Add regression tests (#16853)

Closes #16842

Author: nicolasstucki

tests/neg/i16842.scala

@@ -0,0 +1,25 @@
+sealed trait Expr1
+sealed trait Literal extends Expr1
+
+case class ArrayLiter(elems: List[Expr1]) extends Literal
+
+sealed trait SemanticType {
+  type T // the type with which a literal of this semanticType is represented
+}
+case object SemanticInt extends SemanticType {
+  type T = Int
+}
+
+case class SemanticArray[U <: SemanticType](dim: Int) extends SemanticType {
+  type T = List[U]
+}
+
+sealed trait Expr2[+T]
+class Liter[T <: SemanticType](val ty: T, val value: ty.T) extends Expr2[T]
+
+def typecheckArrayLiter(
+    a: ArrayLiter
+): Liter[SemanticArray[SemanticType]] = {
+  val x: List[Expr2[SemanticInt.type]] = List()
+  Liter(SemanticArray[SemanticInt.type], x) // error // error
+}