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, 4 deletions

diff --git a/compiler/GHC/Hs/Instances.hs b/compiler/GHC/Hs/Instances.hs
index 76ce16948b..7515c37fb5 100644
--- a/compiler/GHC/Hs/Instances.hs
+++ b/compiler/GHC/Hs/Instances.hs
@@ -403,10 +403,15 @@ deriving instance Data (LHsQTyVars GhcPs)
 deriving instance Data (LHsQTyVars GhcRn)
 deriving instance Data (LHsQTyVars GhcTc)
 
--- deriving instance (DataIdLR p p, Data thing) =>Data (HsImplicitBndrs p thing)
-deriving instance (Data thing) => Data (HsImplicitBndrs GhcPs thing)
-deriving instance (Data thing) => Data (HsImplicitBndrs GhcRn thing)
-deriving instance (Data thing) => Data (HsImplicitBndrs GhcTc thing)
+-- deriving instance (Data flag, DataIdLR p p) => Data (HsOuterTyVarBndrs p)
+deriving instance Data flag => Data (HsOuterTyVarBndrs flag GhcPs)
+deriving instance Data flag => Data (HsOuterTyVarBndrs flag GhcRn)
+deriving instance Data flag => Data (HsOuterTyVarBndrs flag GhcTc)
+
+-- deriving instance (DataIdLR p p) => Data (HsSigType p)
+deriving instance Data (HsSigType GhcPs)
+deriving instance Data (HsSigType GhcRn)
+deriving instance Data (HsSigType GhcTc)
 
 -- deriving instance (DataIdLR p p, Data thing) =>Data (HsWildCardBndrs p thing)
 deriving instance (Data thing) => Data (HsWildCardBndrs GhcPs thing)