Add completion for switch cases for `$PSBoundParameters.Keys`

[MartinGC94]

Summary of the new feature / enhancement

I occasionally write functions where I use a switch based on the bound parameters like this:

function Verb-Noun
{
    Param
    (
        [Parameter()]
        [string]
        $Param1,

        [Parameter()]
        [string]
        $Param2
    )

    switch ($PSBoundParameters.Keys)
    {
        Param1 {}
        Param2 {}
    }
}

To ensure I don't make typos I tend to copy+paste the parameters but tab completing them would be much more convenient. But before I try to add this feature, I want to be sure it would actually get accepted. Do other people use this pattern? Is this a pattern we want to encourage? If not, how do other people check if various parameters have been specified?

Proposed technical implementation details (optional)

The identifier/string completion code would be updated to check if it's completing a switch case, and if so, if the switch is on a variable with a property called Keys.
If the variable is $PSBoundParameters it would look for the nearest param block and get results from there.
If the variable is anything else we could try to look for a hashtable definition to that variable and get results from that, but I don't know if it's worth adding this because I don't think this is a particularly common pattern.
Also we could do some type inference on the variable, and if it's an Enum get the completion values from there.

[237dmitry]

I suspect that scripting constructs are not used very often interactively. Personally, I load custom functions using dot-sourcing functions.ps1 from $profile. In interactive mode, I use anonymous script blocks &{ } to limit the scope of variables. But these are not really advanced functions. If such a script block is cumbersome and there is a risk of losing it if I accidentally press ESC, then I edit them in $env:VISUAL by pressing F4.

[MartinGC94]

@237dmitry Editors like VS code get their completion data from the PowerShell engine. This suggestion is mainly thought of to improve the editor experience, not the interactive shell experience.

[237dmitry]

Editors like VS code get their completion data from the PowerShell engine

Then it would be a convenient addition.

[iRon7]

I once had a similar idea for a more common purpose in the form of a switch with a -key or a -property parameter. The issue with this is that it is unclear what you expect the current item ($_) to be in the action script block ({}): the current key/name, the current value or even something like a key-value pair? Knowing that they might be both required for any specific action.

[theJasonHelmick]

The interactive working group has reviewed this issue and would like to see it move forward.

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