Repro steps

1. Create the following F# program:

   open System
   type Foo() = class end
   [<EntryPoint>]
   let main _ =
     let foo = Foo()
     let span = Span<int>.Empty
     foo.CopyTo(span)

2. Try to compile it for netcoreapp3.1 target framework

Expected behavior

Compiler should tell me that the method in question (Foo.CopyTo(Span)) doesn't exist.

Actual behavior

Compiler reports the following:

Program.fs(7,3): error FS0041: No overloads match for method 'CopyTo'.

Known type of argument: Span<int>

Available overloads:
 - (extension) Foo.CopyTo<'T>(destination: Memory<'T>) : unit
 - (extension) Foo.CopyTo<'T>(destination: Span<'T>) : unit

Well, it somewhat does tell me that the method doesn't exists, but it uses a very confusing notation: how's an overload Foo.CopyTo<'T> available? And if it does, then why doesn't it get used here?

I suspect that the compiler actually picks a pair of corresponding System.MemoryExtensions.CopyTo methods which are only available for T[]?, but renders them in a confusing way.

Consider how confusing it was when I've tried to call it on BitArray (which has its own CopyTo method which doesn't work for Spans). I was first fascinated to know that BitArray.CopyTo<'T>(destination: Span<'T>) exists, but then very confused on how to call it.

Known workarounds

No.

Related information

Provide any related information (optional):

• Operating system: Windows 10
• .NET Runtime kind:

  $ dotnet --info
  .NET Core SDK (reflecting any global.json):
   Version:   3.1.302
   Commit:    41faccf259
  
  Runtime Environment:
   OS Name:     Windows
   OS Version:  10.0.19041
   OS Platform: Windows
   RID:         win10-x64
   Base Path:   C:\Program Files\dotnet\sdk\3.1.302\
  
  Host (useful for support):
    Version: 3.1.6
    Commit:  3acd9b0cd1
  
  .NET Core SDKs installed:
    2.1.808 [C:\Program Files\dotnet\sdk]
    2.2.207 [C:\Program Files\dotnet\sdk]
    3.1.302 [C:\Program Files\dotnet\sdk]
  

• Editing Tools: initially I've noticed the same confusing text in Rider