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.

3 files changed, 29 insertions, 0 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T12102b.hs b/testsuite/tests/typecheck/should_fail/T12102b.hs
new file mode 100644
index 0000000000..8478059c8f
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T12102b.hs
@@ -0,0 +1,21 @@
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE GADTs #-}
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TypeFamilies #-}
+module T12102b where
+
+import Data.Kind
+import GHC.TypeLits
+
+type family IsTypeLit a where
+  IsTypeLit Nat    = 'True
+  IsTypeLit Symbol = 'True
+  IsTypeLit a      = 'False
+
+data T :: forall a. (IsTypeLit a ~ 'True) => a -> Type where
+  MkNat    :: T 42
+  MkSymbol :: T "Don't panic!"
+
+deriving instance Show (T a)
diff --git a/testsuite/tests/typecheck/should_fail/T12102b.stderr b/testsuite/tests/typecheck/should_fail/T12102b.stderr
new file mode 100644
index 0000000000..d93b7fda9c
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T12102b.stderr
@@ -0,0 +1,7 @@
+
+T12102b.hs:21:25: error:
+    • Couldn't match expected kind ‘'True’
+                  with actual kind ‘IsTypeLit a0’
+      The type variable ‘a0’ is ambiguous
+    • In the first argument of ‘Show’, namely ‘(T a)’
+      In the stand-alone deriving instance for ‘Show (T a)’
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index d155ca0e8c..fec3a3ae67 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -411,6 +411,7 @@ test('T12083a', normal, compile_fail, [''])
 test('T12083b', normal, compile_fail, [''])
 test('T11974b', normal, compile_fail, [''])
 test('T12102', normal, compile, [''])
+test('T12102b', normal, compile_fail, [''])
 test('T12151', normal, compile_fail, [''])
 test('T7437', normal, compile_fail, [''])
 test('T12177', normal, compile_fail, [''])