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/google-objc-function-naming.m
===================================================================
--- test/clang-tidy/google-objc-function-naming.m
+++ test/clang-tidy/google-objc-function-naming.m
@@ -1,5 +1,13 @@
 // RUN: %check_clang_tidy %s google-objc-function-naming %t
 
+#import <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;
 
 static bool ispositive(int a) { return a > 0; }