“Illegal instance declaration” when declaring instance of IsString

Question

I\'m writing an application that uses UTF-16 strings, and to make use of the overloaded strings extension I tried to make an IsString instance for it:

Answer 1

Why is this rejected?

Because:

  (All instance types must be of the form (T a1 ... an)
   where a1 ... an are *distinct type variables*,
   and each type variable appears at most once in the instance head.

Is there something I've missed?

Yes:

   Use -XFlexibleInstances if you want to disable this.)