No "implementation missing" error for `abstract override`

[auduchinok]

A simplified repro of what has actually failed at runtime with no errors reported by F# compiler:

namespace CSharpLib

{
  public abstract class AbstractClass

  {
    public abstract override string ToString();

  }

}


open CSharpLib



type T() =
    inherit AbstractClass()



[<EntryPoint>]

let hello _ =
    let t = T()
    printfn "Hello %O" t
    0

No errors reported by FCS:

After successful build this code fails at runtime:

"C:\Program Files\dotnet\dotnet.exe"
    
C:/Users/Eugene.Auduchinok/RiderProjects/NewAbstractDefaultMember/NewAbstractDefaultMember/bin/Debug/netcoreapp2.0/NewAbstractDefaultMember.dll
Unhandled Exception: System.TypeLoadException: Method 'ToString' in type 'T' from assembly 'NewAbstractDefaultMember, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' does not have an implementation.
   at NewAbstractDefaultMember.hello(String[] name)

Process finished with exit code -532,462,766.

[cartermp]

Most likely missed (sometime in the past) since that C# code gives an error about a duplicate method if you were to write it in F#.

[abelbraaksma]

I didn't even know abstract override existed and I'm wondering how it's supposed to work. There's already an implementation, so at best this would shadow (i.e. like with new public) the existing member.

But since it's allowed in C#, I guess it should be consumable. Wouldn't PeVerify fail here on the result?

I guess the idea here is to force consumers to implement a method that already has an implementation in its base class?

[dsyme]

I didn't even know abstract override existed and I'm wondering how it's supposed to work.

Likewise, what even is this thing?

[auduchinok]

It's a shadowing definition of an abstract method that requires an implementation in an inheritor 🙂

 .method public hidebysig virtual abstract instance string
    ToString() cil managed
  {
  }

[smoothdeveloper]

it doesn't even seem to be something recent in C#, never heard of it!

https://www.damirscorner.com/blog/posts/20131028-UsingAbstractOverrideInC.html

If I understand right, if introducing an abstract class inheriting a concrete one in a hierarchy, this new class can override such members with abstract member.