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A method's keyword call drops a tuple argument when an earlier default is skipped

The minimal repro — one class, two call paths

unit vunit6;
interface
type
  TC = class
  public
    constructor Create(a: Integer = -1; const s: AnsiString = ''; const v: Variant = 0);
    procedure meth(a: Integer = -1; const s: AnsiString = ''; const v: Variant = 0);
  end;



import 'vunit6.pas' as v6
c = v6.TC(a=0, v=(8, 6))     # OK   -> ctor tag=7
c.meth(a=0, v=(8, 6))        # FAILS -> no overload of meth matches these arguments

Same class, same parameter list, same call, same skipped parameter (s). Only the path differs. That is the whole finding: two argument-binding paths serve one concept and only one of them fills defaults correctly for an object-valued Variant. The fix is presumably to make the method path do what the instantiation path already does, rather than to patch the matcher.

The boundary, measured

Every row is a keyword call with a (8, 6) argument unless stated.

shape result
unit-level procedure, gap before the Variant OK
class instantiation TC(a=0, v=(8,6)), gap before OK
method c.meth(a=0, v=(8,6)), gap before FAILS
method, no gap — every parameter supplied OK
method, gap, argument is a scalar v=8 OK
method, gap, Variant is the first parameter (gap after it) OK
method, gap before a skipped AnsiString target (no Variant) OK
explicit constructor call TC.Make(a=0, v=(8,6)) FAILS
positional first arg, meth(0, v=(8,6)) FAILS
tuple (8,6) vs list [8,6] identical — both fail

So all three conditions are required: a method call, a defaulted parameter left unbound before the argument, and an object-valued Variant (pyvartag 7). Miss any one and it binds.

Note TC.Make(...) — an explicit constructor call — fails while TC(...) succeeds, which says the split is call-path and not constructor-vs-method.

Where it is not

The diagnostic is raised at compiler/pasparser_call.inc:2033, but that is where the failure is reported, not where it is caused: Pascal cannot express a skipped middle default at all, so the gap can only have been created by NilPy's keyword binding when it built the argument list. Start at the NilPy side and at the difference between the two paths, not at the matcher.

Impact

Blocks [[feature-lib-tkinter-grid-pad-accepts-a-two-tuple]], and through it settings.py in [[feature-demo-songformatter-pxx-target]]. It is wider than tkinter, though: any NilPy call into a Pascal facade whose methods take optional Variant options — which is the shape every facade here uses — hits this the moment an application passes a tuple without also supplying every preceding option. pack(padx=(8, 6)) fails for the same reason grid(row=0, column=0, padx=(8, 6)) does.

What is already correct, so nobody re-does it

The tkinter facade needs no change. padx/pady on both grid and pack are already Variant, and TkiOptPad already renders the pair as Tk's braced list. Verified by asking Tk rather than our own formatter — grid info on the live widgets:

pair  : ... -padx {8 6} -pady 2 -sticky e
scalar: ... -padx 8 -pady 2 -sticky e
pady  : ... -padx 1 -pady {3 9} -sticky e

reached by supplying every intermediate option to dodge this bug: grid(row=0, column=0, sticky="e", columnspan=1, rowspan=1, padx=(8, 6), pady=2) compiles and applies asymmetric padding today. That is also the workaround for an application that cannot wait: name every option before the pad.

Gate

Track N's: make test-nilpy green + self-host byte-identical. The repro above must bind through the method path, and the TC(...) / unit-level rows must stay green (they are the control, not the target).

A same-widget, same-method pair in real code (frankwasm, 2026-08-30)

Found while re-running songformatter against HEAD. settings.py calls tk.Label(...).grid(...) twice, nine lines apart, on the same widget class and the same method — and one compiles while the other does not:

sect_label = tk.Label(self.content, text=section, anchor="w", font=(...))
sect_label.grid(row=row, column=0, columnspan=2, sticky="we", padx=8, pady=(10, 2))   # line 178 — COMPILES
...
label = tk.Label(self.content, text=key, anchor="e")
label.grid(row=row, column=0, sticky="e", padx=(8, 6), pady=2)                        # line 183 — FAILS

pascal26:183: error: no overload of grid matches these arguments. Line 178 is before it and produced no error, so it bound.

