Consider extension methods in Space isSameUnapply

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -516,10 +516,14 @@ object SpaceEngine {
    *  We assume that unapply methods are pure, but the same method may
    *  be called with different prefixes, thus behaving differently.
    */
-  def isSameUnapply(tp1: TermRef, tp2: TermRef)(using Context): Boolean =
+  def isSameUnapply(tp1: TermRef, tp2: TermRef)(using Context): Boolean = trace(i"isSameUnapply($tp1, $tp2)") {
+    def isStable(tp: TermRef) =
+      !tp.symbol.is(ExtensionMethod) // The "prefix" of an extension method may be, but the receiver isn't, so exclude
+      && tp.prefix.isStable
     // always assume two TypeTest[S, T].unapply are the same if they are equal in types
-    (tp1.prefix.isStable && tp2.prefix.isStable || tp1.symbol == defn.TypeTest_unapply)
+    (isStable(tp1) && isStable(tp2) || tp1.symbol == defn.TypeTest_unapply)
     && tp1 =:= tp2
+  }
 
   /** Return term parameter types of the extractor `unapp`.
    *  Parameter types of the case class type `tp`. Adapted from `unapplyPlan` in patternMatcher  */

tests/pos/i18601.scala

@@ -0,0 +1,19 @@
+//> using options -Werror
+extension (sc: StringContext)
+  def m: StringContext = sc
+  def unapply(string: String): Option[String] =
+    val pattern = sc.parts.head
+    if string.length == pattern.length then Some(string) else None
+
+class Test:
+  def parse(x: PartialFunction[String, String]) = x
+
+  val pf = parse {
+    case m"x$s"  => s
+    case m"xx$s" => s // was: unreachable
+  }
+
+  // proof that the second case isn't unreachable (matches "ab")
+  def t1 = pf.applyOrElse("a", _ => ".") // "a"
+  def t2 = pf.applyOrElse("ab", _ => ".") // "ab"
+  def t3 = pf.applyOrElse("abc", _ => ".") // "."

tests/pos/i18601b.scala

@@ -0,0 +1,26 @@
+//> using options -Werror
+
+// like pos/i18601
+// but with a dedicated SC class
+// that made the false positive redundancy warning go away
+
+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 then Some(string) else None
+
+class Test:
+  def parse(x: PartialFunction[String, String]) = x
+
+  val pf = parse {
+    case m"x$s"  => s
+    case m"xx$s" => s // was: not unreachable (as a counter-example)
+  }
+
+  // proof that the second case isn't unreachable (matches "ab")
+  def t1 = pf.applyOrElse("a", _ => ".") // "a"
+  def t2 = pf.applyOrElse("ab", _ => ".") // "ab"
+  def t3 = pf.applyOrElse("abc", _ => ".") // "."