Make equality constraints in kinds invisible

Issues #12102 and #15872 revealed something strange about the way GHC
handles equality constraints in kinds: it treats them as _visible_
arguments! This causes a litany of strange effects, from strange
error messages
(https://gitlab.haskell.org/ghc/ghc/issues/12102#note_169035)
to bizarre `Eq#`-related things leaking through to GHCi output, even
without any special flags enabled.

This patch is an attempt to contain some of this strangeness.
In particular:

* In `TcHsType.etaExpandAlgTyCon`, we propagate through the
  `AnonArgFlag`s of any `Anon` binders. Previously, we were always
  hard-coding them to `VisArg`, which meant that invisible binders
  (like those whose kinds were equality constraint) would mistakenly
  get flagged as visible.
* In `ToIface.toIfaceAppArgsX`, we previously assumed that the
  argument to a `FunTy` always corresponding to a `Required`
  argument. We now dispatch on the `FunTy`'s `AnonArgFlag` and map
  `VisArg` to `Required` and `InvisArg` to `Inferred`. As a
  consequence, the iface pretty-printer correctly recognizes that
  equality coercions are inferred arguments, and as a result,
  only displays them in `-fprint-explicit-kinds` is enabled.
* Speaking of iface pretty-printing, `Anon InvisArg` binders were
  previously being pretty-printed like `T (a :: b ~ c)`, as if they
  were required. This seemed inconsistent with other invisible
  arguments (that are printed like `T @{d}`), so I decided to switch
  this to `T @{a :: b ~ c}`.

Along the way, I also cleaned up a minor inaccuracy in the users'
guide section for constraints in kinds that was spotted in
https://gitlab.haskell.org/ghc/ghc/issues/12102#note_136220.

Fixes #12102 and #15872.

1 files changed, 8 insertions, 2 deletions

diff --git a/compiler/iface/ToIface.hs b/compiler/iface/ToIface.hs
index aa4e53cfb4..535c1080f2 100644
--- a/compiler/iface/ToIface.hs
+++ b/compiler/iface/ToIface.hs
@@ -309,8 +309,14 @@ toIfaceAppArgsX fr kind ty_args
         t'  = toIfaceTypeX fr t
         ts' = go (extendTCvSubst env tv t) res ts
 
-    go env (FunTy { ft_res = res }) (t:ts) -- No type-class args in tycon apps
-      = IA_Arg (toIfaceTypeX fr t) Required (go env res ts)
+    go env (FunTy { ft_af = af, ft_res = res }) (t:ts)
+      = IA_Arg (toIfaceTypeX fr t) argf (go env res ts)
+      where
+        argf = case af of
+                 VisArg   -> Required
+                 InvisArg -> Inferred
+                   -- It's rare for a kind to have a constraint argument, but
+                   -- it can happen. See Note [AnonTCB InvisArg] in TyCon.
 
     go env ty ts@(t1:ts1)
       | not (isEmptyTCvSubst env)