Calling a duplicated ordinary method segfaults

Found while writing the regression test for bug-n-a-class-with-two-definitions-of-one-method-hangs-the-compiler-forever. I put this shape in that test, and it crashed for a reason that had nothing to do with the hang — so it is filed rather than folded in.

Repro — 10 lines, complete

class D:
    def m(self, prefix):
        self.prefix = prefix

    def m(self, other, prefix):
        self.prefix = prefix

d = D()
d.m("p", "q")
print(d.prefix)

CPython   -> q
pxx       -> compiles clean, then SIGSEGV (rc=139), no output

Not the hang, and not caused by its fix — measured both ways

Identical on both, so this is pre-existing and the hash-bucket fix neither caused nor cures it. It is also a different failure class: the hang needed three ingredients including a later scope holding a bare same-named local, and produced no binary at all. This one has no third scope, compiles clean, and dies at run time — which makes it the quieter of the two and the one that would survive a suite that only asserts on compiler exit status.

Why it is a bug rather than NilPy laxness

CPython accepts this source — a re-def rebinds and the last definition wins — so a program CPython runs correctly crashes here. That is the upward-compat direction that Track N's rule says is a defect, not the direction it excuses.

Where to look

Both definitions survive registration rather than the second replacing the first — the hang ticket's trace shows two n.ret def@ rows for one name — so a call plausibly binds the arity/signature of one definition and the body or frame of the other. The two definitions here have different arities (2 vs 3 including self), which is the obvious first thing to vary: check whether two same-arity definitions also crash, and whether the surviving d.prefix write lands through the wrong frame.

Gate

make compiler/pascal26 + this repro printing q, and add it to test/test_nilpy_duplicate_method_def.npy, which currently carries a comment saying this shape is deliberately absent because it crashes.