Note [mkRuntimeRepCo]

Given
   class C a where { op :: Maybe a }
we will get an axiom
   axC a :: (C a :: CONSTRAINT r1) ~ (Maybe a :: TYPE r2)
(See Note [Type and Constraint are not apart] in GHC.Builtin.Types.Prim.)

Then we may call mkRuntimeRepCo on (axC ty), and that will return
   mkSelCo (SelTyCon 0 Nominal) (Kind (axC ty)) :: r1 ~ r2

So mkSelCo needs to be happy with decomposing a coercion of kind
   CONSTRAINT r1 ~ TYPE r2

Hence the use of `tyConIsTYPEorCONSTRAINT` in the assertion `good_call`
in `mkSelCo`. See #23018 for a concrete example.  (In this context it's
important that TYPE and CONSTRAINT have the same arity and kind, not
merely that they are not-apart; otherwise SelCo would not make sense.)