Is the below behavior related to this issue? e.g. a pattern based binding of collection expression doesn't seem to handle a generic extension Add in the way I expected.

    [Fact]
    public void Nullability_CollectionExpression_Add_02()
    {
        var src = """
#nullable enable
using System.Collections;
using System.Collections.Generic;

object? oNull = null;
object oNotNull = new object();
MyCollection<object> c = [oNull, oNotNull];

static class E
{
    extension<T>(MyCollection<T> c)
    {
        public void Add(T o) { }
    }
}

public class MyCollection<T> : IEnumerable<T>
{
    IEnumerator<T> IEnumerable<T>.GetEnumerator() => throw null!;
    IEnumerator IEnumerable.GetEnumerator() => throw null!;
}
""";
        //
        var comp = CreateCompilation(src);
        comp.VerifyEmitDiagnostics(
                // (7,26): error CS9215: Collection expression type 'MyCollection<object>' must have an instance or extension method 'Add' that can be called with a single argument.
                // MyCollection<object> c = [oNull, oNotNull];
                Diagnostic(ErrorCode.ERR_CollectionExpressionMissingAdd, "[oNull, oNotNull]").WithArguments("MyCollection<object>").WithLocation(7, 26),
                // (7,26): error CS1929: 'MyCollection<object>' does not contain a definition for 'Add' and the best extension method overload 'E.extension<T>(MyCollection<T>).Add(T)' requires a receiver of type 'MyCollection<T>'
                // MyCollection<object> c = [oNull, oNotNull];
                Diagnostic(ErrorCode.ERR_BadInstanceArgType, "[oNull, oNotNull]").WithArguments("MyCollection<object>", "Add", "E.extension<T>(MyCollection<T>).Add(T)", "MyCollection<T>").WithLocation(7, 26));
    }

Extensions: follow-up on remaining pattern-based binding scenarios #78960

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