PowerShell Integrated Console: "Value cannot be null" for missing mandatory parameter when debugging

Issue Type: Bug

Steps to reproduce

Create a .ps1 script file with the following contents:

class MyNewClass {
    [string] $DummyVar
}

function Get-Details {
    param (
        [Parameter(Mandatory)][MyNewClass] $ClassVar,
        [Parameter(Mandatory)][String] $StringVar
    )
    $ClassVar | Out-Host
}

Get-Details

Run the script from a PowerShell terminal window in VSCode.

Expected behavior

Using an explicit "pwsh" terminal inside VSCode, or an external Windows PowerShell or PowerShell Core shell, the above code gives:

cmdlet Get-Details at command pipeline position 1
Supply values for the following parameters:
ClassVar:

Actual behavior

From the PowerShell Integrated Console, the same script results in:

Value cannot be null.
Parameter name: type
At C:\Users\myuser\Documents\Development\PowerShell\TestParamNull.ps1:14 char:1
+ Get-Details
+ ~~~~~~~~~~~
+ CategoryInfo          : OperationStopped: (:) [], ArgumentNullException
+ FullyQualifiedErrorId : System.ArgumentNullException

If I explicitly call "pwsh" from inside this same integrated console, the script then works as expected.

Best guesses are that the issue is occurring when the debugger is involved, or that your readline is choking because it doesn't know how to request the user enters a custom class?

Environment data

Explicit pwsh

PS> $PSVersionTable
Name                           Value
----                           -----
PSVersion                      6.2.0
PSEdition                      Core
GitCommitId                    6.2.0
OS                             Microsoft Windows 10.0.17763
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

PS> $PSHOME
C:\Program Files\PowerShell\6

PowerShell Integrated Console

PS>$PSVersionTable
Name                           Value
----                           -----
PSVersion                      6.2.0
PSEdition                      Core
GitCommitId                    6.2.0
OS                             Microsoft Windows 10.0.17763
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

PS> $PSHOME
C:\Program Files\PowerShell\6

VSCode PowerShell Debugger Launch Config

        {
            "name": "PowerShell Launch Current File",
            "type": "PowerShell",
            "request": "launch",
            "script": "${file}",
            "args": [],
            "cwd": "${file}"
        }

Extension version: 1.12.1
VS Code version: Code 1.34.0 (a622c65b2c713c890fcf4fbf07cf34049d5fe758, 2019-05-15T21:59:37.030Z)
OS version: Windows_NT x64 10.0.17763

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-7820HQ CPU @ 2.90GHz (8 x 2904)
GPU Status	2d_canvas: enabled checker_imaging: disabled_off flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled multiple_raster_threads: enabled_on native_gpu_memory_buffers: disabled_software rasterization: enabled surface_synchronization: enabled_on video_decode: enabled webgl: enabled webgl2: enabled
Load (avg)	undefined
Memory (System)	31.81GB (14.95GB free)
Process Argv
Screen Reader	no
VM	0%

Reproduce the provided MyNewClass/Get-Details script from the PowerShell Integrated Console and compare it with an explicit pwsh terminal. Start by tracing the debugger and integrated-console parameter-prompting path using the supplied PowerShell Debugger Launch Config. Done means the integrated console prompts for the missing mandatory parameter instead of reporting a null type error.