PSReviewUnusedParameter with script parameter and function using it

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Steps to reproduce

param(
	$a
)


function f
{
  echo $a
}

f

Expected behavior

No warnings

Actual behavior

Invoke-ScriptAnalyzer .\a.ps1

RuleName                            Severity     ScriptName Line  Message
--------                            --------     ---------- ----  -------
PSReviewUnusedParameter             Warning      a.ps1      2     The parameter 'a' has been declared but not used.

Environment data

> $PSVersionTable
Name                           Value
----                           -----
PSVersion                      7.1.3
PSEdition                      Core
GitCommitId                    7.1.3
OS                             Microsoft Windows 10.0.19042
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

> (Get-Module -ListAvailable PSScriptAnalyzer).Version | ForEach-Object { $_.ToString() }
1.19.1

[rjmholt]

Not sure I understand the problem here.

The $a value being referenced in f is not necessarily the $a being passed in (because the $a reference in f, at runtime, looks up the runtime variable scope stack to determine what value to use), so in general this isn’t a usage of the $a parameter and there are no other usages of $a in the immediate scope, so PSSA considers the parameter unused.

It happens that in your scenario, due to dynamic scope, it is used thanks to scope inheritance. But PSScriptAnalyzer is a static analyser; it’s impossible in general for it (or any static analyser) to be able to infer a variable usage that depends on dynamic scope inheritance. In addition to that, most of the time when a parameter is used in the way that you’ve used it, it’s because the author is expecting lexical scope rather than dynamic scope and this represents a bug in their script.

So because analysing variables from dynamic scope isn’t something that PSScriptAnalyzer can feasibly provide guarantees about, and because depending on dynamic scope like this is generally not something that PSScriptAnalyzer is trying to encourage, getting a warning about the $a variable in this scenario is expected. The warning message could probably be improved, but that’s part of a larger work item around improving the way the unused variable logic works in https://github.com/PowerShell/PSScriptAnalyzer/issues/1641.

[rjmholt]

This is actually a valid warning. If you dot-source that script and call f like this:

. ./script.ps1 -a 'x'

f

You’re going to see that second call of f output $null.