Replace HsImplicitBndrs with HsOuterTyVarBndrs

This refactors the GHC AST to remove `HsImplicitBndrs` and replace it with
`HsOuterTyVarBndrs`, a type which records whether the outermost quantification
in a type is explicit (i.e., with an outermost, invisible `forall`) or
implicit. As a result of this refactoring, it is now evident in the AST where
the `forall`-or-nothing rule applies: it's all the places that use
`HsOuterTyVarBndrs`. See the revamped `Note [forall-or-nothing rule]` in
`GHC.Hs.Type` (previously in `GHC.Rename.HsType`).

Moreover, the places where `ScopedTypeVariables` brings lexically scoped type
variables into scope are a subset of the places that adhere to the
`forall`-or-nothing rule, so this also makes places that interact with
`ScopedTypeVariables` easier to find. See the revamped
`Note [Lexically scoped type variables]` in `GHC.Hs.Type` (previously in
`GHC.Tc.Gen.Sig`).

`HsOuterTyVarBndrs` are used in type signatures (see `HsOuterSigTyVarBndrs`)
and type family equations (see `HsOuterFamEqnTyVarBndrs`). The main difference
between the former and the latter is that the former cares about specificity
but the latter does not.

There are a number of knock-on consequences:

* There is now a dedicated `HsSigType` type, which is the combination of
  `HsOuterSigTyVarBndrs` and `HsType`. `LHsSigType` is now an alias for an
  `XRec` of `HsSigType`.
* Working out the details led us to a substantial refactoring of
  the handling of explicit (user-written) and implicit type-variable
  bindings in `GHC.Tc.Gen.HsType`.

  Instead of a confusing family of higher order functions, we now
  have a local data type, `SkolemInfo`, that controls how these
  binders are kind-checked.

  It remains very fiddly, not fully satisfying. But it's better
  than it was.

Fixes #16762. Bumps the Haddock submodule.

Co-authored-by: Simon Peyton Jones <simonpj@microsoft.com>
Co-authored-by: Richard Eisenberg <rae@richarde.dev>
Co-authored-by: Zubin Duggal <zubin@cmi.ac.in>

3 files changed, 5 insertions, 6 deletions

diff --git a/testsuite/tests/dependent/should_fail/BadTelescope5.stderr b/testsuite/tests/dependent/should_fail/BadTelescope5.stderr
index d2ec36e5df..02daf9d742 100644
--- a/testsuite/tests/dependent/should_fail/BadTelescope5.stderr
+++ b/testsuite/tests/dependent/should_fail/BadTelescope5.stderr
@@ -2,10 +2,9 @@
 BadTelescope5.hs:10:81: error:
     • Expected kind ‘k’, but ‘d’ has kind ‘Proxy a’
       ‘k’ is a rigid type variable bound by
-        ‘forall a k (b :: k) (c :: Proxy b) (d :: Proxy a).
-         Proxy c -> SameKind b d’
+        an explicit forall a k (b :: k) (c :: Proxy b) (d :: Proxy a)
         at BadTelescope5.hs:10:17
     • In the second argument of ‘SameKind’, namely ‘d’
       In the type signature:
-        bar :: forall a k (b :: k) (c :: Proxy b) (d :: Proxy a).
-               Proxy c -> SameKind b d
+        bar :: forall a k (b :: k) (c :: Proxy b) (d :: Proxy a). Proxy c
+                                                                  -> SameKind b d
diff --git a/testsuite/tests/dependent/should_fail/T14066.stderr b/testsuite/tests/dependent/should_fail/T14066.stderr
index d958f9a519..240108c296 100644
--- a/testsuite/tests/dependent/should_fail/T14066.stderr
+++ b/testsuite/tests/dependent/should_fail/T14066.stderr
@@ -3,7 +3,7 @@ T14066.hs:15:59: error:
     • Expected kind ‘k0’, but ‘b’ has kind ‘k’
         because kind variable ‘k’ would escape its scope
       This (rigid, skolem) kind variable is bound by
-        ‘forall k (b :: k). SameKind a b’
+        an explicit forall k (b :: k)
         at T14066.hs:15:29-59
     • In the second argument of ‘SameKind’, namely ‘b’
       In the type signature: g :: forall k (b :: k). SameKind a b
diff --git a/testsuite/tests/dependent/should_fail/TypeSkolEscape.stderr b/testsuite/tests/dependent/should_fail/TypeSkolEscape.stderr
index a919095337..d642d6201c 100644
--- a/testsuite/tests/dependent/should_fail/TypeSkolEscape.stderr
+++ b/testsuite/tests/dependent/should_fail/TypeSkolEscape.stderr
@@ -3,7 +3,7 @@ TypeSkolEscape.hs:9:52: error:
     • Expected kind ‘k0’, but ‘a’ has kind ‘TYPE v’
         because kind variable ‘v’ would escape its scope
       This (rigid, skolem) kind variable is bound by
-        ‘forall (v :: RuntimeRep) (a :: TYPE v). a’
+        an explicit forall (v :: RuntimeRep) (a :: TYPE v)
         at TypeSkolEscape.hs:9:12-52
     • In the type ‘forall (v :: RuntimeRep) (a :: TYPE v). a’
       In the type declaration for ‘Bad’