A strawman for aggregate literals

A test case that shows that we can have an "inline type class" that allows
to use a typeclass-based scheme for sequence literals where instances can
be created with macros.

Author: odersky

tests/pos/seqlits.check

@@ -0,0 +1,6 @@
+last was evaluated
+last was evaluated
+Seq List(1, 2, 3, 4)
+Vector Vector(1, 2, 3, 4)
+last was evaluated
+Task with elems List(1, 2, 3, 4)

tests/pos/seqlits.scala

@@ -0,0 +1,41 @@
+import reflect.ClassTag
+
+/** Some delayed computation like a Mill Task */
+case class Task[T](body: () => T)
+
+object SeqLits:
+
+  /** A typeclass to map sequence literals with `T` elements
+   *  to some collection type `C`.
+   */
+  trait FromArray[T, +C]:
+    inline def fromArray(inline xs: IArray[T]): C
+
+  /** An emulation of a sequence literal like [1, 2, 3]. Since we don't have that
+   *  syntax yet, we express it here as `seqLit(1, 2, 3)`.
+   */
+  inline def seqLit[T: ClassTag, C](inline xs: T*)(using inline f: FromArray[T, C]): C =
+    f.fromArray(IArray(xs*))
+
+  /** Straightfoward mapping to Seq */
+  given [T] => FromArray[T, Seq[T]]:
+    inline def fromArray(inline xs: IArray[T]) = Seq(xs*)
+
+  /** A more specific mapping to Vector */
+  given [T] => FromArray[T, Vector[T]]:
+    inline def fromArray(inline xs: IArray[T]) = Vector(xs*)
+
+  /** A delaying mapping to Task */
+  given [T] => FromArray[T, Task[Seq[T]]]:
+    inline def fromArray(inline xs: IArray[T]) = Task(() => Seq(xs*))
+
+  def last: Int = { println("last was evaluated"); 4 }
+
+  val s: Seq[Int] = seqLit(1, 2, 3, last)
+  val v: Vector[Int] = seqLit(1, 2, 3, last)
+  val t: Task[Seq[Int]] = seqLit(1, 2, 3, last)
+
+   @main def Test =
+    println(s"Seq $s")
+    println(s"Vector $v")
+    println(s"${t.getClass.getSimpleName} with elems ${t.body()}")