--- a/mfbt/CheckedInt.h
+++ b/mfbt/CheckedInt.h
@@ -556,17 +556,18 @@ class CheckedInt
 {
   protected:
     T mValue;
     bool mIsValid;
 
     template<typename U>
     CheckedInt(U value, bool isValid) : mValue(value), mIsValid(isValid)
     {
-      MOZ_STATIC_ASSERT(detail::IsSupported<T>::value,
+      MOZ_STATIC_ASSERT(detail::IsSupported<T>::value &&
+                        detail::IsSupported<U>::value,
                         "This type is not supported by CheckedInt");
     }
 
     friend class detail::NegateImpl<T>;
 
   public:
     /**
      * Constructs a checked integer with given @a value. The checked integer is
@@ -579,17 +580,18 @@ class CheckedInt
      * documentation for class CheckedInt, this constructor checks that its
      * argument is valid.
      */
     template<typename U>
     CheckedInt(U value)
       : mValue(T(value)),
         mIsValid(detail::IsInRange<T>(value))
     {
-      MOZ_STATIC_ASSERT(detail::IsSupported<T>::value,
+      MOZ_STATIC_ASSERT(detail::IsSupported<T>::value &&
+                        detail::IsSupported<U>::value,
                         "This type is not supported by CheckedInt");
     }
 
     /** Constructs a valid checked integer with initial value 0 */
     CheckedInt() : mValue(0), mIsValid(true)
     {
       MOZ_STATIC_ASSERT(detail::IsSupported<T>::value,
                         "This type is not supported by CheckedInt");
@@ -750,16 +752,19 @@ struct CastToCheckedIntImpl<T, CheckedIn
 };
 
 } // namespace detail
 
 template<typename T, typename U>
 inline typename detail::CastToCheckedIntImpl<T, U>::ReturnType
 castToCheckedInt(U u)
 {
+  MOZ_STATIC_ASSERT(detail::IsSupported<T>::value &&
+                    detail::IsSupported<U>::value,
+                    "This type is not supported by CheckedInt");
   return detail::CastToCheckedIntImpl<T, U>::run(u);
 }
 
 #define MOZ_CHECKEDINT_CONVENIENCE_BINARY_OPERATORS(OP, COMPOUND_OP)  \
 template<typename T>                                              \
 template<typename U>                                              \
 CheckedInt<T>& CheckedInt<T>::operator COMPOUND_OP(U rhs)         \
 {                                                                 \