If you unload PSReadline and try tab completing or invoke TabExpansion2 manually you get the expected results. I think the issue is that PowerShell looks at the current scope, and I guess $error exists but is empty in the PSReadline module scope.

See #3568 for no success history.

He said:

The original fix I made had a bad assumption as there is a valid $script:error variable.

If that's true then I don't see how this can be fixed without some sort of hack, PowerShell is correctly finding the $Error variable in the PSReadline scope.
So the question becomes, how should the hack be implemented? Should the completion code have a special case for $Error if no results are found? Should the Safe Expression Value code have a special case for $Error within the PSReadline scope? Should PSReadline provide its own completions for $Error?

If I understand correctly current status is that the issue presents only if PSReadline is loaded. If so maybe @daxian-dbw advice.

Interactive completion of $global:Error[0].Fu<tab> works with PSReadLine loaded.

As @MartinGC94 pointed out this is due to PSReadLine's entry point having affinity to its module's session state. The easiest fixes would be in PSReadLine itself to do one of these:

1. Strip session state affinity before exporting (though it may come right back during the export, I can't remember).

   # in the psm1 after the function definition
   $function:PSConsoleHostReadLine = $function:PSConsoleHostReadLine.Ast.GetScriptBlock()

2. Reimplement PSConsoleHostReadLine as a binary command

As a workaround, you can put this in your profile:

. (Get-Module PSReadLine) {
    $function:PSConsoleHostReadLine = $function:PSConsoleHostReadLine.Ast.Body.GetScriptBlock()
}

$function:PSConsoleHostReadLine = $function:PSConsoleHostReadLine.Ast.Body.GetScriptBlock()

(Both seem to be needed when export has already occurred. Not really sure why. Also note that the addition of Body here is needed only after export has occurred)

The above fix could work, but there is also possibility that the PSConsoleHostReadLine function from PSReadLine will depend on state in the module in future, and in that case the function won't work properly if it's disconnected with the PSReadLine session state.

So, maybe the fix needs to happen in the tab completion code, for example, CommandCompletion.CompleteInput offers an additional parameter to allow the caller to choose running it with the top-level session state.

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