Add @Covers annotation

Author: liufengyun

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -923,6 +923,7 @@ class Definitions {
   @tu lazy val FunctionalInterfaceAnnot: ClassSymbol = requiredClass("java.lang.FunctionalInterface")
   @tu lazy val TargetNameAnnot: ClassSymbol = requiredClass("scala.annotation.targetName")
   @tu lazy val VarargsAnnot: ClassSymbol = requiredClass("scala.annotation.varargs")
+  @tu lazy val CoversAnnot: ClassSymbol = requiredClass("scala.annotation.covers")
 
   @tu lazy val JavaRepeatableAnnot: ClassSymbol = requiredClass("java.lang.annotation.Repeatable")
 

library/src/scala/annotation/covers.scala

@@ -0,0 +1,19 @@
+package scala.annotation
+
+/** An annotation specifying that an extractor is irrefutable
+ *  if the scrutinee is a subtype of `T`.
+ *
+ *  For example, the extractor `:+` covers non-empty list (`::[T]`):
+ *
+ *      object :+ {
+ *        def unapply[T](l: List[T]): Option[(List[T], T)] @covers[::[T]] = ...
+ *      }
+ *
+ *      def f(xs: List[Int]) =
+ *        xs match
+ *        case init :+ last => ()
+ *        case Nil => ()
+ *
+ *  Therefore, the pattern match above is exhaustive.
+ */
+final class covers[T] extends StaticAnnotation