Unexpected error "feature 'ref struct interfaces' is not available"

[jjonescz]

Version Used: 4.13.0-3.24612.1 (c30de30)

Steps to Reproduce:

Compile with LangVersion=12:

using System.Diagnostics.CodeAnalysis;
interface I
{
    void M();
}
struct S : I
{
    [UnscopedRef] void I.M() { } // errors here
}

Expected Behavior: Only error CS9102: UnscopedRefAttribute cannot be applied to an interface implementation because implemented member 'I.M()' doesn't have this attribute.

Actual Behavior: Additional error CS9202: Feature 'ref struct interfaces' is not available in C# 12.0. Please use language version 13.0 or greater.

But there is no ref struct implementing an interface.

[AlekseyTs]

C# 12 disallows applying UnscopedRefAttribute to an interface implementation regardless whether the implemented interface member has UnscopedRefAttribute. The "Ref Struct Interfaces" feature allowed applying the attribute to such a member in a struct (https://github.com/dotnet/csharplang/blob/main/proposals/csharp-13.0/ref-struct-interfaces.md#ref-struct-interfaces-1):

When a struct / ref struct member implements an interface member with a [UnscopedRef] attribute, the implementing member may also be decorated with [UnscopedRef] but it is not required.

Hence the language feature check (regardless whether the implementing struct is a ref struct) and the additional error.

The language version check must be performed even when the implemented member has UnscopedRefAttribute, because a different language version can be in effect when it is declared.