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For example, this causes a segmentation fault on the current x64 nightly:

import StdEnv

f :: (Int -> ?Int)
f = ?Just o ((+) 5)

Start = f 1

Whereas this works fine with a custom ADT:

:: M a = J !a

g :: (Int -> M Int)
g = J o ((+) 5)

Start = g 1

Also f = (\x -> ?Just x) o ((+) 5) works fine.

I think ?Just should be defined as a .record in _system.abc, with the corresponding lazy entry for currying, as is done with strict lists and constructors.

Finally, I'm not sure whether it is related or not, but f = ?Just results in a lazy ?^Just being created:

f :: (Int -> ?Int)
f = ?Just

Start = f 1