The tuple is present in both. What differs is only which keyword carries it — pady at 178, padx at 183 — plus the presence of columnspan at 178, which changes how many earlier defaults are skipped. That is a controlled pair the synthetic repro cannot express: same receiver type, same method, same signature, tuple in both, opposite outcomes.

So the defect is not "a tuple argument is rejected". It is positional: a tuple is rejected at some argument slots and accepted at others, as a function of which earlier defaulted parameters were left unbound — which is what this ticket's title already says, now with real-code evidence for the accepted half. Any fix should assert both lines, since a fix that makes tuples work everywhere would pass a test built only from line 183.

This is the wall songformatter is standing at. It blocks settings.py, convertrawtext.py and SongFormatter.py, all three at a grid(padx=(...)) call. key_analysis.py compiles clean.

Grant, 2026-08-30 — frank-coordinator to the wasm/N lane: land it in pasparser_call.inc

The defect is not in pyparser.inc. It is FindUMethOverloadAhead in compiler/pasparser_call.inc:1733 — the speculative type-aware method-overload probe. PyKwArgIndex and PyBindKwArgs are correct and never run: the probe refuses the call before binding is reached.

Mechanism. The probe steps over a NilPy name=expr keyword deliberately, so the name is not parsed as an expression, and its own comment states the assumption:

"The probe only needs the argument's TYPE; which parameter it binds to is settled later by PyKwArgIndex."

True of what the loops below want to know, false of how they index — both are positional, Procs[pi].Params[base + j] (lines 1939, 1946). So meth(a=0, v=(8,6)) against meth(a; s: AnsiString; v: Variant) judges argument 1 against s, a tuple lands on a string slot, the reference-shaped-argument guard fires, and a legal call is refused. Predicted before measuring, then confirmed on every row: tuple→string FAILS, tuple→int FAILS, no-gap OK, scalar OK — and it explains why the ctor and the unit-level proc pass, since neither path runs the probe.

Second exposure, same root, and it is the more serious one. The ranking loop indexes positionally too, so an overloaded method called with keywords that skip a default is ranked against the wrong parameters — silent wrong-overload selection, not a diagnostic. Under CLAUDE.md's table that is the silent-wrong-behaviour escape, and it is why this is one fix rather than a patch.

Granted: land it, with A's gate. pasparser_call.inc is a Pascal-frontend file shared with Track A, so the fixedpoint alone is not enough — tools/gate.sh quick is required on top of it. Verified free rather than assumed free: tools/fleet_dirt.sh across 15 discovered checkouts shows only ~/pxx-songfmt holding the file; frankA is in pasparser_stmt.inc, frank-rust in pasparser_generic.inc.

The lane holder correctly refused to push it under Track N and held a built, verified, uncommitted fix pending placement rather than assuming. That is the right call and the reason the grant is cheap to make.

Fix shape: OverloadArgParamIdx records the keyword name as a token index and resolves it per candidate, used by both loops, abstaining rather than rejecting when the name is not a declared parameter, so **kwargs callees are unaffected. Both halves of the boundary asserted — settings.py:178 (which already worked) as well as :183.

Follow-up filed separately: the code is in the wrong file by name

The broken code sits behind if isNilPy inside a file named for the Pascal parser. the-substrate-is-ast-and-ir-not-the-parser.md is being violated by filename: a NilPy calling-convention concern living in pasparser_call.inc is exactly why a Track N ticket routed its holder into a file Track N does not own, and why the collision question had to be asked at all. Worth its own ticket independently of who lands this fix — the isNilPy branching in that file is dense (lines 613, 634, 1147, 1767, 1851, 1888, 2131), so this is a carve-out question, not a one-line move.

