Improve implied lambda and delegate argument names

[kerams]

Alleviates the problem in #11898 (without direct delegates) and improves fsharp/fslang-suggestions#1083 (comment).

> System.IO.File.OpenText;;
val it: path: string -> System.IO.StreamReader

> let handler (x: string) (y: int) =
-     System.Console.WriteLine(x, y)
-
- let invokeHandler (foo: System.Action<string, int>) =
-     for parameter in foo.Method.GetParameters() do
-         printfn "%s " parameter.Name
-
- invokeHandler (System.Action<_,_>(handler));;
x
y
val handler: x: string -> y: int -> unit
val invokeHandler: foo: System.Action<string,int> -> unit
val it: unit = ()

> let z = Ok;;
val z: ResultValue: 'a -> Result<'a,'b>

instead of

> System.IO.File.OpenText;;
val it: arg00: string -> System.IO.StreamReader

> let handler (x: string) (y: int) =
-     System.Console.WriteLine(x, y)
-
- let invokeHandler (foo: System.Action<string, int>) =
-     for parameter in foo.Method.GetParameters() do
-         printfn "%s " parameter.Name
-
- invokeHandler (System.Action<_,_>(handler));;
delegateArg0
delegateArg1
val handler: x: string -> y: int -> unit
val invokeHandler: foo: System.Action<string,int> -> unit
val it: unit = ()

> let z = Ok;;
val z: arg0: 'a -> Result<'a,'b>

[kerams]

Is Test Unoptimized Declarations Project1 using different lang versions between WindowsCompressedMetadata desktop_release and WindowsCompressedMetadata coreclr_release or am I going crazy?

[vzarytovskii]

Is Test Unoptimized Declarations Project1 using different lang versions between WindowsCompressedMetadata desktop_release and WindowsCompressedMetadata coreclr_release or am I going crazy?

Uh, it should run differently built fcs, I can look into it tomorrow.

[kerams]

Got it, desktop test options do not specify preview for the checking of scripts, so I forced it for the 2 tests in question.

[T-Gro]

Got it, desktop test options do not specify preview for the checking of scripts, so I forced it for the 2 tests in question.

Yep, the current passing of langversion:preview looks good to me.

[psfinaki]

@kerams awesome job, thanks a lot for the testing and examples!

If you want to continue working on this (refactoring, perf) - go ahead, otherwise feel free to resolve the comments and merge as-is.

[0-gravity]

Will it be released any time soon?

[vzarytovskii]

Will it be released any time soon?

Probability should be in the next vs/dotnet release