PR Summary

Delay type parameter-binding of unclosed (self-referential) type references in generic interface declarations

PR Context

We currently support implementing typed generic interfaces, but only if the type parameters can be resolved in the defining scope prior to defining the new type.

This makes the following implementation impossible in PowerShell:

    class SortableThing : IComparable, IComparable[SortableThing]
    {
        [int]Value

        [int]
        CompareTo([SortableThing]$obj)
        {
            return $this.Value.CompareTo($obj.Value)
        }

        [int]
        CompareTo($obj)
        {
            if($obj -isnot [SortableThing]){
                throw [ArgumentException]::new()
            }
            return $this.CompareTo([SortableThing]$obj)
        }
    }

To this end, the following changes have been made to the DefineTypeHelper in PSType:

• For declared interfaces, GetBaseTypes now outputs a list of late-binding InterfaceExpression's rather than a list of Type instances describing concrete interfaces
• We add the interfaces to the new type after defining the type builder, allowing us to resolve self-referential type parameters against the still-unclosed-type instead of throwing TypeNotFound

At some point we will need to re-think how we resolve generic type arguments, perhaps refactor GenericTypeName completely to natively support late binding against unclosed types, but I'll leave that an exercise for the future for now.

PR Checklist

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• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
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• User-facing changes
  • Not Applicable
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  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
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    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode (which runs in a different PS Host).
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