PowerShell autocompletion script fails with runtime errors on empty or single-item completions

[VedantMadane]

Description

Cobra's generated PowerShell completion script fails with runtime exceptions in various edge cases due to how it parses and filters the completion output array ($Out):

1. Null Pointer/MethodNotFound Exceptions on Empty Outputs:
   If a command execution returns no output (for example, if the CLI binary crashes, fails or simply yields no completions and stdout is empty), the array $Out is empty. The generated script calls $Out[-1].TrimStart(':') without checking if $Out contains any elements. This throws a fatal PowerShell error:

   You cannot call a method on a null-valued expression.
   

2. System.Char vs. String Type Confusion:
   PowerShell pipelines automatically unwrap arrays containing exactly one element into a scalar object (such as System.String). In the next execution phase, querying $Out[-1] returns a System.Char instead of a System.String. Since System.Char does not have a TrimStart method, calling it throws:

   Method invocation failed because [System.Char] does not contain a method named 'TrimStart'.
   

3. Filtering Collision on Identical String Values:
   To strip the directive (which is the last element of the output, such as :4), the script uses:

   $Out = $Out | Where-Object { $_ -ne $Out[-1] }

   Instead of removing only the last item by index, this filters out all items matching the value of the last item. If a command legitimately outputs a completion item that matches the directive string (such as when a flag or argument completes to the string ":0"), it is incorrectly stripped.

Root Cause Analysis

In powershell_completions.go inside the shell script template, the generated script assumes $Out is always a populated array with string elements:

    #call the command store the output in $out and redirect stderr and stdout to null
    Invoke-Expression -OutVariable out "$RequestComp" 2>&1 | Out-Null

    # get directive from last line
    [int]$Directive = $Out[-1].TrimStart(':')
    if ($Directive -eq "") {
        $Directive = 0
    }

    # remove directive (last element) from out
    $Out = $Out | Where-Object { $_ -ne $Out[-1] }

Proposed Fix

Modify the template inside powershell_completions.go to safely check the size of the array, convert the directive to an integer defensively and strip the last item cleanly by index using Select-Object:

	$Directive = 0
	if ($Out.Count -gt 0) {
		$DirectiveStr = $Out[-1].TrimStart(':')
		if ($DirectiveStr -ne "") {
			$Directive = [int]$DirectiveStr
		}
		$Out = $Out | Select-Object -First ($Out.Count - 1)
	}
	__%[1]s_debug "The completion directive is: $Directive"
	__%[1]s_debug "The completions are: $Out"