Powershell: ValidateSet/Register-ArgumentCompleter/Enum completions does not work correctly on Arrays

[PrzemyslawKlys]

Steps to reproduce

function New-Function {
    [cmdletBinding()]
    param(
        [validateSet('New', 'Old', 'Other')][string[]] $Test1,
        [validateSet('New', 'Old', 'Other')][string[]] $Test2
    )
}
New-Function -Test1 New -Test2 New
New-Function -Test1 New, Old -Test2 New
New-Function -Test1 New, Old -Test2 New

Paste in VSCode. Try to autocomplete Test2 parameter on line 8 (first instance in the script). It will work for 1st element of array, but if you try adding 2nd or 3rd element autocomplete fails.

It works correctly if you start from top to bottom as in

function New-Function {
    [cmdletBinding()]
    param(
        [validateSet('New', 'Old', 'Other')][string[]] $Test1,
        [validateSet('New', 'Old', 'Other')][string[]] $Test2
    )
}
New-Function -Test1 New -Test2 New

It will work on line 8

function New-Function {
    [cmdletBinding()]
    param(
        [validateSet('New', 'Old', 'Other')][string[]] $Test1,
        [validateSet('New', 'Old', 'Other')][string[]] $Test2
    )
}
New-Function -Test1 New -Test2 New
New-Function -Test1 New, Old -Test2 New
New-Function -Test1 New, Old -Test2 New

It will work on line 10 but fails for lines 8 and 9.

Originally reported in VSCode PowerShell extension where more discussion is present: https://github.com/PowerShell/vscode-powershell/issues/2982

Expected behavior

Autocomplete working on arrays in any order. On first line, last line, middle line.

Actual behavior

Environment data

Name                           Value
----                           -----
PSVersion                      7.1.0-rc.1
PSEdition                      Core
GitCommitId                    7.1.0-rc.1
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

[SeeminglyScience]

Simplified repo

Steps to Reproduce

$withoutTrailingCommands = @'
function New-Function {
    [CmdletBinding()]
    param(
        [ValidateSet('New', 'Old', 'Other')]
        [string[]] $Test
    )
}

New-Function -Test New, Old,
'@

$withTrailingCommands = @'
function New-Function {
    [CmdletBinding()]
    param(
        [ValidateSet('New', 'Old', 'Other')]
        [string[]] $Test
    )
}
New-Function -Test New, Old,
anythinghere
'@

$cursor = $withoutTrailingCommands.Length

(TabExpansion2 -inputScript $withoutTrailingCommands -cursorColumn $cursor).CompletionMatches
'----'
(TabExpansion2 -inputScript $withTrailingCommands -cursorColumn $cursor).CompletionMatches

Expected behavior

Both return the same results

CompletionText ListItemText     ResultType ToolTip
-------------- ------------     ---------- -------
New            New          ParameterValue New
Old            Old          ParameterValue Old
Other          Other        ParameterValue Other
----
New            New          ParameterValue New
Old            Old          ParameterValue Old
Other          Other        ParameterValue Other

Actual behavior

Only the first tabexpansion call actually worked. Validate set string array argument completion works only when there are no following statements. Or following parameters it seems.

CompletionText ListItemText     ResultType ToolTip
-------------- ------------     ---------- -------
New            New          ParameterValue New
Old            Old          ParameterValue Old
Other          Other        ParameterValue Other
----
.\Documents    Documents    ProviderContainer etc
.\Downloads    Downloads    ProviderContainer etc
...etc

[PrzemyslawKlys]

It's even worse than I thought it is. Thank you

[PrzemyslawKlys]

I just wanted to add I often have issue with autocomplete for 1st element as well

Especially when using Register-ArgumentCompleter

        [parameter(ParameterSetName = "FontAwesomeBrands")]
        [parameter(ParameterSetName = "FontAwesomeRegular")]
        [parameter(ParameterSetName = "FontAwesomeSolid")][string] $IconColor,

....


Register-ArgumentCompleter -CommandName New-HTMLTab -ParameterName IconColor -ScriptBlock $Script:ScriptBlockColors

I wasn't able to reproduce it easily so there must be something in play that impacts how things are autocomplete when used the way I use it.

[PrzemyslawKlys]

It would seem for PowerShell 5.1 this fails very often

function Test-Me1 {
    [cmdletBinding()]
    param(
        [string[]] $IconColor,
        [string[]] $TextColor
    )
}

function Test-Me2 {
    [cmdletBinding()]
    param(
        [string[]] $IconColor
    )
}


$ScriptBlock = {
    param($commandName, $parameterName, $wordToComplete, $commandAst, $fakeBoundParameters)
    'One', 'Two', 'Three' | Where-Object { $_ -like "*$wordToComplete*" }
}

Register-ArgumentCompleter -CommandName Test-Me1 -ParameterName IconColor -ScriptBlock $ScriptBlock
Register-ArgumentCompleter -CommandName Test-Me1 -ParameterName TextColor -ScriptBlock $ScriptBlock


Register-ArgumentCompleter -CommandName Test-Me2 -ParameterName IconColor -ScriptBlock $ScriptBlock

Test-Me1 -IconColor One -TextColor One

Test-Me2 -IconColor One

In PowerShell 7 it seems to work (for 1st entry only, not for 2nd. So I guess somewhere along the road it was fixed partially.