Powershell: Improper usage of $input as a function parameter is silently ignored

[petrSchreiber]

As I learned the hard way today, $input is PowerShell automatic variable.

PowerShell seems to ignore obviously improper usage of the $input, leaving the programmer confused about the parameter not being propagated further.

Steps to reproduce

There are two identical (non-optimal, I admit) implementations of Ensure-Array.
They both differ only in parameter name, nothing more.

function Ensure-ArrayA($input)
{
    $result = @()
    if ($input -is [system.array])
    {
        $result = $input
    }
    else
    {
        $result = @($input)
    }
    return [Array]$result
}

function Ensure-ArrayB($Unknown)
{
    $result = @()
    if ($Unknown -is [system.array])
    {
        $result = $Unknown
    }
    else
    {
        $result = @($Unknown)
    }
    return [Array]$result
}

$x = Ensure-ArrayA 1
$y = Ensure-ArrayB 1

$x.Count
$y.Count

Expected behavior

I would presume PowerShell would fire an exception regarding improper usage of the $input as function parameter in Ensure-ArrayA.

Actual behavior

Instead, PowerShell silently ignores this incorrect code, pretending everything is all right. Of course, using $input will result in incorrect output as consequence.

The script above will output simply:

1

Environment data

> $PSVersionTable

Name                           Value                                                                                                                                                          
----                           -----                                                                                                                                                          
PSVersion                      5.1.14393.1198                                                                                                                                                 
PSEdition                      Desktop                                                                                                                                                        
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}                                                                                                                                        
BuildVersion                   10.0.14393.1198                                                                                                                                                
CLRVersion                     4.0.30319.42000                                                                                                                                                
WSManStackVersion              3.0                                                                                                                                                            
PSRemotingProtocolVersion      2.3                                                                                                                                                            
SerializationVersion           1.1.0.1 

[iSazonov]

@petrSchreiber Thanks for your report!

Related #3695.

/cc @jpsnover

[mklement0]

Also: #3061, which tackles the issue comprehensively.

[SteveL-MSFT]

@PowerShell/powershell-committee discussed this and agree that the parser will enforce not allowing writing to $input

[iSazonov]

Set-Variable can write to the variable.

[SteveL-MSFT]

@iSazonov I think the intent here is to address common usage where the user isn't aware that the variable they are using could be clobbered by PowerShell. Set-Variable doesn't seem like it would fall under this, but I'm not opposed to special casing it in that cmdlet.

[mklement0]

See this documentation issue for the necessary attendant change to the documentation.

[SteveL-MSFT]

Although a breaking change, I think we can address this in 6.1.0