Index: clang-tidy/google/AvoidCStyleCastsCheck.cpp
===================================================================
--- clang-tidy/google/AvoidCStyleCastsCheck.cpp
+++ clang-tidy/google/AvoidCStyleCastsCheck.cpp
@@ -110,6 +110,10 @@
   // compiled as C++.
   if (getCurrentMainFile().endswith(".c"))
     return;
+  // Ignore casts between Objective-C types.
+  if (SourceType->isObjCObjectPointerType() ||
+      DestType->isObjCObjectPointerType())
+    return;
 
   SourceManager &SM = *Result.SourceManager;
 
Index: test/clang-tidy/google-readability-casting.mm
===================================================================
--- /dev/null
+++ test/clang-tidy/google-readability-casting.mm
@@ -0,0 +1,42 @@
+// RUN: clang-tidy %s -checks=-*,google-readability-casting -- \
+// RUN:   -xobjective-c++ -fobjc-abi-version=2 -fobjc-arc | count 0
+
+// Note: this test expects no diagnostics, but FileCheck cannot handle that,
+// hence the use of | count 0.
+
+#define nil 0
+
+@interface Foo
+@end
+
+@protocol Proto
+@end
+
+@interface Bar : Foo <Proto>
+@end
+
+@interface Baz : Foo <Proto>
+@end
+
+void foo() {
+  id nilObj = nil;
+  Foo *foo = (Foo *)nilObj;
+  Bar *bar = (Bar *)nilObj;
+  id ego = (id)foo;
+  foo = (Foo *)bar;
+  foo = (id)bar;
+  id<Proto> nilProto = (id<Proto>)bar;
+  ego = (id)nilProto;
+  bar = (Bar *)nilProto;
+  Foo<Proto> *fooProto = (Foo<Proto> *)bar;
+  Baz *baz = (Baz *)bar;
+  Class klass = (Class)nilObj;
+  ego = (id)klass;
+  void *voidStar = nullptr;
+  foo = (__bridge Foo *)voidStar;
+  nilProto = (__bridge id<Proto>)voidStar;
+  klass = (__bridge Class)voidStar;
+  voidStar = (__bridge void *)foo;
+  voidStar = (__bridge void *)nilProto;
+  voidStar = (__bridge void *)klass;
+}