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, 7 insertions, 3 deletions

diff --git a/testsuite/tests/typecheck/should_fail/T15799.stderr b/testsuite/tests/typecheck/should_fail/T15799.stderr
index 6823cadb8c..161cfe026a 100644
--- a/testsuite/tests/typecheck/should_fail/T15799.stderr
+++ b/testsuite/tests/typecheck/should_fail/T15799.stderr
@@ -1,5 +1,9 @@
 
 T15799.hs:46:62: error:
-    Couldn't match kind ‘TypeLits.Natural’ with ‘Op Nat’
-    • Expected kind ‘Op Nat’, but ‘UnOp b’ has kind ‘Nat’
-    • In the first argument of ‘(<=)’, namely ‘UnOp b’
+    Expected a constraint, but ‘UnOp b <= a’ has kind ‘*’
+
+T15799.hs:46:67: error:
+    • Couldn't match kind ‘TypeLits.Natural’ with ‘Op Nat’
+      Expected kind ‘Op (Op Nat)’, but ‘b’ has kind ‘Op Nat’
+    • In the first argument of ‘UnOp’, namely ‘b’
+      In the first argument of ‘(<=)’, namely ‘UnOp b’