Implementing interface method returning byref<'t> cannot just throw

For an interface method with a byref<'t> return, it's not possible to provide an implementation that just throws an exception

Repro steps

open System

type Ii<'T> =
    abstract GetSpan : int -> ReadOnlySpan<'T>

[<Struct>]
type X(value : int) =
    interface Ii<int> with
        member __.GetSpan n = ReadOnlySpan()

[<Struct>]
type Y(value : int) =
    interface Ii<int> with
        member __.GetSpan n = failwith "Nope"

Expected behavior

Compiles

Actual behavior

The line

member __.GetSpan n = failwith "Nope"

reports the error
FS0412 A type instantiation involves a byref type. This is not permitted by the rules of Common IL.

Known workarounds

Include an unreachable dummy return value

        member __.GetSpan n = failwith "Nope";  ReadOnlySpan()

Related information

Visual F# Tools 10.2 for F# 4.5 15.8.0.0. Commit Hash: 39fd7b8.

Thanks, that definitely looks like a bug.

It's by design. failwith is a generic function instantiated with the return type. The same would apply to Unchecked.defaultof<_>. Byrefs are just whacky wierd on .NET

You can record a language suggestion if you like

We'll have to strongly consider doing work here. Since both raise and failwith are generic functions, you cannot throw an exception like you can in C#:

    interface Ii<T>
    {
        int GetSpan(ReadOnlySpan<T> sp);
    }

    struct Y : Ii<int>
    {
        public int GetSpan(ReadOnlySpan<int> sp)
        {
            throw new Exception("Nope");
        }
    }

Yes, it would make sense to do something here, though it is a bit of a corner case. It's shame .NET generics have this limitation on byref instanatiations.

Could the 'something' also encompass the pipe operator? (like #5286)

Could the 'something' also encompass the pipe operator? (like #5286)

No. I'm very loathe to go down that long slippery slope since it's a bug farm.

Anything we do here would be for raise and nothing else. Not even for failwith.

No. I'm very loathe to go down that long slippery slope since it's a bug farm.

Fair enough :-)

@dsyme

@dsyme Do you still think a language suggestion is necessary, or should we just use this to track and update the RFC?

@dsyme

@dsyme Do you still think a language suggestion is necessary, or should we just use this to track and update the RFC?

We can use this to track I suppose, special casing raise is fine

Filed: fsharp/fslang-suggestions#872

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Implementing interface method returning byref<'t> cannot just throw #5776

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Implementing interface method returning byref<'t> cannot just throw #5776

Description

Repro steps

Expected behavior

Actual behavior

Known workarounds

Related information

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cartermp commented on Oct 16, 2018

dsyme commented on Oct 18, 2018

cartermp commented on Oct 18, 2018

dsyme commented on Oct 19, 2018

marklam commented on Oct 19, 2018

dsyme commented on Oct 19, 2018

marklam commented on Oct 19, 2018

cartermp commented on Oct 19, 2018

dsyme commented on Oct 25, 2018

cartermp commented on May 26, 2020

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