Why does C# call a 'less better' overload?

[AArnott]

Consider this example:

using System;

ReadOnlyMemory<char> memory = new Char[] { 'a' };
Console.WriteLine("a".Equals(memory));
Console.WriteLine(Extensions.Equals("a", memory));

static class Extensions{
    internal static bool Equals(this string value, ReadOnlyMemory<char> memory) => true;
}

This produces "False, True". Why? Because the C# compiler binds the "a".Equals method call to string.Equals(object?) instead of the Extensions.Equals(this string, ReadOnlyMemory<char>) extension method. Why? The extension method clearly matches the type better.

Sharplab

[HaloFour]

Extensions aren't overloads, they're only considered if there are no instance members that match, even if they require type coercion.

[bernd5]

See: 11.7.8.3 Extension method invocations

if the normal processing of the invocation finds no applicable methods, an attempt is made to process the construct as an extension method invocation. If «expr» or any of the «args» has compile-time type dynamic, extension methods will not apply.

[AArnott]

Thank you for your spec-based answer.