Add is type to Predef

Author: odersky

library/src/scala/runtime/stdLibPatches/Predef.scala

@@ -61,4 +61,16 @@ object Predef:
     inline def ne(inline y: AnyRef | Null): Boolean =
       !(x eq y)
 
+  /** A type supporting Self-based type classes.
+   *
+   *    A is TC
+   *
+   *  expands to
+   *
+   *    TC { type Self = A }
+   *
+   *  which is what is needed for a context bound `[A: TC]`.
+   */
+  infix type is[A <: AnyKind, B <: {type Self <: AnyKind}] = B { type Self = A }
+
 end Predef

tests/pos/parsercombinators-arrow.scala

@@ -1,8 +1,6 @@
 //> using options -language:experimental.modularity -source future
 import collection.mutable
 
-infix type is[A <: AnyKind, B <: {type Self <: AnyKind}] = B { type Self = A }
-
 /// A parser combinator.
 trait Combinator:
 

tests/pos/typeclasses-arrow.scala

@@ -1,8 +1,5 @@
 //> using options -language:experimental.modularity -source future
 
-// this should go in Predef
-infix type is[A <: AnyKind, B <: {type Self <: AnyKind}] = B { type Self = A }
-
 class Common:
 
   trait Ord:

tests/pos/typeclasses-this.scala

@@ -1,8 +1,5 @@
 //> using options -language:experimental.modularity -source future
 
-// this should go in Predef
-infix type is[A <: AnyKind, B <: {type Self <: AnyKind}] = B { type Self = A }
-
 class Common:
 
   trait Ord:

tests/pos/typeclasses.scala

@@ -26,8 +26,6 @@ class Common:
       def flatMap[B](f: A => Self[B]): Self[B]
       def map[B](f: A => B) = x.flatMap(f `andThen` pure)
 
-  infix type is[A <: AnyKind, B <: {type Self <: AnyKind}] = B { type Self = A }
-
 end Common
 
 
@@ -132,7 +130,6 @@ instance Sheep: Animal with
     override def talk(): Unit =
       println(s"$name pauses briefly... $noise")
 */
-import Instances.is
 
 // Implement the `Animal` trait for `Sheep`.
 given (Sheep is Animal) with