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>

1 files changed, 9 insertions, 8 deletions

diff --git a/testsuite/tests/parser/should_compile/T15323.stderr b/testsuite/tests/parser/should_compile/T15323.stderr
index 8539599660..ae19f0a174 100644
--- a/testsuite/tests/parser/should_compile/T15323.stderr
+++ b/testsuite/tests/parser/should_compile/T15323.stderr
@@ -42,14 +42,15 @@
             (Unqual
              {OccName: TestParens}))]
           ({ T15323.hs:6:20-54 }
-           (True))
-          [({ T15323.hs:6:27 }
-            (UserTyVar
-             (NoExtField)
-             (SpecifiedSpec)
-             ({ T15323.hs:6:27 }
-              (Unqual
-               {OccName: v}))))]
+           (HsOuterExplicit
+            (NoExtField)
+            [({ T15323.hs:6:27 }
+              (UserTyVar
+               (NoExtField)
+               (SpecifiedSpec)
+               ({ T15323.hs:6:27 }
+                (Unqual
+                 {OccName: v}))))]))
           (Just
            ({ T15323.hs:6:31-36 }
             [({ T15323.hs:6:31-36 }