Regression: "Unreachable case" warning in pattern match on `StringContext`

[sirthias]

Compiler version

3.3.1

Minimized code

extension (sc: StringContext)
  def m: StringContext = sc
  def unapply(string: String): Option[String] =
    val pattern = sc.parts.head
    if (string.length == pattern.length) Some(string) else None

def parse(x: PartialFunction[String, String]) = x

val pf = parse {
  case m"x$s" => s
  case m"xx$s" => s
}

pf.applyOrElse("a", _ => ".") // "a"
pf.applyOrElse("ab", _ => ".") // "ab"
pf.applyOrElse("abc", _ => ".") // "."

Output

-- [E030] Match case Unreachable Warning: --------------------------------------
17 |  case m"xx$s" => s
   |       ^^^^^^^
   |       Unreachable case

Expectation

No warning, as with compiler version 3.3.0

Notes

There is no regression (and no warning) here:

extension (sc: StringContext) def m: SC = SC(sc)

class SC(sc: StringContext):
  def unapply(string: String): Option[String] =
    val pattern = sc.parts.head
    if (string.length == pattern.length) Some(string) else None

def parse(x: PartialFunction[String, String]) = x

val pf = parse {
  case m"x$s" => s
  case m"xx$s" => s
}

pf.applyOrElse("a", _ => ".") // "a"
pf.applyOrElse("ab", _ => ".") // "ab"
pf.applyOrElse("abc", _ => ".") // "."

[scala-center-bot]

This issue was picked for the Issue Spree No. 37 of 03 October 2023 which takes place in 7 days. @dwijnand, @KuceraMartin, @natsukagami, @yilinwei will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.

[sirthias]

Awesome! Thank you, @dwijnand ! 👍