Make Callback.Equals() only work when comparing against the same type.

base/callback_unittest.nc

Index: base/callback_unittest.nc
diff --git a/base/callback_unittest.nc b/base/callback_unittest.nc
new file mode 100644
index 0000000000000000000000000000000000000000..9bddd1f8367cd7af4fc359455c443df00b5767d5
--- /dev/null
+++ b/base/callback_unittest.nc
@@ -0,0 +1,50 @@
+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/callback.h"
+
+namespace base {
+
+class Parent {
+};
+
+class Child : Parent {
+};
+
+#if defined(NCTEST_EQUALS_REQUIRES_SAMETYPE)  // [r"no matching function for call to 'base::Callback<void\(\)>::Equals\(base::Callback<int\(\)>&\)'"]
+
+// Attempting to call comparison function on two callbacks of different type.
+//
+// This should be a compile time failure because each callback type should be
+// considered distinct.
+void WontCompile() {
+  Closure c1;
+  Callback<int(void)> c2;
+  c1.Equals(c2);
+}
+
+#elif defined(NCTEST_CONSTRUCTION_FROM_SUBTYPE)  // [r"conversion from 'base::Callback<base::Parent\(\)>' to non-scalar type 'base::Callback<base::Child\(\)>'"]
+
+// Construction of Callback<A> from Callback<B> if A is supertype of B.
+//
+// While this is technically safe, most people aren't used to it when coding
+// C++ so if this is happening, it is almost certainly an error.
+void WontCompile() {
+  Callback<Parent(void)> cb_a;
+  Callback<Child(void)> cb_b = cb_a;
+}
+
+#elif defined(NCTEST_ASSIGNMENT_FROM_SUBTYPE)  // [r"no match for 'operator=' in 'cb_a = cb_b'"]
+
+// Assignment of Callback<A> from Callback<B> if A is supertype of B.
+// See explanation for NCTEST_CONSTRUCTION_FROM_SUBTYPE
+void WontCompile() {
+  Callback<Parent(void)> cb_a;
+  Callback<Child(void)> cb_b;
+  cb_a = cb_b;
+}
+
+#endif
+
+}  // namespace base