`HsDataDefn` allows newtypes with multiple constructors.
Now, it is possible to construct a HsDataDefn
with dd_ND = NewType
but (dd_cons = _:_:_
or dd_cons = []
), which is wrong, since a newtype
must have exactly 1 constructor.
Now, it is possible to construct a HsDataDefn
with dd_ND = NewType
but (dd_cons = _:_:_
or dd_cons = []
), which is wrong, since a newtype
must have exactly 1 constructor.