diff --git a/clang-tools-extra/clangd/ParsedAST.cpp b/clang-tools-extra/clangd/ParsedAST.cpp
--- a/clang-tools-extra/clangd/ParsedAST.cpp
+++ b/clang-tools-extra/clangd/ParsedAST.cpp
@@ -493,6 +493,7 @@
     // We restore the original severity in the level adjuster.
     // FIXME: It would be better to have a real API for this, but what?
     for (auto ID : {diag::ext_implicit_function_decl_c99,
+                    diag::ext_implicit_lib_function_decl,
                     diag::ext_implicit_lib_function_decl_c99,
                     diag::warn_implicit_function_decl}) {
       OverriddenSeverity.try_emplace(
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
@@ -1465,11 +1465,12 @@
 TEST(IncludeFixerTest, HeaderNamedInDiag) {
   Annotations Test(R"cpp(
     $insert[[]]int main() {
-      [[printf]](""); // error-ok
+      [[printf]]("");
     }
   )cpp");
   auto TU = TestTU::withCode(Test.code());
-  TU.ExtraArgs = {"-xc"};
+  TU.ExtraArgs = {"-xc", "-std=c99",
+                  "-Wno-error=implicit-function-declaration"};
   auto Index = buildIndexWithSymbol({});
   TU.ExternalIndex = Index.get();
 
@@ -1482,13 +1483,22 @@
                              "declarations"),
           withFix(Fix(Test.range("insert"), "#include <stdio.h>\n",
                       "Include <stdio.h> for symbol printf")))));
+
+  TU.ExtraArgs = {"-xc", "-std=c89"};
+  EXPECT_THAT(
+      *TU.build().getDiagnostics(),
+      ElementsAre(AllOf(
+          Diag(Test.range(), "implicitly declaring library function 'printf' "
+                             "with type 'int (const char *, ...)'"),
+          withFix(Fix(Test.range("insert"), "#include <stdio.h>\n",
+                      "Include <stdio.h> for symbol printf")))));
 }
 
 TEST(IncludeFixerTest, CImplicitFunctionDecl) {
-  Annotations Test("void x() { [[foo]](); /* error-ok */ }");
+  Annotations Test("void x() { [[foo]](); }");
   auto TU = TestTU::withCode(Test.code());
   TU.Filename = "test.c";
-  TU.ExtraArgs.push_back("-std=c99");
+  TU.ExtraArgs = {"-std=c99", "-Wno-error=implicit-function-declaration"};
 
   Symbol Sym = func("foo");
   Sym.Flags |= Symbol::IndexedForCodeCompletion;
@@ -1509,6 +1519,13 @@
                "support implicit function declarations"),
           withFix(Fix(Range{}, "#include \"foo.h\"\n",
                       "Include \"foo.h\" for symbol foo")))));
+
+  TU.ExtraArgs = {"-std=c89", "-Wall"};
+  EXPECT_THAT(*TU.build().getDiagnostics(),
+              ElementsAre(AllOf(
+                  Diag(Test.range(), "implicit declaration of function 'foo'"),
+                  withFix(Fix(Range{}, "#include \"foo.h\"\n",
+                              "Include \"foo.h\" for symbol foo")))));
 }
 
 TEST(DiagsInHeaders, DiagInsideHeader) {