GHC HEAD regression: Template variable unbound in rewrite rule
hybrid-vectors-0.2.1
currently fails to build with GHC HEAD. I've attached a minimized repro file with no dependencies (it's 410 lines long, since I had to copy-paste a lot of code from vector
in order to reproduce this). The attached file builds on GHC 7.10.3 and 8.0.2, but fails on GHC HEAD with this panic:
$ ~/Software/ghc2/inplace/bin/ghc-stage2 -O2 -fforce-recomp Bug.hs
[1 of 1] Compiling Data.Vector.Hybrid.Internal ( Bug.hs, Bug.o )
Bug.hs:389:10: warning: [-Wmissing-methods]
• No explicit implementation for
‘gmbasicOverlaps’, ‘gmbasicInitialize’, and ‘gmbasicUnsafeRead’
• In the instance declaration for ‘GMVector (MVector u v) (a, b)’
|
389 | instance (GMVector u a, GMVector v b) => GMVector (MVector u v) (a, b) where
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Bug.hs:407:10: warning: [-Wmissing-methods]
• No explicit implementation for
‘gbasicUnsafeFreeze’, ‘gbasicUnsafeThaw’, ‘gbasicLength’,
‘gbasicUnsafeSlice’, and ‘gbasicUnsafeIndexM’
• In the instance declaration for ‘GVector (Vector u v) (a, b)’
|
407 | instance (GVector u a, GVector v b) => GVector (Vector u v) (a, b) where
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ghc-stage2: panic! (the 'impossible' happened)
(GHC version 8.3.20170310 for x86_64-unknown-linux):
Template variable unbound in rewrite rule
Variable: sc_s8FV
Rule "SC:$j0"
Rule bndrs: [sc_s8FV, sc_s8FW, sc_s8FX, sg_s8FY, sc_s8FU]
LHS args: [sc_s8FU,
(MV
@ (GMutable u_a4oM)
@ (GMutable v_a4oO)
@ (PrimState (ST RealWorld))
@ (a_a4oN, b_a4oP)
@ a_a4oN
@ b_a4oP
@~ (<(a_a4oN, b_a4oP)>_N
:: ((a_a4oN, b_a4oP) :: *) ~# ((a_a4oN, b_a4oP) :: *))
sc_s8FW
sc_s8FX)
`cast` (sg_s8FY
:: (MVector
(GMutable u_a4oM)
(GMutable v_a4oO)
(PrimState (ST RealWorld))
(a_a4oN, b_a4oP) :: *)
~R#
(GMutable
(Vector u_a4oM v_a4oO) (PrimState (ST RealWorld)) c_a4oQ :: *))]
Actual args: [sc_s8FS,
wild_X3x
`cast` (Sub
(Sym (D:R:GMutableVector[0] <u_a4oM>_N <v_a4oO>_N)) <PrimState
(ST
RealWorld)>_N (Sym
cobox_a4Gz)
:: (MVector
(GMutable u_a4oM)
(GMutable v_a4oO)
(PrimState (ST RealWorld))
(a_a4oN, b_a4oP) :: *)
~R#
(GMutable
(Vector u_a4oM v_a4oO) (PrimState (ST RealWorld)) c_a4oQ :: *))]
Call stack:
CallStack (from HasCallStack):
prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1191:58 in ghc:Outputable
callStackDoc, called at compiler/utils/Outputable.hs:1195:37 in ghc:Outputable
pprPanic, called at compiler/specialise/Rules.hs:579:19 in ghc:Rules
Some important things to note:
-
-O2
is required to trigger this bug; using-O1
or below works fine. - The problematic code is the
Read (Vector u v c)
instance at the very bottom. In particular, commenting out the last linereadPrec = greadPrec
makes the panic go away. - Moreover, there's a particular
INLINE
pragma in theGMVector (MVector u v) (a, b)
instance that is critical to triggering the panic (it's on line 395; I've added a commentRemoving this INLINE pragma makes it compile
above it).