isUnliftedType GHC panic on pattern with True :: Maybe

{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ScopedTypeVariables #-}
module Oops where

pattern What = True :: Maybe

Fails on 8.4.3 with:

ghc: panic! (the 'impossible' happened)
  (GHC version 8.4.3 for x86_64-unknown-linux):
	isUnliftedType
  (Maybe |> {co_a1Dg3}) :: TYPE t_a1Dg9[tau:1]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1150:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1939:10 in ghc:Type

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

I'm currently cloning from git, and will try to get around to check it with HEAD over the next days. BTW, it's not a show-stopper for me.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture	aarch64

changed milestone to %8.6.1

changed weight to 3

added Tbug Trac import typechecker labels

Trac metadata

Trac field	Value
Architecture	aarch64 → Unknown/Multiple

This also panics on GHC HEAD:

$ /opt/ghc/head/bin/ghc Bug2.hs
[1 of 1] Compiling Oops             ( Bug2.hs, Bug2.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.5.20180616 for x86_64-unknown-linux):
        isUnliftedType
  (Maybe |> {co_aWy}) :: TYPE t_aWE[tau:0]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1164:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1934:10 in ghc:Type

This appears to be a regression from GHC 8.2.2, in which the same program simply gives an error message:

$ /opt/ghc/8.2.2/bin/ghc Bug2.hs
[1 of 1] Compiling Oops             ( Bug2.hs, Bug2.o )

Bug2.hs:5:16: error:
    • Couldn't match expected type ‘Maybe’ with actual type ‘Bool’
    • In the pattern: True
      In the pattern: True :: Maybe
      In the declaration for pattern synonym ‘What’
  |
5 | pattern What = True :: Maybe
  |                ^^^^

Bug2.hs:5:16: error:
    • Couldn't match expected type ‘Maybe’ with actual type ‘Bool’
    • In the expression: True
      In an equation for ‘What’: What = True
  |
5 | pattern What = True :: Maybe
  |                ^^^^^^^^^^^^^

Bug2.hs:5:24: error:
    • Expecting one more argument to ‘Maybe’
      Expected a type, but ‘Maybe’ has kind ‘* -> *’
    • In the type ‘Maybe’
      In a pattern type signature: Maybe
      In the pattern: True :: Maybe
  |
5 | pattern What = True :: Maybe
  |                        ^^^^^

This was introduced in commit 594879dc (Fix floating of equalities).

mentioned in commit 7b8dcd90

mentioned in commit 2896082e

Simon, is this a merge candidate for 8.6.1?

It seems like a simple enough patch and this is clearly a bug so I'm happy to merge.

added 1 deleted label

Yes, probably worth merging

closed

Merged in 149d7912.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

added compiler crash label

added Plow label