Merge pull request #4896 from dotty-staging/fix-#4891

Make subclasses of scala.quoted.Expr covariant

Author: Blaisorblade

library/src/scala/quoted/Expr.scala

@@ -31,14 +31,14 @@ object Expr {
  */
 object Exprs {
   /** An Expr backed by a pickled TASTY tree */
-  final class TastyExpr[T](val tasty: Pickled, val args: Seq[Any]) extends Expr[T] {
+  final class TastyExpr[+T](val tasty: Pickled, val args: Seq[Any]) extends Expr[T] {
     override def toString: String = s"Expr(<pickled tasty>)"
   }
 
   /** An Expr backed by a lifted value.
    *  Values can only be of type Boolean, Byte, Short, Char, Int, Long, Float, Double, Unit, String or Null.
    */
-  final class LiftedExpr[T](val value: T) extends Expr[T] {
+  final class LiftedExpr[+T](val value: T) extends Expr[T] {
     override def toString: String = s"Expr($value)"
   }
 
@@ -54,7 +54,7 @@ object Exprs {
   }
 
   /** An Expr representing `'{(~f).apply(~x)}` but it is beta-reduced when the closure is known */
-  final class FunctionAppliedTo[T, U](val f: Expr[T => U], val x: Expr[T]) extends Expr[U] {
+  final class FunctionAppliedTo[T, +U](val f: Expr[T => U], val x: Expr[T]) extends Expr[U] {
     override def toString: String = s"Expr($f <applied to> $x)"
   }
 }