C# compiler chooses inaccessible extension method over accessible one

[LeonidVasilyev]

Version Used:
Microsoft (R) Visual C# Compiler version 2.6.0.62329 (5429b35)

Steps to Reproduce:

1. Create new console application with following code in Program.cs:

using System;
using System.Collections.Generic;
using System.Linq;

public class Program
{
    static void Main(string[] args)
    {
        var a = new[] { 0 };
        var b = new[] { 1, 2, 3 };
        Console.WriteLine(b.Count(a.Contains));
    }
}

public static class Extensions
{
    // NOTE: private access modifier simulates internal class public method in referenced assembly.
    private static bool Contains<T>(this IEnumerable<T> a, T value) =>
        throw new NotImplementedException();
}

2. Build project or wait full solution analysis to complete

Expected Behavior:
Project compiled successfully. System.Linq.Enumerable.Contains() used as an argument of System.Linq.Enumerable.Count() function.

Actual Behavior:
Compiler error CS0122: 'Extensions.Contains(IEnumerable, T)' is inaccessible due to its protection level

Original case is described in C# compiler chooses wrong extension method question on StackOverflow. CS0122 wrong member inaccessible when overload does not exist issue looks related.

[Joe4evr]

Should be something to be improved as part of dotnet/csharplang#98

@gafter It doesn't seem like this is part of those improvements. Is this perhaps a regression? If not, can this still be added as part of the improved overload resolution?

[jcouv]

Assigning to 15.7 milestone for now, we should confirm whether this is a 15.7 regression.
Never mind, the repro is in 15.5 (compiler version 2.6).
I'll let @gafter triage to appropriate milestone.

[gafter]

VS2015 behaves exactly the same way.

[gafter]

Looks like a bug. Inaccessible members should not affect overload resolution.

[gafter]

See https://github.com/dotnet/csharplang/blob/master/spec/expressions.md#extension-method-invocations . In the terminology of the spec, we are letting non-eligible candidates short-circuit finding the right method group.

[gafter]

In review in #26345