#14435: GHC 8.2.1 regression: -ddump-tc-trace hangs forever
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Reporter: RyanGlScott | Owner: (none)
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 8.2.1
(Type checker) |
Keywords: | Operating System: Unknown/Multiple
Architecture: | Type of failure: Compile-time
Unknown/Multiple | performance bug
Test Case: | Blocked By:
Blocking: | Related Tickets:
Differential Rev(s): | Wiki Page:
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Thanks to Christiaan Baaij for noticing this. Take this program:
{{{#!hs
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE UndecidableInstances #-}
module Bug where
import Data.Type.Equality
import GHC.TypeLits
type family Replicate (n :: Nat) (x :: a) :: [a] where
Replicate 0 _ = '[]
Replicate n x = x : Replicate (n - 1) x
replicateSucc :: (Replicate (k + 1) x) :~: (x : Replicate k x)
replicateSucc = Refl
}}}
Note that this program does not typecheck (nor should it). In GHC 8.0.2
and 8.2.1, if you compile this with no tracing flags, it'll give the
error:
{{{
$ /opt/ghc/8.2.1/bin/ghci Bug.hs
GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug ( Bug.hs, interpreted )
Bug.hs:16:17: error:
• Couldn't match type ‘Replicate (k + 1) x’
with ‘x : Replicate k x’
Expected type: Replicate (k + 1) x :~: (x : Replicate k x)
Actual type: (x : Replicate k x) :~: (x : Replicate k x)
• In the expression: Refl
In an equation for ‘replicateSucc’: replicateSucc = Refl
• Relevant bindings include
replicateSucc :: Replicate (k + 1) x :~: (x : Replicate k x)
(bound at Bug.hs:16:1)
|
16 | replicateSucc = Refl
| ^^^^
}}}
Things become interesting when you compile this program with `-ddump-tc-
trace`. In GHC 8.0.2, it'll trace some extra output and eventually reach
the same error as above. In 8.2.1, however, the tracing hangs, causing
compilation to never terminate! Here is the output right before it hangs:
{{{
lk1 :
tc_infer_args (invis) @a_11
tc_infer_args (vis)
[anon] a_11
x_a1Dt
lk1 x_a1Dt
u_tys
tclvl 1
k0_a1GD[tau:1] ~ a0_a1GF[tau:1]
arising from a type equality k0_a1GD[tau:1] ~ a0_a1GF[tau:1]
found filled tyvar k_a1GD[tau:1] :-> a0_a1GE[tau:1]
u_tys
tclvl 1
* ~ *
arising from a kind equality arising from
a0_a1GE[tau:1] ~ a0_a1GF[tau:1]
u_tys
tclvl 1
'GHC.Types.LiftedRep ~ 'GHC.Types.LiftedRep
arising from a kind equality arising from
a0_a1GE[tau:1] ~ a0_a1GF[tau:1]
u_tys yields no coercion
u_tys yields no coercion
writeMetaTyVar a_a1GE[tau:1] :: * := a0_a1GF[tau:1]
u_tys yields no coercion
checkExpectedKind
k0_a1GD[tau:1]
a0_a1GF[tau:1]