[FS-1045] F# 4.x: Add ToFSharpFunc overloads for Func<...>, especially for .NET Core

[dsyme]

RFC https://github.com/fsharp/fslang-design/blob/master/RFCs/FS-1045-func-to-fsharpfunc-overloads.md

Addresses #1847 - ToFSharpFunc(Func) missing in .NET Core.

Adds

    type FuncConvert = 
        static member  inline ToFSharpFunc       : converter:Func<'T,'U>          -> ('T -> 'U)
        static member  inline ToFSharpFunc       : converter:Func<'T1,'T2,'U>          -> ('T1 * 'T2 -> 'U)
        static member  inline ToFSharpFunc       : converter:Func<'T1,'T2,'T3,'U>          -> ('T1 * 'T2 * 'T3 -> 'U)
        static member  inline ToFSharpFunc       : converter:Func<'T1,'T2,'T3,'T4,'U>          -> ('T1 * 'T2 * 'T3 * 'T4 -> 'U)
        static member  inline ToFSharpFunc       : converter:Func<'T1,'T2,'T3,'T4,'T5,'U>          -> ('T1 * 'T2 * 'T3 * 'T4 * 'T5 -> 'U)
        static member  inline ToFSharpFunc       : converter:Func<'T1,'T2,'T3,'T4,'T5,'T6,'U>          -> ('T1 * 'T2 * 'T3 * 'T4 * 'T5 * 'T6 -> 'U)
        static member  inline ToFSharpFunc       : converter:Func<'T1,'T2,'T3,'T4,'T5,'T6,'T7,'U>          -> ('T1 * 'T2 * 'T3 * 'T4 * 'T5 * 'T6 * 'T7 -> 'U)

There was no good way to create an FSharpFunc object from C# on .NET core with a method call.

[dsyme]

@cartermp @neoeinstein Just to note this will need an RFC since this also adds more overloads for ToFSharpFunc on .NET Framework

Also adds this on the Converter overload

    [<Obsolete("Please use the FuncConvert.ToFSharpFunc(Func<...>) overload of this method")>]

[KevinRansom]

Surface area change ... FSharp 4.2?

[dsyme]

Hmmm... Yes in principle. But we should get the missing functionality into our FSharp.Core for .NET Core sooner than that I think. I'll look if we can add just the one overload somehow

[KevinRansom]

@dsyme I'm okay with reving the version number, there are a number of PR's we would take if we do this.

[KevinRansom]

@dsyme can you resolve the conflicts pls.

[dsyme]

@KevinRansom resolved

[dsyme]

@cartermp @KevinRansom I've written the RFC https://github.com/fsharp/fslang-design/blob/master/RFCs/FS-1045-func-to-fsharpfunc-overloads.md

However as noted in the RFC this is a breaking change for some C# code, so don't merge this until we resolve that design question

[dsyme]

See comment here and in the RFC - I think we should not go ahead with this because it is a breaking change, and instead we should provide a .NET Standard 2.0 build of FSHarp.Core.dll where System.Converter is available. Then we can keep the FuncConvert API the same as it has always been.

[0x53A]

Everything except System.Func<'T,'U> can be safely taken, right?

In that case, why not just skip it?

IF the user proves an inline lambda, then the C# compiler will happily map it to a Converter instead of a Func.

IF the user wants to pass-through an existing Func delegate, then he would need to wrap it once - but I don't think that would be a killer-downside, and it wouldn't be worse than the current state.

[KevinRansom]

@dsyme ... what do you want to do with this PR?

[brettfo]

@dsyme, can you retarget this to master? I deleted the dev15.6 branch (which this was targeting) because it's old and we have a tag for it.

[dsyme]

@brettfo Are you sure you don't want this at dev15.8? It needs to target a branch that can bump the FSharp.Core version number

[dsyme]

Continued at #4815