Is it possible to add a closing parenthesis on tab-completed methods with no arguments?

Yes, it's possible to customize TabExpansion2 to automatically append a closing parenthesis ) when completing a method with no parameters. This answer provides 2 different ways to do it:

1. Creating new CompletionResult objects.
2. Updating the existing CompletionResult objects via reflection.

Before providing the options, first few clarifications:

Why .ToolTip.EndsWith('()') ?

This is sort of a hacky way to determine if a method has parameters or not, the default TabExpansion2 returns a CompletionResult object for each completion, and its .ToolTip property describes the member, including all method overloads if applicable. For methods, the overload taking no arguments, if applicable, is always listed first in the ToolTip. If a method has only a no-parameter signature, the ToolTip ends with (), making .ToolTip.EndsWith('()') a reliable check in most cases.

For example:

(TabExpansion2 "''.GetT").CompletionMatches[0]
CompletionText ListItemText ResultType ToolTip
-------------- ------------ ---------- -------
GetType(       GetType      Method     type GetType()
(TabExpansion2 "''.GetT").CompletionMatches[0].ToolTip.EndsWith('()')
True
(TabExpansion2 "''.IndexOf").CompletionMatches[0].ToolTip.EndsWith('()')
False

What Becomes the Completion?

The CompletionText property of the CompletionResult object is what PowerShell inserts as the final completion text. Both solutions below modify this property, either by creating a new CompletionResult with an updated CompletionText or by altering the existing object's CompletionText to append the closing ).

Approach 1: Creating new objects

This approach creates a new CompletionResult object for no-parameter methods, appending the closing parenthesis. It's safer and more maintainable since it doesn’t rely on internal fields, though it may use slightly more memory.

function TabExpansion2 {
    [CmdletBinding(DefaultParameterSetName = 'ScriptInputSet')]
    [OutputType([System.Management.Automation.CommandCompletion])]
    param(
        [Parameter(ParameterSetName = 'ScriptInputSet', Mandatory = $true, Position = 0)]
        [AllowEmptyString()]
        [string] $inputScript,
        [Parameter(ParameterSetName = 'ScriptInputSet', Position = 1)]
        [int] $cursorColumn = $inputScript.Length,
        [Parameter(ParameterSetName = 'AstInputSet', Mandatory = $true, Position = 0)]
        [System.Management.Automation.Language.Ast] $ast,
        [Parameter(ParameterSetName = 'AstInputSet', Mandatory = $true, Position = 1)]
        [System.Management.Automation.Language.Token[]] $tokens,
        [Parameter(ParameterSetName = 'AstInputSet', Mandatory = $true, Position = 2)]
        [System.Management.Automation.Language.IScriptPosition] $positionOfCursor,
        [Parameter(ParameterSetName = 'ScriptInputSet', Position = 2)]
        [Parameter(ParameterSetName = 'AstInputSet', Position = 3)]
        [Hashtable] $options = $null
    )
    end {
        if ($psCmdlet.ParameterSetName -eq 'ScriptInputSet') {
            $toComplete = [System.Management.Automation.CommandCompletion]::CompleteInput(
                  $inputScript,
                 $cursorColumn,
                      $options)
        }
        else {
            $toComplete = [System.Management.Automation.CommandCompletion]::CompleteInput(
                              $ast,
                           $tokens,
                 $positionOfCursor,
                          $options)
        }
        for ($i = 0; $i -lt $toComplete.CompletionMatches.Count; $i++) {
            $match = $toComplete.CompletionMatches[$i]
            if ($match.ToolTip.EndsWith('()')) {
                $toComplete.CompletionMatches[$i] =
                    [System.Management.Automation.CompletionResult]::new(
                        $match.CompletionText + ')',
                        $match.ListItemText,
                        $match.ResultType,
                        $match.ToolTip)
            }
        }
        $toComplete
    }
}

Approach 2: Using Reflection

This approach uses reflection to modify the CompletionText of existing CompletionResult objects, it’s potentially more performant and memory-efficient but riskier due to reliance on private fields that could change in future PowerShell versions.

function TabExpansion2 {
    [CmdletBinding(DefaultParameterSetName = 'ScriptInputSet')]
    [OutputType([System.Management.Automation.CommandCompletion])]
    param(
        [Parameter(ParameterSetName = 'ScriptInputSet', Mandatory = $true, Position = 0)]
        [AllowEmptyString()]
        [string] $inputScript,
        [Parameter(ParameterSetName = 'ScriptInputSet', Position = 1)]
        [int] $cursorColumn = $inputScript.Length,
        [Parameter(ParameterSetName = 'AstInputSet', Mandatory = $true, Position = 0)]
        [System.Management.Automation.Language.Ast] $ast,
        [Parameter(ParameterSetName = 'AstInputSet', Mandatory = $true, Position = 1)]
        [System.Management.Automation.Language.Token[]] $tokens,
        [Parameter(ParameterSetName = 'AstInputSet', Mandatory = $true, Position = 2)]
        [System.Management.Automation.Language.IScriptPosition] $positionOfCursor,
        [Parameter(ParameterSetName = 'AstInputSet', Position = 3)]
        [Parameter(ParameterSetName = 'ScriptInputSet', Position = 2)]
        [Hashtable] $options = $null
    )
    end {
        if ($psCmdlet.ParameterSetName -eq 'ScriptInputSet') {
            $toComplete = [System.Management.Automation.CommandCompletion]::CompleteInput(
                $inputScript, $cursorColumn, $options)
        }
        else {
            $toComplete = [System.Management.Automation.CommandCompletion]::CompleteInput(
                $ast, $tokens, $positionOfCursor, $options)
        }
        # Cache the FieldInfo for performance
        if (-not $script:fieldInfo) {
            $fieldName = 'completionText'
            if ($IsCoreCLR) {
                # In PowerShell 7 the field starts with 
                $fieldName = 'completionText'
            }
            $script:fieldInfo = [System.Management.Automation.CompletionResult].GetField(
                $fieldName,
                [System.Reflection.BindingFlags] 'Instance, NonPublic')
        }
        foreach ($match in $toComplete.CompletionMatches) {
            if ($match.ToolTip.EndsWith('()')) {
                $script:_fieldInfo.SetValue($match, $match.CompletionText + ')')
            }
        }
        $toComplete
    }
}