PR Summary

Fixes a bug where a parameter declared with different Position values across multiple parameter sets would be bound twice during positional argument processing, overwriting the correct value.

#2212

Root Cause

EvaluateUnboundPositionalParameters() correctly inserts a parameter into the sorted positional dictionary once per parameter set it belongs to — meaning a parameter that is Position=0 in set Two and Position=1 in set One appears at both dictionary keys. When BindPositionalParametersInSet() iterates these keys in sorted order, it would bind the parameter at position 0 (correctly), then encounter the same parameter again at position 1 and re-bind it — overwriting $First with the argument intended for $Second, and then also binding $Second to that same argument.

Fix

Added a BoundParameters.ContainsKey guard at the top of the outer foreach loop in BindPositionalParametersInSet() (ParameterBinderController.cs) so that a parameter already bound at an earlier position is skipped. The equivalent guard was added to BindPseudoPositionalParameterInSet() in PseudoParameterBinder.cs so that tab-completion behavior stays consistent with runtime binding.

Files Changed

Testing

All 5 new CI-tagged tests pass (Tests Passed: 44, Failed: 0) when run against the fixed build on Windows x64.

Reproduction from the original issue:

function Test-It {
    [CmdletBinding()]
    param(
        [Parameter(ParameterSetName = 'Two', Position = 0)]
        [Parameter(ParameterSetName = 'One', Position = 1)]
        [string] $First,

        [Parameter(ParameterSetName = 'Two', Position = 1)]
        [string] $Second
    )
    "$First / $Second"
}

Test-It Hello World   # Before fix: "World / World"  After fix: "Hello / World"

PR Checklist

• PR has a meaningful title
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• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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• Breaking changes
  • None
• User-facing changes
  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • Make sure you've added a new test if existing tests do not effectively test the code changed