[Bug]: Key error from failed pattern matching

[da-woods]

Describe the bug

def f(arg):
    match arg:
        case {"a": a, "b": b}:
            print(f"case ab: {a} {b}")
        case {"x": x, "y": y, **keys}:
            print(f"case xy: {x} {y} {keys}")
       case {**keys}:
            print(f"case keys: {keys}")
       case _:
            print("Unmatched")

>>> f(dict(p=1, q=2))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in f
KeyError: 'x'

(The traceback comes from the line case {"x": x, "y": y, **keys}: but my quick copy-paste demo from the REPL doesn't show that very well).

The expected result is that this should be matched by case {**keys}:

Operating system

Linux

CPU architecture

x86_64

GraalPy version

25.1.3 / Python 3.12.8 (Tue Jun 23 15:26:01 UTC 2026)

JDK version

No response

Context configuration

No response

Steps to reproduce

See description section

Expected behavior

See description section

Stack trace

Additional context

This started failing in the Cython test suite when run as a pure-Python (i.e. not compiled) test (it runs file compiled, but that isn't your problem)

[msimacek]

Thank you for the report and sorry about the regression. I already noticed it in our cython downstream test, unfortunately I saw it too late for the release (our downstream test was not working for a while due to CI issues).