Unboxed tuples/sum error message on `a = show 5` in expression evaluation and interactive modes

With GHC 8.4.3 (on both Ubuntu 16.04.5 and Ubuntu 18.04.1) and recent GHC (not HEAD but close at ff29fc84c03c800cfa04c2a00eb8edf6fa5f4183 on Ubuntu 16.04.5), I get errors for a = show 5.

I run the following commands, showing that show 5 is usually fine:

$ ghc -fobject-code -O2 -e 'show 5'
"5"

$ ghc -fobject-code -O2 -e 'let a = show 5 in a'
"5"

But not with a = show 5:

$ ghc -fobject-code -O2 -e 'a = show 5'
<interactive>: 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.

Running with ghci gives the same error:

$ ghci -fobject-code -O2
GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
Prelude> a = show 5
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.
Prelude> 
Leaving GHCi.

Both errors stop when optimization is turned off:

$ ghc -fobject-code -O0 -e 'a = show 5'

$ ghci -fobject-code -O0
GHCi, version 8.4.3: http://www.haskell.org/ghc/  :? for help
Prelude> a = show 5
Prelude> 
Leaving GHCi.