T18191.hs:6:11: error:
    GADT constructor type signature cannot contain nested ‘forall’s or contexts
    In the definition of data constructor ‘MkT’

T18191.hs:9:11: error:
    GADT constructor type signature cannot contain nested ‘forall’s or contexts
    In the definition of data constructor ‘MkS’

T18191.hs:12:11: error:
    GADT constructor type signature cannot contain nested ‘forall’s or contexts
    In the definition of data constructor ‘MkU’

T18191.hs:15:21: error:
    GADT constructor type signature cannot contain nested ‘forall’s or contexts
    In the definition of data constructor ‘MkZ1’

T18191.hs:16:19: error:
    GADT constructor type signature cannot contain nested ‘forall’s or contexts
    In the definition of data constructor ‘MkZ2’