Resolution, 2026-08-30 (frankwasm), under the grant at e268f9990

It was never the keyword binder

PyKwArgIndex and PyBindKwArgs are correct and the call never reaches them. The refusal comes from FindUMethOverloadAhead (compiler/pasparser_call.inc:1733), the speculative type-aware method-overload probe, which runs first.

The probe parses the argument list speculatively to learn argument types. For a NilPy name=expr it steps over the keyword name — deliberately, so the name is not parsed as an expression and reported as an undefined variable — and its own comment records the assumption:

      { NilPy `name=expr`: step over the keyword before probing, or the name is
        parsed as an expression and reported as an undefined variable — an
        OVERLOADED façade method could not be called with keyword arguments at
        all (`canvas.create_window((0,0), window=w)`). The probe only needs the
        argument's TYPE; which parameter it binds to is settled later by
        PyKwArgIndex. }

That is true of what the two loops below it want to know and false of how they index, which is positional — Procs[pi].Params[base + j]. So for meth(a=0, v=(8,6)) against meth(a: Integer; s: AnsiString; v: Variant), argument 1 is judged against s, a reference-shaped argument lands on a string slot, the guard fires, and a legal call is refused.

Predicted from the code, then measured before any edit. Every row of the ticket's boundary table follows from it, including why the constructor and the unit-level procedure pass: different paths, no probe.

shape predicted measured
tuple's positional slot is s (AnsiString) refuse FAILS
tuple's positional slot is a (Integer) refuse FAILS
no gap — tuple lands on its own Variant slot bind OK
scalar argument (not reference-shaped) bind OK

The second exposure, which is the more serious half

The ranking loop indexes positionally too. With more than one candidate, an overloaded method called with keywords that skip a default is ranked against the wrong parameters — that is silent wrong-overload selection, not a diagnostic. Same root, so one fix closes both; a fix aimed only at the reported symptom would have repaired the refusal and left the silent one in place.

The fix

A helper OverloadArgParamIdx(pi, base, j, kwTok) resolves argument j to its declared parameter: base + j when positional, and the named parameter when the argument was name=. The probe now records the keyword name as a token index instead of discarding it, and both loops index through the helper.

It abstains (returns -1, callers skip that argument) when the name matches no declared parameter, because a callee with **kwargs legitimately takes it — and an abstention can only ever drop a check, never manufacture a refusal. That is this file's own standing rule: an unsound refusal of a legal program is worse than the looseness it replaces. A wrong keyword name is still rejected, by PyKwArgIndex, with its proper diagnostic.

Verification

New gated test test_nilpy_keyword_call_tuple_on_a_skipped_default.npy with kwpadprobe.pas, a facade shaped like grid() — ordinal, then string, then Variant, so a skipped option lands the tuple on the string's slot. It is verified to fail on the baseline (no overload of grid matches these arguments) as well as pass here.

Both halves are asserted, which is the point of the test rather than a flourish. The refused shapes and the accepted ones (sticky supplied, no gap) are pinned, because a fix repairing the refusal while breaking its neighbour would pass any test that only watches the failure — and songformatter grids two identical widgets nine lines apart, one of each. Also pinned: a leading positional argument then keywords, scalars, lists, and the free-procedure control that never runs the probe.

Attribution measured against HEAD-without-this-patch (f8f879988222) rather than an older sha, both compilers built from the same base.

Songformatter

settings.py compiles — both line 178 (accepted) and line 183 (refused). key_analysis.py compiles. convertrawtext.py and SongFormatter.py now reach a later, unrelated wall reported at key_analysis.py:82; that one is pre-existing and not from this fix — with the tuple-pad trigger removed from a copy of the app so the baseline can reach the same depth, f8f879988222 hits the identical failure. Filed separately.

Gate

make compiler/pascal26 fixedpoint bcb428ba25ac; tools/gate.sh quick GREEN (A's gate, as the grant requires — this is Pascal-frontend ground shared with Track A, so the fixedpoint alone is not sufficient).

Log