Vladislav Zavialov authored Aug 02, 2024 and Zubin committed Sep 27, 2024

Add support for wildcard binders in type declarations:

	type Const a b = a   -- BEFORE: the `b` had to be named
	                     --         even if unused on the RHS

	type Const a _ = a   -- AFTER: the compiler accepts
	                     --        a wildcard binder `_`

The new feature is part of GHC Proposal #425 "Invisible binders
in type declarations", and more specifically its amendment #641.

Just like a named binder, a wildcard binder `_` may be:

	* plain:      _
	* kinded:    (_ :: k -> Type)
	* invisible, plain:  @_
	* invisible, kinded: @(_ :: k -> Type)

Those new forms of binders are allowed to occur on the LHSs of
data, newtype, type, class, and type/data family declarations:

	data D _ = ...
	newtype N _ = ...
	type T _ = ...
	class C _ where ...
	type family F _
	data family DF _

(Test case: testsuite/tests/typecheck/should_compile/T23501a.hs)

However, we choose to reject them in forall telescopes and
type family result variable binders (the latter being part
of the TypeFamilyDependencies extension):

	type family Fd a = _    -- disallowed  (WildcardBndrInTyFamResultVar)
	fn :: forall _. Int     -- disallowed  (WildcardBndrInForallTelescope)

(Test case: testsuite/tests/rename/should_fail/T23501_fail.hs)

See the new Notes:
	* Note [Type variable binders]
	* Note [Wildcard binders in disallowed contexts]

To accommodate the new forms of binders, HsTyVarBndr was changed
as follows (demonstrated without x-fields for clarity)

	-- BEFORE (ignoring x-fields and locations)
	data HsTyVarBndr flag
	  = UserTyVar   flag Name
	  | KindedTyVar flag Name HsKind

	-- AFTER (ignoring x-fields and locations)
	data HsTyVarBndr flag = HsTvb flag HsBndrVar HsBndrKind
	data HsBndrVar  = HsBndrVar Name | HsBndrWildCard
	data HsBndrKind = HsBndrNoKind | HsBndrKind LHsKind

The rest of the patch is downstream from this change.

To avoid a breaking change to the TH AST, we generate fresh
names to replace wildcard binders instead of adding a dedicated
representation for them (as discussed in #641).

And to put a cherry on top of the cake, we now allow wildcards in
kind-polymorphic type variable binders in constructor patterns,
see Note [Type patterns: binders and unifiers] and the tyPatToBndr
function in GHC.Tc.Gen.HsType; example:

	fn (MkT @(_ :: forall k. k -> Type) _ _) = ...

(Test case: testsuite/tests/typecheck/should_compile/T23501b.hs)