Teach SpaceEngine that generic tuples are irrefutable

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

@@ -306,6 +306,7 @@ object SpaceEngine {
       val isEmptyTp = extractorMemberType(unappResult, nme.isEmpty, NoSourcePosition)
       isEmptyTp <:< ConstantType(Constant(false))
     }
+    || unappResult.derivesFrom(defn.NonEmptyTupleClass)
   }
 
   /** Is the unapply or unapplySeq irrefutable?

tests/neg/i15991.abstract.scala

@@ -0,0 +1,20 @@
+object Foo:
+  def unapply[T <: Tuple](tup: T): String *: String *: T =
+    "a" *: "b" *: tup
+
+// like {pos,neg}/i15991, but with an abstract tuple tail
+class Test:
+  val tup2: String *: String *: EmptyTuple = ("c", "d")
+
+  def test3 =
+    val Foo(x, y, z) = tup2 // error: Wrong number of argument patterns for Foo; expected: (String, String, String, String)
+    x + y + z
+
+  def test3a =
+    val x1x = tup2 match
+      case Foo(x, y, z) => // error: Wrong number of argument patterns for Foo; expected: (String, String, String, String)
+        (x, y, z)
+    val x = x1x._1
+    val y = x1x._2
+    val z = x1x._3
+    x + y + z

tests/neg/i15991.scala

@@ -0,0 +1,7 @@
+object Foo:
+  def unapply(x: Any): String *: String *: EmptyTuple = ("a", "b")
+
+class Test:
+  def test =
+    val Foo(x, y, z) = 1 // error: Wrong number of argument patterns for Foo; expected: (String, String)
+    x + y + z

tests/pos/i15991.abstract.scala

@@ -0,0 +1,20 @@
+object Foo:
+  def unapply[T <: Tuple](tup: T): String *: String *: T =
+    "a" *: "b" *: tup
+
+// like {pos,neg}/i15991, but with an abstract tuple tail
+class Test:
+  val tup2: String *: String *: EmptyTuple = ("c", "d")
+
+  def test2 =
+    val Foo(x, y, _, _) = tup2
+    x + y
+
+  // like test2, but as the desugaring of what PatternDef's become
+  def test2b =
+    val x1x = tup2 match
+      case Foo(x, y, _, _) =>
+        (x, y)
+    val x = x1x._1
+    val y = x1x._2
+    x + y

tests/pos/i15991.orig.scala

@@ -0,0 +1,9 @@
+class Foo
+
+object Foo:
+//  def unapply(f: Foo): (Int, Int) = ???  // does not raise a warning
+  def unapply(f: Foo): Int *: Int *: EmptyTuple = ???
+
+@main def example =
+  val Foo(x, y) = new Foo
+  println(x)

tests/pos/i15991.scala

@@ -0,0 +1,8 @@
+object Foo:
+  def unapply(x: Any): String *: String *: EmptyTuple =
+    ("a", "b")
+
+class Test:
+  def test =
+    val Foo(x, y) = 1
+    x + y