mpz_invert returns wrong result

[Benjamin Eltzner]

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>> * The return value of mpz_invert is wrongly different from 0,
>> when the inverse cannot be calculated (cf. documentation, section
>> 5.9). In the example, the modulus by 1 yields 0, which cannot be
>> inverted. Thus, the result should be 0 but is 1.
> 
> https://gmplib.org/gmp6.0.html
> 
> First item of the changes in 6.0.0:
> 
> * The function mpz_invert now considers any number invertible in
> Z/1Z.
> 
> (because 0 * x == 1 modulo 1 whatever x)

Sorry for not parsing that. Still the current behavior contradicts the
documentation
(https://gmplib.org/manual/Number-Theoretic-Functions.html#Number-Theoretic-Functions):

"If the inverse exists, the return value is non-zero and rop will
satisfy 0 < rop < abs(op2)."

In the example code I sent, rop = 0 but if I understand your reasoning
correctly, it should be 1.

Also I would like to add, that "taking mod 1" constitutes a mapping to
the zero ring, the only ring with only one element, such that 0=1. In
a case where "everything is 0" AND "everything is 1" it seems very
strange to me to prefer 1. Not only to me: the generally accepted
convention is "n mod 1 = 0" and I do not think it is a good idea to go
differently on this.

Best regards, Benjamin
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