Consider extension methods in Space isSameUnapply

Author: dwijnand

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  */