GHC HEAD-only panic with PartialTypeSignatures
The following program typechecks on GHC 8.8.1:
{-# LANGUAGE PartialTypeSignatures #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Bug where
import Data.Proxy
f :: forall a (x :: a). Proxy (x :: _)
f = Proxy
However, it panics on GHC HEAD:
$ ~/Software/ghc5/inplace/bin/ghc-stage2 Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
Bug.hs:8:37: warning: [-Wpartial-type-signatures]ghc-stage2: panic! (the 'impossible' happened)
(GHC version 8.9.0.20190522:
No skolem info:
[a_avg[sk:1]]
Call stack:
CallStack (from HasCallStack):
callStackDoc, called at compiler/utils/Outputable.hs:1173:37 in ghc:Outputable
pprPanic, called at compiler/typecheck/TcErrors.hs:2927:5 in ghc:TcErrors
This regression was introduced in commit 80dfcee6. cc'ing @simonpj, the author of that patch.