Part of #8345

• This PR follows our AI policy

Summary

SyntaxError's members were stored as None class-attribute placeholders plus per-instance __dict__ entries, so SyntaxError('m').__dict__ was non-empty ({'print_file_and_line': None}, plus filename/lineno/offset/text for the (msg, tuple) form) unlike CPython, and they weren't backed by descriptors. They're now struct fields on PySyntaxError (#[repr(C)] + base field, the same approach as SystemExit in #8282 and StopIteration in #8301), exposed via #[pygetset] — writable, deletable, independent from args, with an empty __dict__ as in CPython. IndentationError, TabError, and _IncompleteInputError inherit the fields.

Re-initialization now resets end_lineno/end_offset, so a 4-element location tuple after a 6-element one no longer keeps stale values (matches CPython's gh-146250 fix):

e = SyntaxError("msg", ("file.py", 1, 2, "txt", 2, 3))
e.__init__("new", ("new.py", 4, 5, "t"))
print(e.end_lineno, e.end_offset)
# before: 2 3    after: None None   (matches CPython)

new_exception is also routed through the real constructor for exception types that carry additional payload (now including SyntaxError), instead of building a base-sized PyBaseException. Without this, the compile-error path (new_exception_msg(syntax_error, …), ~15 call sites) would allocate a wrong-sized payload — a debug-assert panic, and out-of-bounds reads in release.

Test plan

Verified against CPython 3.14.6: msg/filename/lineno/offset/text/end_lineno/end_offset for SyntaxError(), the 4-tuple, and the 6-tuple forms; write/delete; re-init reset; empty __dict__; IndentationError/TabError inheritance; and the compile-error path (compile("1 +", ...) renders the caret and sets lineno/msg).

• test_exceptions: no regressions — test_syntax_error_memory_leak now passes; the single test_badisinstance failure is pre-existing on main (an unrelated empty-message RecursionError).
• test_syntax / test_compile: pass.
• cargo fmt / cargo clippy: clean (no new warnings).
• Unmarks test_exceptions's test_syntax_error_memory_leak (removes its @expectedFailure) — it now passes.

Notes:

• Like StopIteration.value in Store StopIteration.value in a struct field instead of the instance dict #8301, the members are getset_descriptors rather than CPython's member_descriptors; both are data descriptors and behave the same for get/set/delete.
• msg is initialized in the constructor (not only __init__) so TabError still renders its message. TabError's __init__ slot is not reached during construction — a pre-existing limitation for two-level exception subclasses — so its location members (lineno/offset/…) are not populated from a direct TabError(msg, tuple) call; the compile path sets them via attribute assignment, so raised TabErrors are unaffected. Fixing that inheritance is out of scope here.

Assisted-by: Claude Code:claude-fable-5

Summary by CodeRabbit

• Bug Fixes
  • Improved SyntaxError handling for message, filename, line number, offset, source text, and end-position fields.
  • Added stricter validation for supported location argument shapes, including proper end-position constraints.
  • Improved exception instantiation compatibility when exception types have different internal payload layouts.