Illegal "yield (from)" python syntax accepted

Originally reported as https://github.com/astral-sh/ruff/issues/4992.

I'm not quite sure where the incorrectness here is, but I think this bug indicates an incorrect parsing of the yield from expression. The following is a python snippet with illegal syntax:

def e():
  x = yield from y = 5

We can verify that it is indeed illegal syntax with:

$ python yield-elision.py 
  File "/tmp/tmp.1z4PScoI6T/yield-elision.py", line 2
    x = yield from y = 5
        ^^^^^^^^^^^^
SyntaxError: assignment to yield expression not possible

Ruff, however, believes this is valid syntax:

$ ruff yield-elision.py --show-source --show-fixes
yield-elision.py:2:3: F841 [*] Local variable `x` is assigned to but never used
  |
2 | def e():
3 |   x = yield from y = 5
  |   ^ F841
  |
  = help: Remove assignment to unused variable `x`

yield-elision.py:2:18: F821 Undefined name `y`
  |
2 | def e():
3 |   x = yield from y = 5
  |                  ^ F821
  |

Found 2 errors.

And will attempt to fix it, leading to a self-reported parsing error:

$ ruff yield-elision.py  --fix
error: Autofix introduced a syntax error in `yield-elision.py` with rule codes F841: invalid syntax. Got unexpected token '=' at byte offset 24
---
def e():
  yield from y = 5

---
yield-elision.py:2:3: F841 Local variable `x` is assigned to but never used
yield-elision.py:2:18: F821 Undefined name `y`
Found 2 errors.

I believe this indicates a parsing bug.

Commencez par reproduire l’exemple yield from de l’issue et suivez la façon dont le parser traite l’expression. C’est terminé lorsque la syntaxe Python invalide signalée par Ruff est rejetée, au lieu d’être traitée comme du code valide ou de produire un autofix qui introduit une erreur de syntaxe.