int() accepts strings with underscores in free-threading build (should raise ValueError)

[DengYuncheng27]

Bug report

Bug description:

In the experimental free-threading build of python-freethreaded.3.13.1 , the int() function unexpectedly accepts strings containing underscores (e.g., '1806_1') and silently converts them to integers by ignoring the underscores — returning 18061.
print(int('1806_1')) # 18061

This behavior violates the documented and long-standing specification of int(), which should raise a ValueError for such input.

CPython versions tested on:

3.13

Operating systems tested on:

Windows

[skirpichev]

This behavior violates the documented and long-standing specification of int(), which should raise a ValueError for such input.

This is documented behavior for int's constructor:

If the argument is not a number or if base is given, then it must be a string, bytes, or bytearray instance representing an integer in radix base. Optionally, the string can be preceded by + or - (with no space in between), have leading zeros, be surrounded by whitespace, and have single underscores interspersed between digits.

This happens for ordinary builds as well, of course.