WildcardInADTContext2.hs:1:53: warning:
    -XDatatypeContexts is deprecated: It was widely considered a misfeature, and has been removed from the Haskell language.

WildcardInADTContext2.hs:4:10: error:
    Not in scope: type variable ‘_a’
    Perhaps you meant ‘a’ (line 4)