feat: introduce a static cast to scala.compiletime

Author: hamzaremmal

library/src/scala/compiletime/package.scala

@@ -193,3 +193,15 @@ def byName[T](x: => T): T = x
   */
 extension [T](x: T)
   transparent inline def asMatchable: x.type & Matchable = x.asInstanceOf[x.type & Matchable]
+
+extension (inline self: Any)
+  /**
+    * TODO: Write the scaladoc here and how it works
+    * ???
+    * @param self ???
+    * @param T ???
+    * @return ???
+    */
+  transparent inline def as[T]: T = inline self match
+    case self: T => self
+    case _ => error("Cannot statically prove the correctness of the cast")

tests/neg/static-cast.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/static-cast.scala:11:18 ----------------------------------------------------------------------------
+11 |val _ = singletons[B, (A.A1.type, A.A2.type, A.A3.type)] // error
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |        Cannot statically prove the correctness of the cast

tests/neg/static-cast.scala

@@ -0,0 +1,11 @@
+import scala.compiletime.*
+
+trait B
+enum A { case A1, A2, A3 }
+
+private inline def singletons[T, Elem <: Tuple]: Seq[T] =
+  inline erasedValue[Elem] match
+    case _: EmptyTuple => Seq.empty
+    case _: (h *: t) => valueOf[h].as[T] +: singletons[T, t]
+
+val _ = singletons[B, (A.A1.type, A.A2.type, A.A3.type)] // error

tests/pos/static-cast.scala

@@ -0,0 +1,10 @@
+import scala.compiletime.*
+
+enum A { case A1, A2, A3 }
+
+private inline def singletons[T, Elem <: Tuple]: Seq[T] =
+  inline erasedValue[Elem] match
+    case _: EmptyTuple => Seq.empty
+    case _: (h *: t) => valueOf[h].as[T] +: singletons[T, t]
+
+val x = singletons[A, (A.A1.type, A.A2.type, A.A3.type)]