Overload resolution prefers Span<T> over ReadOnlySpan<T> for collection expression

[cston]

Version Used:

Steps to Reproduce:

Compile and run:

using System;

class Program
{
    static void Main()
    {
        M([1, 2, 3]);
    }
    
    static void M(Span<int> span) { Console.WriteLine("Span<int>"); }
    static void M(ReadOnlySpan<int> span) { Console.WriteLine("ReadOnlySpan<int>"); }
}

See sharplab.io

Expected Behavior:
ReadOnlySpan<int>

Actual Behavior:
Span<int>

Reported by @stephentoub

[CyrusNajmabadi]

Isn't span the more specific type, since it can be converted to ROS?

[jnm2]

We crossed the bridge of saying that some things are more important than "which is more specific" when we preferred ReadOnlySpan<T> over T[]. In ReadOnlySpan<T> vs T[], performance was more important.

In the case of ReadOnlySpan<T> over Span<T>, performance could again be more important than specificity; assembly data can be used with ReadOnlySpan and not Span.