raise ValueError when discrete_log does not exist

[DaveWitteMorris]

Steps To Reproduce

sage: Kp = GF(5)
sage: g = Kp(3)
sage: a = Kp(1)
sage: dl = discrete_log(a, g, bounds=(1, 2), operation="*")
sage: print(g ** dl == a, dl)
False 2

Expected Behavior

A ValueError should be raised, because there is no solution within the specified bounds.

Actual Behavior

discrete_log returns a value that is not a valid logarithm.

Additional Information

Reported in this sage_devel thread.

Environment

Various.

Checklist

• I have searched the existing issues for a bug report that matches the one I want to file, without success.
• I have read the documentation and troubleshoot guide

[grhkm21]

sage: K = GF(2^61 - 1)
sage: d = discrete_log(K(1), K(3), bounds=(1,2^30), algorithm="lambda", operation="*")
sage: d <= 2^30
# False

this too

[DaveWitteMorris]

Probably the "Expected Behavior" that I suggested is not completely correct. I think bounds should be considered to be an aid to the algorithm, not a restriction on the output. So the method should either return a valid logarithm, or raise a ValueError (because there is no logarithm within the specified bounds). And this behaviour should be documented.