fix: ensure valid indexing of arguments array

[manit2004]

Resolves #15921

Description

What is the purpose of this pull request?

This pull request:

• Fixes mask-arguments and pick-arguments, which both fall back to a generic loop (nary()) once 6 or more argument positions are selected. That loop looped arguments.length times instead of idx.length times; i.e. it used "how many arguments the wrapper happens to be called with" instead of "how many positions were actually selected." Calling the wrapper with more arguments than positions selected leaked extra stray undefineds through to the wrapped function. Adds regression tests covering both the more-args-than-selected and fewer-args-than-selected cases, since the existing nary tests always happened to call with exactly as many arguments as positions selected, which is why this went unnoticed.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• None

Questions

Any questions for reviewers of this pull request?

No.

Other

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Repro:

var pickArguments = require( '@stdlib/utils/pick-arguments' );

function foo() {
    return arguments.length;
}

var bar = pickArguments( foo, [ 0, 1, 2, 3, 4, 5 ] );

bar( 1, 2, 3, 4, 5, 6, 7, 8 ); // returned 8, now returns 6

Verified both fixes against the exact repro (before: 8/7, after: 6/6 for pick-arguments/mask-arguments respectively), and confirmed the new tests fail on the unfixed code and pass on the fixed code.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

AI Assistance

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• Yes
• No

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• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding

---

@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
utils/mask-arguments	$\\color{green}207/207$ $\\color{green}+100.00\\%$	$\\color{green}20/20$ $\\color{green}+100.00\\%$	$\\color{green}8/8$ $\\color{green}+100.00\\%$	$\\color{green}207/207$ $\\color{green}+100.00\\%$
utils/pick-arguments	$\\color{green}207/207$ $\\color{green}+100.00\\%$	$\\color{green}21/21$ $\\color{green}+100.00\\%$	$\\color{green}8/8$ $\\color{green}+100.00\\%$	$\\color{green}207/207$ $\\color{green}+100.00\\%$

The above coverage report was generated for the changes in this PR.

[kgryte]

LGTM. Thanks for fixing, @manit2004!