Describe the bug

io.StringIO() type-checks initial_value before it validates newline. When both are invalid, GraalPy reports the initial_value error with TypeError, while CPython reports the illegal newline with ValueError.

Operating system

macOS

CPU architecture

ARM64

GraalPy version

master a3009f9 (Python 3.13.14); also reproduced on 25.1.3 (Python 3.12.8)

JDK version

No response

Context configuration

No response

Steps to reproduce

import io
io.StringIO(b'', 'x')

GraalPy master:

TypeError: StringIO() argument must be str, not bytes

Expected behavior

CPython 3.13.14 (same on 3.12.13):

ValueError: illegal newline value: 'x'

Stack trace

Additional context

io.StringIO(1, 'x') behaves the same way.

Root Cause

StringIOBuiltins.InitNode converts initial_value with the TString Argument Clinic conversion (StringIOBuiltins.java#L281-L293), which raises before the method body reaches validateNewline (#L318-L320). CPython's _io_StringIO___init___impl checks newline first and initial_value last (stringio.c#L670-L698).

Fix Suggestion

Take initial_value as a plain Object and convert it after validateNewline, raising initial_value must be str or None, not %p when it is not a str.