CPP, 27

main=print[1{-/*-},2{-*/-}]

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Prints [1] with -XCPP and [1,2] otherwise.

Credits: This is inspired by Laikoni's answer, but instead of a #define it simply uses C comments.

CPP, 27 25

main=print({-/*-}1{-*/-})

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Prints () for -XCPP and 1 for -XNoCPP

Previous version:

main=print[1{-/*-},2{-*/-}]

Try it online!

Prints [1] with -XCPP and [1,2] otherwise.

Credits: This is inspired by Laikoni's answer, but instead of a #define it simply uses C comments.