"Arity decrease" warnings when compiling Haddock
I'm getting warnings of the form
WARNING: file compiler/GHC/Core/Op/Simplify/Utils.hs, line 1528
Arity decrease: lvl_s4yB 3 0 0
lvl_s4XI
`cast` (Sym (N:Builder[0])
:: (forall r. BuildStep r -> BuildStep r) ~R# Builder)
Call stack:
CallStack (from HasCallStack):
callStackDoc, called at compiler/utils/Outputable.hs:1289:29 in ghc:Outputable
warnPprTrace, called at compiler/GHC/Core/Op/Simplify/Utils.hs:1526:11 in ghc:GHC.Core.Op.Simplify.Utils
Having looked at prepareRhs
lately, I suspect it has to do that we don't transfer lvl_s4yB
's arityInfo
on to lvl_s4XI
's sanitised_info
in the makeTrivialWithInfo
case.
As I said in #17673 (comment 267431), we probably want to share the logic with WW, which doesn't have this weakness.