[3.10] gh-85267: Improvements to inspect.signature __text_signature__ handling (GH-98796) (#100393)

This makes a couple related changes to inspect.signature's behaviour
when parsing a signature from `__text_signature__`.

First, `inspect.signature` is documented as only raising ValueError or
TypeError. However, in some cases, we could raise RuntimeError.  This PR
changes that, thereby fixing GH-83685.

(Note that the new ValueErrors in RewriteSymbolics are caught and then
reraised with a message)

Second, `inspect.signature` could randomly drop parameters that it
didn't understand (corresponding to `return None` in the `p` function).
This is the core issue in GH-85267. I think this is very surprising
behaviour and it seems better to fail outright.

Third, adding this new failure broke a couple tests. To fix them (and to
e.g. allow `inspect.signature(select.epoll.register)` as in GH-85267), I
add constant folding of a couple binary operations to RewriteSymbolics.

(There's some discussion of making signature expression evaluation
arbitrary powerful in GH-68155. I think that's out of scope. The
additional constant folding here is pretty straightforward, useful, and
not much of a slippery slope)

Fourth, while GH-85267 is incorrect about the cause of the issue, it turns
out if you had consecutive newlines in __text_signature__, you'd get
`tokenize.TokenError`.

Finally, the `if name is invalid:` code path was dead, since
`parse_name` never returned `invalid`..
(cherry picked from commit 79311cbfe718f17c89bab67d7f89da3931bfa2ac)

Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com>

1 files changed, 6 insertions, 0 deletions

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+Several improvements to :func:`inspect.signature`'s handling of ``__text_signature``.

+- Fixes a case where :func:`inspect.signature` dropped parameters

+- Fixes a case where :func:`inspect.signature` raised :exc:`tokenize.TokenError`

+- Allows :func:`inspect.signature` to understand defaults involving binary operations of constants

+- :func:`inspect.signature` is documented as only raising :exc:`TypeError` or :exc:`ValueError`, but sometimes raised :exc:`RuntimeError`. These cases now raise :exc:`ValueError`

+- Removed a dead code path