ModuleOrigin SemiGroup instance looks incorrect
An issue was reported on IRC:
08:38 <hololeap> there's a transient bug that we're getting while compiling some packages using ghc-9.0.2 in gentoo. here's the full build output: http://sprunge.us/N4z2Ni?text
08:38 <hololeap> you can see the panic at line 114
08:38 <hololeap> this is using ghc patched with this commit: https://gitlab.haskell.org/ghc/ghc/-/commit/c7e801991dc9d4f702e92caf6d6347bbff6a8f3c
08:39 <hololeap> I'm wondering what could be the underlying cause of this panic and how we can debug it further
08:48 <hololeap> it looks like the ModuleOrigins would have to both be ModUnusable to produce this output: https://hackage.haskell.org/package/ghc-9.0.2/docs/src/GHC.Unit.State.html#line-197
08:48 <hololeap> how can we determine what is broken and how to fix it in this scenario?See also: https://github.com/gentoo-haskell/gentoo-haskell/issues/1250
The relevant part of the build output is:
112 Preprocessing library for singletons-th-3.0..
113 Building library for singletons-th-3.0..
114 ghc: panic! (the 'impossible' happened)
115 (GHC version 9.0.2:
116 ModOrigin: hidden module redefined
117 x: unusable module
118 y: unusable module
119 Call stack:
120 CallStack (from HasCallStack):
121 callStackDoc, called at compiler/GHC/Utils/Outputable.hs:1230:37 in ghc:GHC.Utils.Outputable
122 pprPanic, called at compiler/GHC/Unit/State.hs:239:14 in ghc:GHC.Unit.State
123
124 Please report this as a GHC bug: https://www.haskell.org/ghc/reportabug#16996 (closed) and #11050 (closed) report similar issues.
The failures are all due to the partiality of the Semigroup instance of ModuleOrigin which was added in 20787529
- it doesn't take into account combining two
ModHidden(#11050 (closed)) - it doesn't take into account
ModUnusableat all that was added in df0f148f (this issue and #16996 (closed))
Its Monoid instance looks dubious too (mempty is a ModOrigin...).
I would suggest removing both instances and then defining and using:
newtype ModuleOrigins
= ModuleOrigins [ModuleOrigin]
deriving (Monoid,Semigroup)And then let lookupModule* functions deal with hidden/unusable/multiple module/units as they already do to some extent.