Suppressing False Incomplete Pattern Matching Warnings for Polymorphic Pattern Synonyms

[Sylvain Henry]

> In the case where all the patterns are polymorphic, a user must
> provide a type signature but we accept the definition regardless of
> the type signature they provide. 

Currently we can specify the type *constructor* in a COMPLETE pragma:

pattern J :: a -> Maybe apattern J a = Just apattern N :: Maybe apattern 
N = Nothing{-# COMPLETE N, J :: Maybe #-}


Instead if we could specify the type with its free vars, we could refer 
to them in conlike signatures:

{-# COMPLETE N, [J:: a -> Maybe a ] :: Maybe a #-}

The COMPLETE pragma for LL could be:

{-# COMPLETE [LL :: HasSrcSpan a => SrcSpan -> SrcSpanLess a -> a ] :: a 
#-}


I'm borrowing the list comprehension syntax on purpose because it would 
allow to define a set of conlikes from a type-list (see my request [1]):

{-# COMPLETE [V :: (c :< cs) => c -> Variant cs | c <- cs ] :: Variant 
cs #-}


>    To make things more formal, when the pattern-match checker
> requests a set of constructors for some data type constructor T, the
> checker returns:
>
>    * The original set of data constructors for T
>    * Any COMPLETE sets of type T
>
> Note the use of the phrase *type constructor*. The return type of all
> constructor-like things in a COMPLETE set must all be headed by the
> same type constructor T. Since `LL`'s return type is simply a type
> variable `a`, this simply doesn't work with the design of COMPLETE
> sets.

Could we use a mechanism similar to instance resolution (with 
FlexibleInstances) for the checker to return matching COMPLETE sets instead?


--Sylvain


[1] https://mail.haskell.org/pipermail/ghc-devs/2018-July/016053.html

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