Refactor (~) to reduce the superclass stack

Currently (see Note [The equality types story] in TysPrim) we have

  -- Hetereogeneous equality
  class a ~# b => a ~~ b
  instance a ~# b => a ~~ b

  -- Homogeneous equality
  class a ~~ b => (a :: k) ~ (b :: k)
  instance a ~~ b => a ~ b

Note that (~#) is a superclass of (~~), and (~~) is a superclass of (~). This means that in the common case of using (~) we need two superclass selections to get to the (~#) we want.

Nothing really wrong with that, but it bloats programs, and is confusing to read when debugging. I propose to change this to

  -- Homogeneous equality
  class a ~# b => (a :: k) ~ (b :: k)
  instance a ~# b => a ~ b

That is, implement (~) in precisely the same way as (~~).

This makes (~) a tiny bit more baked-in to the compiler, but in exchange it behaves in the same way as (~~), instead of behaving in a slightly different way.

There should be no observable effect for users.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture