Index: clang-tidy/google/FunctionNamingCheck.cpp
===================================================================
--- clang-tidy/google/FunctionNamingCheck.cpp
+++ clang-tidy/google/FunctionNamingCheck.cpp
@@ -93,12 +93,16 @@
   if (!getLangOpts().ObjC)
     return;
 
-  // Match function declarations that are not in system headers and are not
-  // main.
+  // Enforce Objective-C function naming conventions on all functions except:
+  // • Functions defined in system headers.
+  // • C++ member functions.
+  // • Namespaced functions.
+  // • Implicitly defined functions.
+  // • The main function.
   Finder->addMatcher(
       functionDecl(
           unless(anyOf(isExpansionInSystemHeader(), cxxMethodDecl(),
-                       hasAncestor(namespaceDecl()), isMain(),
+                       hasAncestor(namespaceDecl()), isMain(), isImplicit(),
                        matchesName(validFunctionNameRegex(true)),
                        allOf(isStaticStorageClass(),
                              matchesName(validFunctionNameRegex(false))))))
Index: test/clang-tidy/Inputs/Headers/stdio.h
===================================================================
--- test/clang-tidy/Inputs/Headers/stdio.h
+++ test/clang-tidy/Inputs/Headers/stdio.h
@@ -0,0 +1,18 @@
+//===--- stdio.h - Stub header for tests ------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _STDIO_H_
+#define _STDIO_H_
+
+// A header intended to contain C standard input and output library
+// declarations.
+
+int printf(const char *, ...);
+
+#endif // _STDIO_H_
+
Index: test/clang-tidy/google-objc-function-naming.m
===================================================================
--- test/clang-tidy/google-objc-function-naming.m
+++ test/clang-tidy/google-objc-function-naming.m
@@ -1,4 +1,12 @@
-// RUN: %check_clang_tidy %s google-objc-function-naming %t
+// RUN: %check_clang_tidy %s google-objc-function-naming %t -- -- -isystem %S/Inputs/Headers
+
+#include <stdio.h>
+
+static void TestImplicitFunctionDeclaration(int a) {
+  // Call a builtin function so that the compiler generates an implicit
+  // function declaration.
+  printf("%d", a);
+}
 
 typedef _Bool bool;