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This bug prevents lol-apps' tests and benchmarks from building with GHC 8.4.1-alpha2. This is as much as I'm able to minimize the issue:

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE PartialTypeSignatures #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# OPTIONS_GHC -Wno-partial-type-signatures #-}
module Bug (bench_mulPublic) where

data Cyc r
data CT zp r'q
class Reduce a b
type family LiftOf b

bench_mulPublic :: forall z zp zq . (z ~ LiftOf zq, _) => Cyc zp -> Cyc z -> IO (zp,zq)
bench_mulPublic pt sk = do
  ct :: CT zp (Cyc zq) <- encrypt sk pt
  undefined ct

encrypt :: forall z zp zq. Reduce z zq => Cyc z -> Cyc zp -> IO (CT zp (Cyc zq))
encrypt = undefined

On GHC 8.2.2, this compiles without issue. But on GHC 8.4.1-alpha2, this errors with:

$ /opt/ghc/8.4.1/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:15:1: error:
    • Could not deduce (Reduce fsk0 zq)
      from the context: (z ~ LiftOf zq, Reduce fsk zq)
        bound by the inferred type for ‘bench_mulPublic’:
                   forall z zp zq fsk.
                   (z ~ LiftOf zq, Reduce fsk zq) =>
                   Cyc zp -> Cyc z -> IO (zp, zq)
        at Bug.hs:(15,1)-(17,14)
      The type variable ‘fsk0’ is ambiguous
    • In the ambiguity check for the inferred type for ‘bench_mulPublic’
      To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
      When checking the inferred type
        bench_mulPublic :: forall z zp zq fsk.
                           (z ~ LiftOf zq, Reduce fsk zq) =>
                           Cyc zp -> Cyc z -> IO (zp, zq)
   |
15 | bench_mulPublic pt sk = do
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^...

Trac metadata

Trac field	Value
Version	8.4.1-alpha1
Type	Bug
TypeOfFailure	OtherFailure
Priority	highest
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture