Proposal: Allow Func<T, T, bool> where IEqualityComparer<T> is expected

[dersia]

I saw a Api design in the runtime repo, where there are bunch of methods on MemoryExtensions are receiving an IEqualityComparer<T> overload so that T can be unconstrained on those methods.

this made me think, if it would be a good idea to allow Func<T, T, bool> to be used as parameters where an IEqualityComparer<T> is expected. This would allow a scenario where for

public static bool Contains<T>(this ReadOnlySpan<T> span, T value, IEqualityComparer<T>? comparer = null);

the following would be allowed:

var c = [1, 2, 3, 4, 5];

c.Contains(5, (a, b) => a == b);

(I know what this is a very simple example and can be solved differently, but I hope the idea is clear.)

this would be implemented by emitting a anonymous class that implements the interface and uses the Func<T, T, bool> in the equals method.
this would mean, that I don't have to create comparer classes just for those use cases, where I am not reusing the comparer for anything else.

I am not sure, if I am the only one with this issue, but I guess, this would simplify a lot of codebases.

[HaloFour]

See: #2517

[ufcpp]

And #728

[TahirAhmadov]

That interface has Equals and GetHashCode, does the language have to run around and look for matching signatures? What if there are 2 methods with identical signatures? I think this is a bad idea.
Declaring a class like DelegateEqualityComparer<T> is trivial (and it's probably already available in some general purpose helper Nuget).

[Trarizon]

Actually it exists in BCL. EqualityComparer.Create

[dersia]

I wasn't aware of IEqualityComparer.Create. thanks for that.

@TahirAhmadov for some reason it slipped my mind to think of a general compare class, that could be used that way. thanks for pointing at the obvious solution. 🤦