ETS ToString() script methods aren't honored in expandable strings and casts

The only type where a custom .ToString() is consistently honored appears to be [pscustomobject].

With all others - irrespective of whether the script method is added at the type level (Update-TypeData) or the instance level (Add-Member) - only explicit .ToString() calls honor the override; string interpolation and casts do not.

Steps to reproduce

# Override the .ToString() method for [hashtable]
# to output a compressed JSON representation.
Update-TypeData -Force -TypeName System.Collections.Hashtable -MemberName ToString -MemberType ScriptMethod -Value { $this | ConvertTo-Json -Compress }

$ht = @{ one = 1 }
$htAsJson = $ht | ConvertTo-Json -Compress

# Explicit .ToString() call: OK
$ht.ToString() | Should -Be $htAsJson

# !! BROKEN: [string] cast
[string] $ht | Should -Be $htAsJson

# !! BROKEN: string interpolation
"$ht" | Should -Be $htAsJson

Expected behavior

All tests should pass.

Actual behavior

Tests 2 and 3 fail, because the .ToString() override wasn’t honored.

Strings differ at index 0. Expected: '{"one":1}' But was:  'System.Collections.Hashtable'

Environment data

PowerShell Core 7.2.0-preview.2

[vexx32]

Engine WG discussed this issue today. Our opinion is that the requested change is not likely to be trivial, as these different methods of casting happen at different points in the engine.

We agree it would make sense for these three casting methods to be consistently calling the ETS ToString() method, but it would be a breaking change to the existing behaviour at this point.

We would need to see a compelling use case where this is necessary to consider changing the existing behaviour and risk breaking existing scripts that may unintentionally be relying on the behaviour.

[iSazonov]

We have an issue with a request to support extension methods. Many find that offer very desirable. But if we want extension methods we will fall in the issue as result.