Different GHCi error messages for similar scenarios

With the following code:

{-# LANGUAGE UnboxedTuples #-}
module Test where

data UnboxedTupleData = MkUTD (# (),() #)

doThings :: UnboxedTupleData -> ()
doThings (MkUTD t) = ()

This is accepted and compiled with a simple ghc --make Test.hs. However, with ghci Test.hs, you get an ugly error message:

GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Test             ( Test.hs, interpreted )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.2.2 for x86_64-unknown-linux):
	bcIdPrimRep
  t_s1ro :: (# (), () #)
  Call stack:
      CallStack (from HasCallStack):
        prettyCurrentCallStack, called at compiler/utils/Outputable.hs:1133:58 in ghc:Outputable
        callStackDoc, called at compiler/utils/Outputable.hs:1137:37 in ghc:Outputable
        pprPanic, called at compiler/ghci/ByteCodeGen.hs:1582:5 in ghc:ByteCodeGen

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

Replacing (MkUTD t) with just t obtains a different error, with a nicer message:

GHCi, version 8.2.2: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Test             ( Test.hs, interpreted )
Error: bytecode compiler can't handle unboxed tuples and sums.
  Possibly due to foreign import/export decls in source.
  Workaround: use -fobject-code, or compile this module to .o separately.

True to its word, GHCi accepts it with -fobject-code, but it also accepts the unwrapping one, with the worse error message, with -fobject-code.

It only happens with unboxed tuples in GHCi: if you replace the unboxed tuple with an unboxed int, GHCi will happily accept it. GHC cheerfully compiles everything.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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