@iSazonov Sorry for the late response (I'm currently on vacation :)).
Yes, I thought about implicit cast to/from Span/ReadOnlySpan. The implicit cast doesn't help regular conversion operation in PowerShell for the same root reason -- the target ByRef-like type cannot be boxed.

But when calling methods via Expression with jitting, Expression.Convert can be used to implicit cast an argument to the Span types before passing to the call (see an example below).

var arg = @"e:\abc\def";
var method = typeof(Path).GetMethod(nameof(Path.IsPathRooted), new Type[] { typeof(ReadOnlySpan<char>) });

var body = Expression.Call(method, Expression.Convert(Expression.Constant(arg), typeof(ReadOnlySpan<char>)));
var func = Expression.Lambda<Func<bool>>(body, null).Compile();
var rest = func();
Console.WriteLine(rest);

> True

PowerShell evaluates with interpretation by default, which bacially translating Expression tree to pre-defined C# code, so the Expression.Convert might not work like when Expression tree is jitted (haven't looked into it, will investigate that). If it works with interpreter too (or we can update the interpreter to make it work), we will need to update the method resolution. We currently use the same "figuring-out-conversion" method for regular conversion in powershell as well as when resolving the best matching method. The implicit cast for ByRef-like target types should continue to be "no-conversion" for regular conversion operation, but acceptable for method resolution.

Anyway, this should be done in a separate PR. I will open an issue to track it. The issue #7596 was opened to track it.

@daxian-dbw Thanks for depth comment! I think support dynamically converting Span based methods is important because main direction in .Net Core is to implement methods with Span parameters and other methods only wrap them. Later we can get APIs without the wrapped methods in CoreFX or third party libraries and PowerShell will not be able tocall them. So my fisrt think was that we could generate the wrap methods in runtime.

@lzybkr and @iSazonov Your comments have been addressed. Please take another look when you have time. Thanks!

Will it resolve:

 [System.Text.Encoding]::GetEncoding(0)
format-default : Cannot create boxed ByRef-like values.
+ CategoryInfo          : NotSpecified: (:) [format-default], InvalidProgramException
+ FullyQualifiedErrorId : System.InvalidProgramException,Microsoft.PowerShell.Commands.FormatDefaultCommand

I get this on 6.1.0 RC1.

Here is what it looks with changes from this PR:

PS:1> [System.Text.Encoding]::GetEncoding(0)


Preamble          :
BodyName          :
EncodingName      : Western European (Windows)
HeaderName        :
WebName           : windows-1252
WindowsCodePage   :
IsBrowserDisplay  :
IsBrowserSave     :
IsMailNewsDisplay :
IsMailNewsSave    :
IsSingleByte      : True
EncoderFallback   : System.Text.InternalEncoderBestFitFallback
DecoderFallback   : System.Text.InternalDecoderBestFitFallback
IsReadOnly        : True
CodePage          : 1252

Instance property access doesn't throw in powershell, even in strict mode. So accessing Preamble in the formatting code won't break it like it did previously.

BTW, the AppVeyor CI PR failed with the following error

TEST FAILURES
Description: Enable-PSSession Cmdlet creates a default PSSession configuration untied to a specific PowerShell version.
Name:        Validate Enable-PSSession Cmdlet.Enable-PSSession Cmdlet creates a default PSSession configuration untied to a specific PowerShell version.
message:
Expected a value, but got $null or empty.
stack-trace:
at <ScriptBlock>, C:\projects\powershell\test\powershell\Modules\Microsoft.PowerShell.Core\PSSessionConfiguration.Tests.ps1: line 855
855:             $matchedEndpoint | Should -Not -BeNullOrEmpty
1 tests in test/powershell failed

The same failure is happening in the daily build, so it's not caused by chnages in this PR.

I guess that remoting enpoints is created when we install PowerShell Core on base CI system but then test a preview version. So possible fix is to remove endpoints with Get-PSSessionConfiguration | Unregister-PSSessionConfiguration

    Describe "Validate Enable-PSSession Cmdlet" -Tags @("Feature", 'RequireAdminOnWindows') {
        BeforeAll {
            if ($IsNotSkipped) {
                Get-PSSessionConfiguration  | Unregister-PSSessionConfiguration
                Enable-PSRemoting
            }
        }