Cross-package function using `arguments` returns undefined (synthetic-arguments param inflates cross-module arity) — #321 gateway

[proggeramlug]

Summary

A cross-package (compilePackages) function that uses arguments returns
undefined for calls with fewer args than the inflated param count.

Perry appends a synthetic arguments param (name: "arguments", is_rest: true)
to any function whose body references arguments (#677). Cross-package, the
importer's param-count collection (compile.rs ~606, func.params.len())
counts that synthetic param as a real one, inflating the arity, and the
cross-module call mishandles it (the synthetic arguments is treated as a
trailing-rest collecting only args after the fixed params, instead of the full
argument list). Under-filled calls then return undefined.

Minimal repro (compilePackages)

node_modules/mypkg/src/index.ts:

export function usesArgs(a: number, b?: number): number {
  return arguments.length === 1 ? a : a + (b as number);
}
export function noArgs(a: number, b?: number): number {
  return b === undefined ? a : a + b;
}

entry (perry.compilePackages: ["mypkg"]):

import { usesArgs, noArgs } from "mypkg";
console.log(usesArgs(5), usesArgs(5, 3), noArgs(5), noArgs(5, 3));

	node	perry
usesArgs(5)	5	undefined ✗
usesArgs(5,3)	8	8
noArgs(5)	5	5
noArgs(5,3)	8	8

--print-hir on the entry: usesArgs (params: 3) vs noArgs (params: 2) — the
synthetic arguments param inflates usesArgs to 3. Single-file (non-package)
arguments-using functions work; only the cross-package path is broken.

Impact — the #321 (Effect framework) gateway blocker

effect's pipe and dual (the two most-used combinators) both use arguments,
so cross-package they return undefined. This cascades:

• pipe(a, fn) → undefined
• Effect.map / Effect.pipe → _op of undefined in the fiber runtime
• S.Struct({...}) decode → _tag of undefined in ParseResult
• effect's runtime init: liveServices = pipe(Context.empty(), Context.add(...)…)
  → undefined → currentServices.initial undefined →
  FiberRuntime.refreshRefCache → .unsafeMap of undefined.

The effect DoD (deep import, Schema init, S.Number decode, import { Effect }
barrel Effect.runSync(Effect.succeed(42))) already works (#1804/#1809/#1810/
#1811/#1812/#1815) because those paths don't depend on a pipe/arguments
result. This bug is the gateway to the rest of effect.

Fix direction

The cross-module param count + call must treat the synthetic arguments param
specially: exclude it from the user-visible arity, and populate it with the
full argument list (as the same-module synthetic-arguments path does), not
as a trailing rest. Sites: compile.rs exported-func param-count collection +
the cross-module call lowering's rest/arity handling. Broad blast radius
(all cross-module calls to arguments-using functions) — needs a thorough
regression sweep.

Refs #321, #1758, #677.

[proggeramlug]

Verified fixed by PR #1817 (merged 2026-05-26). Reproduced the exact compilePackages shape from the issue body:

$ /tmp/repro1816_out
5 8 5 8

All four cases (usesArgs(5), usesArgs(5,3), noArgs(5), noArgs(5,3)) match Node.

Cross-module shape locked in test-files/test_gap_cross_module_arguments.ts; cross-package shape (compilePackages → node_modules) locked in test-files/issue_1069/. Closing as fixed.