Simon Peyton Jones authored Jul 18, 2020 and Marge Bot committed Jul 24, 2020

Issue #18451 showed that we could get an infinite type, through
over-use of occCheckExpand in the kind of an /occurrence/ of a
type variable.

See Note [Occurrence checking: look inside kinds] in GHC.Core.Type

This patch fixes the problem by making occCheckExpand less eager
to expand synonyms in kinds.

It also improves pretty printing of kinds, by *not* suppressing
the kind on a tyvar-binder like
    (a :: Const Type b)
where type Const p q = p.  Even though the kind of 'a' is Type,
we don't want to suppress the kind ascription.  Example: the
error message for polykinds/T18451{a,b}. See GHC.Core.TyCo.Ppr
Note [Suppressing * kinds].