[jseop-lim]

Description

Fixes #1257

io.StringIO() converted initial_value through an Argument Clinic TString conversion, which ran before newline was validated, so a call with both arguments invalid raised TypeError for initial_value instead of ValueError for newline. CPython's _io_StringIO___init___impl checks newline first and initial_value last (stringio.c#L670-L698).

AS-IS

>>> io.StringIO(b'', 'x')
TypeError: StringIO() argument must be str, not bytes
>>> io.StringIO(1, 'x')
TypeError: StringIO() argument must be str, not int

TO-BE

>>> io.StringIO(b'', 'x')
ValueError: illegal newline value: x
>>> io.StringIO(1, 'x')
ValueError: illegal newline value: x
>>> io.StringIO(b'')
TypeError: initial_value must be str or None, not bytes

(exception classes matching CPython; the newline message wording was already different on master)

Changes

• Drop the clinic conversion of initial_value from StringIOBuiltins.InitNode and convert it with CastToTruffleStringNode after validateNewline, treating None and a missing argument as the empty string.
• Add ErrorMessages.INITIAL_VALUE_MUST_BE_STR_OR_NONE_NOT_P for a non-str initial_value.
• Add test_stringio_validates_newline_before_initial_value to tests/test_io.py.

Testing

mx python-jvm on darwin-aarch64, then mx graalpytest test_io.py (29 passed, 1 skipped). test.test_memoryio (161 passed) and test.test_univnewlines (34 passed) from lib-python pass. The new test fails on the unpatched build. Fourteen StringIO() argument combinations now raise the same exception classes as CPython 3.13.14.