Show that this solves scala#10929

Author: odersky

tests/pos/i10929-new-syntax.scala

@@ -0,0 +1,22 @@
+//> using options -language:experimental.modularity -source future
+trait TupleOf[+A]:
+  type Self
+  type Mapped[+A] <: Tuple
+  def map[B](x: Self)(f: A => B): Mapped[B]
+
+object TupleOf:
+
+  given EmptyTuple is TupleOf[Nothing]:
+    type Mapped[+A] = EmptyTuple
+    def map[B](x: EmptyTuple)(f: Nothing => B): Mapped[B] = x
+
+  given [A, Rest <: Tuple : TupleOf[A]] => A *: Rest is TupleOf[A]:
+    type Mapped[+A] = A *: Rest.Mapped[A]
+    def map[B](x: A *: Rest)(f: A => B): Mapped[B] =
+      (f(x.head) *: Rest.map(x.tail)(f))
+
+def foo[T: TupleOf[Int]](xs: T): T.Mapped[Int] = T.map(xs)(_ + 1)
+
+@main def test =
+  foo(EmptyTuple): EmptyTuple // ok
+  foo(1 *: EmptyTuple): Int *: EmptyTuple // now also ok

tests/pos/i10929.scala

@@ -0,0 +1,21 @@
+//> using options -language:experimental.modularity -source future
+infix abstract class TupleOf[T, +A]:
+  type Mapped[+A] <: Tuple
+  def map[B](x: T)(f: A => B): Mapped[B]
+
+object TupleOf:
+
+  given TupleOf[EmptyTuple, Nothing] with
+    type Mapped[+A] = EmptyTuple
+    def map[B](x: EmptyTuple)(f: Nothing => B): Mapped[B] = x
+
+  given [A, Rest <: Tuple](using tracked val tup: Rest TupleOf A): TupleOf[A *: Rest, A] with
+    type Mapped[+A] = A *: tup.Mapped[A]
+    def map[B](x: A *: Rest)(f: A => B): Mapped[B] =
+      (f(x.head) *: tup.map(x.tail)(f))
+
+def foo[T](xs: T)(using tup: T TupleOf Int): tup.Mapped[Int] = tup.map(xs)(_ + 1)
+
+@main def test =
+  foo(EmptyTuple): EmptyTuple // ok
+  foo(1 *: EmptyTuple): Int *: EmptyTuple // now also ok