Unicode Case Folding

While working on cats-uri, I ran into an issue with how CIString was handling certain unicode values which led me to notice it wasn't respecting Caseless matching from the Unicode standard. As it turns out, neither does String.equalsIgnoreCase.

I'd just about completed a branch to implement full case folding as defined by the Unicode standard when I ran across this test.

  test("character based equality") {
    assert(CIString("ß") != CIString("SS"))
  }

Since under the Unicode standard's caseless matching these two strings would compare equal, I'm beginning to think we are intentionally not following the standard here. Is that the case? If so, why? Is it to maintain parity with what the Java standard library is doing with methods like equalsIgnoresCase?

Start with tests/shared/src/test/scala/org/typelevel/ci/CIStringSuite.scala, especially the character-based equality test, and read the linked Unicode caseless-matching section. Compare the existing behavior with the full-unicode-case-folding branch mentioned in the issue. Done means the project’s intended case-folding semantics are decided and the relevant tests and implementation consistently reflect that decision.