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A nested def capturing a rebound parameter uses the parameter's type

Repro

def o(x: int):
    x /= 2
    def inner():
        return x + 1
    return inner()
print(o(5))                     # CPython 3.5



v383: pascal26:5: error: invalid IR node reference in store_sym
HEAD: pascal26:5: error: invalid IR node reference in store_sym

...and the same shape rebinding to a string compiles and then fails at run time:

def o(x: int):
    x = "s"
    def inner():
        return x + "t"
    return inner()
print(o(5))                     # CPython "st"



v383: TypeError: unsupported operand type(s) for +: 'int' and 'str'
HEAD: TypeError: expected a number, got str

The boundary, which is what points at the cause

shape verdict
capture a plain float local works — 2.5
capture a plain int local works — 3
capture a plain str local works — st
capture a rebound variant parameter works — 2
capture a rebound typed parameter broken, both shapes above

Capturing works for every ordinary local, and works for a rebound VARIANT parameter — which is the one case that has had a private slot since [[bug-nilpy-rebinding-a-list-parameter-aliases-the-callers-list]]. So the capture scan handles a private slot fine; it does not find this one.

The likely difference is WHEN the slot is created. The variant slot is allocated before PyCollectLocalsAST; the typed slot the sibling fix adds is allocated after it (it needs the constraint table to know the type) and the pass is then re-run. Moving the typed slot earlier — with a provisional type, patched afterwards — would make the two identical and is the first thing to try. Measure it; do not assume it.

Gate

Both repros match CPython, plus the four control rows in the table above, plus a nonlocal rebinding of a captured parameter.

Resolution — the capture's TYPE was decided on the trial parse, like the by-ref flag beside it

The ticket's hypothesis was right about when, and the mechanism turned out to be one step over from where it was looking: not the cells, but the nested def's proc registration.

PyQueueNestedDef registers the nested def with a trailing capture parameter per captured name, typed from FindSym(cname). The enclosing body's local-typing trial parse reaches that first — and at that moment a rebound TYPED parameter's private slot does not exist yet, because it is allocated after PyCollectLocalsAST (it needs the constraint table to know its type), while a rebound VARIANT parameter's slot is allocated before. So:

That branch is already the home of exactly this correction — it re-applies the by-ref decision there, with a comment saying it "was therefore taken without them". The capture's TYPE has the identical problem and now sits beside it, re-resolved by name:

capSym := FindSym(PyCapName[procIdx * PY_MAX_CAPS + j]);
if (capSym >= 0) and (Syms[capSym].Kind in [skLocal, skParam]) and ... then
begin
  capNow := Syms[capSym].TypeKind;
  if TypeIsPromoInt(capNow) then capNow := tyVariant;
  ...
end;

The promo boxing mirrors the registration arm's own rule so the two spellings of one decision cannot drift. The parameter's symbol is retyped with the TParam — they are one declaration in two records, and leaving the symbol behind is the silent-ABI-mismatch shape this file keeps meeting.

The nonlocal row is why the re-apply is not restricted to by-value captures

The first cut skipped captures already marked by-ref, on the theory that a cell carries a pointer and needs no retyping. Measured: the gate's nonlocal row still failed, TypeError: expected a number, got type — and it fails identically on v387 pinned, so it is pre-existing and in the same family (the untyped-parameter spelling of it works, which is the same boundary again). Dropping the guard fixes it and makes the rule simpler rather than more special: by-ref-ness is carried by IsRef, not by the type, so there was never a reason for the two to disagree about which type the capture has.

Gate — met, plus three shapes it did not ask for

row pxx CPython
o(5) — capture a rebound typed param 3.5 3.5
ostr(5) — the same rebinding to a str st st
CONTROL — rebound UNTYPED param 3.0 3.0
CONTROL — a plain local 3.5 3.5
nonlocal write through the captured slot 12.5 12.5
the same shape inside a METHOD 7.0 7.0
captured by a LAMBDA 104.5 104.5
two rebound typed params of different types 3.0z 3.0z

The four control rows of the ticket's boundary table all still hold. 24 named closure / nested-def / lambda-capture canaries green (nested_def_capture, nested_def_default_capture, escaping_closure_many_captures, nonlocal_escaping_closure, closure_lifetime, captured_class, lambda_sibling_capture, lifted_lambda_return_value, …). Self-host fixedpoint verified, converged after 1 round(s).

Test: test/test_nilpy_nested_def_captures_a_rebound_parameter.npy (+.expected, registered) — the eight rows above.

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