--- a/mfbt/TypeTraits.h
+++ b/mfbt/TypeTraits.h
@@ -22,18 +22,58 @@ namespace mozilla {
  * mozilla::IsBaseOf<A, C>::value is false;
  */
 template<class Base, class Derived>
 class IsBaseOf
 {
   private:
     static char test(Base* b);
     static int test(...);
+
   public:
-    static const bool value = (sizeof(test(static_cast<Derived*>(0))) == sizeof(char));
+    static const bool value =
+      sizeof(test(static_cast<Derived*>(0))) == sizeof(char);
+};
+
+/*
+ * IsConvertible determines whether a value of type From will implicitly convert
+ * to a value of type To.  For example:
+ *
+ *   struct A {};
+ *   struct B : public A {};
+ *   struct C {};
+ *
+ * mozilla::IsConvertible<A, A>::value is true;
+ * mozilla::IsConvertible<A*, A*>::value is true;
+ * mozilla::IsConvertible<B, A>::value is true;
+ * mozilla::IsConvertible<B*, A*>::value is true;
+ * mozilla::IsConvertible<C, A>::value is false;
+ * mozilla::IsConvertible<A, C>::value is false;
+ * mozilla::IsConvertible<A*, C*>::value is false;
+ * mozilla::IsConvertible<C*, A*>::value is false.
+ *
+ * For obscure reasons, you can't use IsConvertible when the types being tested
+ * are related through private inheritance, and you'll get a compile error if
+ * you try.  Just don't do it!
+ */
+template<typename From, typename To>
+struct IsConvertible
+{
+  private:
+    static From create();
+
+    template<typename From1, typename To1>
+    static char test(To to);
+
+    template<typename From1, typename To1>
+    static int test(...);
+
+  public:
+    static const bool value =
+      sizeof(test<From, To>(create())) == sizeof(char);
 };
 
 /*
  * Conditional selects a class between two, depending on a given boolean value.
  *
  * mozilla::Conditional<true, A, B>::Type is A;
  * mozilla::Conditional<false, A, B>::Type is B;
  */

--- a/mfbt/tests/Makefile.in
+++ b/mfbt/tests/Makefile.in
@@ -8,16 +8,17 @@ srcdir = @srcdir@
 VPATH = @srcdir@
 
 include $(DEPTH)/config/autoconf.mk
 
 STL_FLAGS =
 
 CPP_UNIT_TESTS = \
   TestCheckedInt.cpp \
+  TestTypeTraits.cpp \
   $(NULL)
 
 # in order to prevent rules.mk from trying to link to libraries that are
 # not available to MFBT, we have to reset these MOZ_GLUE*_LDFLAGS before including it
 # and LIBS_ after including it. For WRAP_LDFLAGS, it shouldn't matter.
 # See later comments in bug 732875.
 
 MOZ_GLUE_PROGRAM_LDFLAGS=

new file mode 100644
--- /dev/null
+++ b/mfbt/tests/TestTypeTraits.cpp
@@ -0,0 +1,55 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "mozilla/Assertions.h"
+#include "mozilla/TypeTraits.h"
+
+using mozilla::IsConvertible;
+
+class A { };
+class B : public A { };
+class C : private A { };
+class D { };
+
+static void
+TestIsConvertible()
+{
+  // Pointer type convertibility
+  MOZ_ASSERT((IsConvertible<A*, A*>::value),
+             "A* should convert to A*");
+  MOZ_ASSERT((IsConvertible<B*, A*>::value),
+             "B* should convert to A*");
+  MOZ_ASSERT((!IsConvertible<A*, B*>::value),
+             "A* shouldn't convert to B*");
+  MOZ_ASSERT((!IsConvertible<A*, C*>::value),
+             "A* shouldn't convert to C*");
+  MOZ_ASSERT((!IsConvertible<A*, D*>::value),
+             "A* shouldn't convert to unrelated D*");
+  MOZ_ASSERT((!IsConvertible<D*, A*>::value),
+             "D* shouldn't convert to unrelated A*");
+
+  // Instance type convertibility
+  MOZ_ASSERT((IsConvertible<A, A>::value),
+             "A is A");
+  MOZ_ASSERT((IsConvertible<B, A>::value),
+             "B converts to A");
+  MOZ_ASSERT((!IsConvertible<D, A>::value),
+             "D and A are unrelated");
+  MOZ_ASSERT((!IsConvertible<A, D>::value),
+             "A and D are unrelated");
+
+  // These cases seem to require C++11 support to properly implement them, so
+  // for now just disable them.
+  //MOZ_ASSERT((!IsConvertible<C*, A*>::value),
+  //           "C* shouldn't convert to A* (private inheritance)");
+  //MOZ_ASSERT((!IsConvertible<C, A>::value),
+  //           "C doesn't convert to A (private inheritance)");
+}
+
+int
+main()
+{
+  TestIsConvertible();
+}