`inspect` does not correctly handle over-length `__defaults__` for functions

Bug report

Bug description:

This is largely a re-report of #81244 which I believe was inappropriately dismissed. This is a bug because it causes functions to be incorrectly documented and understood. For example, it breaks pydoc and therefore the built-in help.

Python explicitly allows for the __defaults__ of a function to be re-assigned. Presumably this was for a reason. Having modified a function this way, it would make sense to have an accurate signature for the result.

However, consider the output of the following (3.8+; requires modification for far-EOL versions):

import inspect

def example(a, b, /, c, d): return (a, b, c, d)

for count in range(10):
    example.__defaults__ = tuple(range(count))
    signature = inspect.signature(example)
    result = example(*'abcd'[:max(0, 4-count)])
    print(f'with {count} defaults: {signature=!s:23}, {result=}')

We get:

with 0 defaults: signature=(a, b, /, c, d)        , result=('a', 'b', 'c', 'd')
with 1 defaults: signature=(a, b, /, c, d=0)      , result=('a', 'b', 'c', 0)
with 2 defaults: signature=(a, b, /, c=0, d=1)    , result=('a', 'b', 0, 1)
with 3 defaults: signature=(a, b=0, /, c=1, d=2)  , result=('a', 0, 1, 2)
with 4 defaults: signature=(a=0, b=1, /, c=2, d=3), result=(0, 1, 2, 3)
with 5 defaults: signature=(a, b, /, c, d=0)      , result=(1, 2, 3, 4)
with 6 defaults: signature=(a, b, /, c=0, d=1)    , result=(2, 3, 4, 5)
with 7 defaults: signature=(a, b=0, /, c=1, d=2)  , result=(3, 4, 5, 6)
with 8 defaults: signature=(a=0, b=1, /, c=2, d=3), result=(4, 5, 6, 7)
with 9 defaults: signature=(a=0, b=1, /, c=2, d=3), result=(5, 6, 7, 8)

As can be seen, when there are more __defaults__ than parameters, inspect's logic for pairing default values with parameters differs from what the runtime does. With up to 2N values it behaves as though the last N were missing; with even more, it behaves as if all but the first N were missing. Whereas the runtime consistently uses the last N values of the tuple. There is no distinction between positional-only and positional-or-keyword arguments.

Consequently, help(example) shows an incorrect signature as well. In my testing, there has been an issue with help used this way since 3.4, though it works correctly in 2.7 and 3.3.

If this is not deemed a bug in inspect's logic (why does it not simply have access to the interpreter's logic?), then there is a problem with the runtime. Arguably, it should reject attempts to set __defaults__ to an over-long tuple, since its actual behaviour (taking only the last N values) is not unambiguously the right way to handle it. "In the face of ambiguity, refuse the temptation to guess."

N.B. From further testing, it appears that inspect.signature is still mistaken in 3.3 (the version where it was added), but the builtin help() still works, even though pydoc still imports inspect. Presumably a legacy method was used to compute the signature for help output, which was then changed in 3.4.

CPython versions tested on:

3.3 (pydoc/help() works but inspect.signature does not), 3.4 .. 3.14 inclusive (not working)

(pydoc/help() works in 2.7; unclear if/how inspect is involved)

Operating systems tested on:

Linux (but most likely platform-agnostic)

The issue identifies inspect.signature, pydoc, and help() as affected entry points; start by tracing how inspect pairs defaults with parameters. Verify behavior for over-long defaults against the runtime, then add coverage showing that the displayed signature uses the last values consistently.