Postconditions on user-defined conversion parameters are not respected

[RikkiGibson]

Version Used: dc6f316

Steps to Reproduce:

#nullable enable

using System.Diagnostics.CodeAnalysis;

public static class Program
{
    public static void M1(string? s)
    {
        AssertNotNull(s);
        s.ToString();
    }

    public static void M2(string? s)
    {
        C c = s;
        s.ToString(); // 1
    }

    public static void AssertNotNull([AllowNull, NotNull] string s) { throw null!; }

    public class C
    {
        public static implicit operator C([AllowNull, NotNull] string s) { throw null!; }
    }
}

SharpLab

Expected Behavior: No diagnostics

Actual Behavior: Diagnostic indicated by // 1.