As part of #27229, we want all the relevant comparers for strings to implement IAlternateEqualityComparer<ReadOnlySpan<char>, string>. The initial implementation does so for all of the StringComparer singletons, but not for EqualityComparer<string>.Default. This means that the alternate lookup support for spans on dictionaries/sets will only work when those collections are explicitly given a supported comparer. This issue tracks implementing the interface on the default comparer for string, as well.