Binding a parameter via the pipeline can break implicit parameter-set selection

[mklement0]

@ili101 discovered that binding a parameter via the pipeline can cause PowerShell to select the wrong parameter set under certain circumstances.

The issue can be resolved by designating a default parameter set.

Arguably, PowerShell shouldn't pick a default parameter set at all in this case, but it does:

• defensibly with direct arguments,
• seemingly incorrectly with conceptually equivalent pipeline input

Note that it is legitimate use case to have an explicit parameter set that has no mandatory parameter (set A below): You may want to group multiple parameters as logically related, even if none of them is individually mandatory.

Steps to reproduce

function foo
{
  [CmdletBinding()] # DefaultParameterSetName='A' would make the issue go away
  Param(
    [Parameter(ParameterSetName = 'A')]            $A1,
    [Parameter(ParameterSetName = 'A')]            $A2,
    [Parameter(ParameterSetName = 'B', Mandatory)] $B1,
    [Parameter(ParameterSetName = 'B')]            $B2,
    [Parameter(ValueFromPipeline)]                 $InputObject
  )
  $PSCmdlet.ParameterSetName
}

# OK: Argument-less invocation selects 'A'
foo  | Should -Be A
# OK: Binding -InputObject by direct argument also selects 'A' 
foo -InputObject 1 | Should -Be A
# FAILS: Binding -InputObject via the pipeline inexplicably selects 'B'
# Just press Enter to answer the prompt.
& { 1 | foo }      | Should -Be A

Expected behavior

All tests should pass.

Actual behavior

The 3rd test fails: It unexpectedly selects set B prompts for a value for -B1.

Expected strings to be the same, but they were different. 
String lengths are both 1. Strings differ at index 0. 
Expected: 'A' But was:  'B'

Environment data

PowerShell Core 7.0.0-preview.6

[microsoft-github-policy-service]

This issue has not had any activity in 6 months, if this is a bug please try to reproduce on the latest version of PowerShell and reopen a new issue and reference this issue if this is still a blocker for you.

[microsoft-github-policy-service]

This issue has not had any activity in 6 months, if this is a bug please try to reproduce on the latest version of PowerShell and reopen a new issue and reference this issue if this is still a blocker for you.

[microsoft-github-policy-service]

This issue has not had any activity in 6 months, if this is a bug please try to reproduce on the latest version of PowerShell and reopen a new issue and reference this issue if this is still a blocker for you.

[aolszowka]

Still repros for me; following exactly what this user has written:

> $PSVersionTable

Name                           Value
----                           -----
PSVersion                      7.3.9
PSEdition                      Core
GitCommitId                    7.3.9
OS                             Microsoft Windows 10.0.19045
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}       
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

function foo {
    [CmdletBinding()] # DefaultParameterSetName='A' would make the issue go away
    Param(
        [Parameter(ParameterSetName = 'A')]            $A1,
        [Parameter(ParameterSetName = 'A')]            $A2,
        [Parameter(ParameterSetName = 'B', Mandatory)] $B1,
        [Parameter(ParameterSetName = 'B')]            $B2,
        [Parameter(ValueFromPipeline)]                 $InputObject
    )
    $PSCmdlet.ParameterSetName
}

# OK: Argument-less invocation selects 'A'
foo
# OK: Binding -InputObject by direct argument also selects 'A'
foo -InputObject 1
# FAILS: Binding -InputObject via the pipeline inexplicably selects 'B'
# Just press Enter to answer the prompt.
& { 1 | foo }

[microsoft-github-policy-service]

This issue has been marked as "No Activity" as there has been no activity for 6 months. It has been closed for housekeeping purposes.