/clangd/TUSchedulerTests.cpp
===================================================================
--- clang-tools-extra/trunk/unittests/clangd/TUSchedulerTests.cpp
+++ clang-tools-extra/trunk/unittests/clangd/TUSchedulerTests.cpp
@@ -308,6 +308,51 @@
               UnorderedElementsAre(Foo, AnyOf(Bar, Baz)));
 }
 
+TEST_F(TUSchedulerTests, EmptyPreamble) {
+  TUScheduler S(
+      /*AsyncThreadsCount=*/4, /*StorePreambleInMemory=*/true,
+      PreambleParsedCallback(),
+      /*UpdateDebounce=*/std::chrono::steady_clock::duration::zero(),
+      ASTRetentionPolicy());
+
+  auto Foo = testPath("foo.cpp");
+  auto Header = testPath("foo.h");
+
+  Files[Header] = "void foo()";
+  Timestamps[Header] = time_t(0);
+  auto WithPreamble = R"cpp(
+    #include "foo.h"
+    int main() {}
+  )cpp";
+  auto WithEmptyPreamble = R"cpp(int main() {})cpp";
+  S.update(Foo, getInputs(Foo, WithPreamble), WantDiagnostics::Auto,
+           [](std::vector<Diag>) {});
+  S.runWithPreamble("getNonEmptyPreamble", Foo,
+                    [&](llvm::Expected<InputsAndPreamble> Preamble) {
+                      // We expect to get a non-empty preamble.
+                      EXPECT_GT(cantFail(std::move(Preamble))
+                                    .Preamble->Preamble.getBounds()
+                                    .Size,
+                                0u);
+                    });
+  // Wait for the preamble is being built.
+  ASSERT_TRUE(S.blockUntilIdle(timeoutSeconds(1)));
+
+  // Update the file which results in an empty preamble.
+  S.update(Foo, getInputs(Foo, WithEmptyPreamble), WantDiagnostics::Auto,
+           [](std::vector<Diag>) {});
+  // Wait for the preamble is being built.
+  ASSERT_TRUE(S.blockUntilIdle(timeoutSeconds(1)));
+  S.runWithPreamble("getEmptyPreamble", Foo,
+                    [&](llvm::Expected<InputsAndPreamble> Preamble) {
+                      // We expect to get an empty preamble.
+                      EXPECT_EQ(cantFail(std::move(Preamble))
+                                    .Preamble->Preamble.getBounds()
+                                    .Size,
+                                0u);
+                    });
+}
+
 TEST_F(TUSchedulerTests, RunWaitsForPreamble) {
   // Testing strategy: we update the file and schedule a few preamble reads at
   // the same time. All reads should get the same non-null preamble.