Note [Using typesAreApart when calling mightMentionIP]

We call 'mightMentionIP' in two situations:

  (1) to check that a predicate does not contain any implicit parameters
      IP str ty, for a fixed literal str and any type ty,
  (2) to check that a predicate does not contain any HasCallStack or
      HasExceptionContext constraints.

In both of these cases, we want to be sure, so we should be conservative:

  For (1), the predicate might contain an implicit parameter IP Str a, where
  Str is a type family such as:

    type family MyStr where MyStr = "abc"

  To safeguard against this (niche) situation, instead of doing a simple
  type equality check, we use 'typesAreApart'. This allows us to recognise
  that 'IP MyStr a' contains an implicit parameter of the form 'IP "abc" ty'.

  For (2), we similarly might have

    type family MyCallStack where MyCallStack = CallStack

  Again, here we use 'typesAreApart'. This allows us to see that

    (?foo :: MyCallStack)

  is indeed a CallStack constraint, hidden under a type family.