long_subtype_new underallocates for zero #101037

When creating int instances, we always allocate space for and initialise at least one digit, since the medium_value function that's used for fast paths assumes the existence of that digit. This change was introduced for Python 3.11 in PR #27832.

However, the corresponding change in long_subtype_new was missed; this results in us asking for zero digits in the case of creating an instance of a subclass of int with value zero - e.g.,

class MyInt(int):
    pass

myint = MyInt()  # problem

That's then a problem if myint is used in a context where medium_value might be called on it, for example as in int(myint).

We've got away with this so far because of a compensating overallocation issue: see #81381; this bug blocked an attempt to address #81381 in #100855. But I believe this should still be fixed.

I'll make a PR shortly.

Linked PRs

• gh-101037: Fix potential memory underallocation for zeros of int subtypes #101038
• [3.11] gh-101037: Fix potential memory underallocation for zeros of int subtypes (GH-101038) #101219