Recompiling results in a missing instance

The attached program compiles without error using cabal.

import Bar

main :: IO ()
main = do
  let x = True   -- line 5
  --x <- True    -- line 6
  foo x bar

foo :: Bool -> IO () -> IO ()
foo _ _ = undefined

However:

1. cabal build. This succeeds. [This step isn't required to reproduce the bug, but it shows that the program should compile.]
2. Replace line 5 with line 6 in Main.hs
3. cabal build. GHC correctly reports a compile error (Couldn't match expected type 'IO Bool' with actual type 'Bool')
4. Fix the problem by reinstating line 5 and removing line 6.
5. cabal build. This step *should* compile without errors, but instead I get a GHC error about a missing instance related to the monad-classes library.
6. cabal build all day. The missing instance error still occurs until you clean temporary files and start from scratch.

I suspect the error in step 5 is itself a (different?) GHC bug, but for now it is reported here. This GHC bug report is specifically about compiling with identical source code (steps 1 and 5) and getting different outcomes (step 1 compiles, step 5 has an error), which shouldn't happen regardless of other bugs.

[Note I can produce the missing instance issue without having to recompile GHC, which is why I think it might be independent of this bug.]

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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