"mkPluginUsage: file not found" warning in haddock generation on GHC-8.8

[msakai]

(from #63)

Haddock issued the following warning on GHC-8.8.

haddock: panic! (the 'impossible' happened)
  (GHC version 8.8.3 for x86_64-unknown-linux):
	mkPluginUsage: file not found
Data.UnitsOfMeasure.Plugin /tmp/.haddock-16403/Data/UnitsOfMeasure/Plugin.o
Call stack:
    CallStack (from HasCallStack):
      callStackDoc, called at compiler/utils/Outputable.hs:1159:37 in ghc:Outputable
      pprPanic, called at compiler/deSugar/DsUsage.hs:235:15 in ghc:DsUsage
Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug
Warning: Failed to build documentation for uom-plugin-0.3.0.0.

It seems the problem was already reported at https://gitlab.haskell.org/ghc/ghc/issues/16093 and some workarounds are suggested there.

[adamgundry]

Looks like this is resolved (in 9.4.x, to be precise).