> PyDec_CheckExact(type) always return 0. Should be PyDec_CheckExact(result).

'result' is always an exact decimal, because your enum example won't work otherwise.


> And what about other calls of PyDecType_FromFloatExact()? Can they produce broken instance of Decimal subtype?

The PyDecType* functions handle calling the correct allocator, they don't handle calling __new__().  Why should they? The regular non-class-method enum examples worked.