Address review comments

 - Move $throws to scala.runtime
 - Add comment

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1261,13 +1261,15 @@ object desugar {
       makeOp(left, right, Span(left.span.start, op.span.end, op.span.start))
   }
 
-  /** Translate throws type `A throws E1 | ... | En`
+  /** Translate throws type `A throws E1 | ... | En` to
+   *  $throws[... $throws[A, E1] ... , En].
    */
   def throws(tpt: Tree, op: Ident, excepts: Tree)(using Context): AppliedTypeTree = excepts match
     case InfixOp(l, bar @ Ident(tpnme.raw.BAR), r) =>
       throws(throws(tpt, op, l), bar, r)
     case e =>
-      AppliedTypeTree(cpy.Ident(op)(tpnme.THROWS), tpt :: excepts :: Nil)
+      AppliedTypeTree(
+        TypeTree(defn.throwsAlias.typeRef).withSpan(op.span), tpt :: excepts :: Nil)
 
   /** Translate tuple expressions of arity <= 22
    *

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

@@ -834,6 +834,7 @@ class Definitions {
       CanEqualClass.companionModule.requiredMethod(methodName)
 
   @tu lazy val CanThrowClass: ClassSymbol = requiredClass("scala.CanThrow")
+  @tu lazy val throwsAlias: Symbol = ScalaRuntimePackageVal.requiredType(tpnme.THROWS)
 
   @tu lazy val TypeBoxClass: ClassSymbol = requiredClass("scala.runtime.TypeBox")
     @tu lazy val TypeBox_CAP: TypeSymbol = TypeBoxClass.requiredType(tpnme.CAP)

library/src-bootstrapped/scala/CanThrow.scala

@@ -10,15 +10,7 @@ import annotation.{implicitNotFound, experimental}
 @implicitNotFound("The capability to throw exception ${E} is missing.\nThe capability can be provided by one of the following:\n - A using clause `(using CanThrow[${E}])`\n - A `throws` clause in a result type such as `X throws ${E}`\n - an enclosing `try` that catches ${E}")
 erased class CanThrow[-E <: Exception]
 
-/** A helper type to allow syntax like
- *
- *    def f(): T throws Ex1 | Ex2
- *
- *  Used in desugar.throws.
- */
-@experimental
-infix type $throws[R, +E <: Exception] = CanThrow[E] ?=> R
-
 @experimental
 object unsafeExceptions:
   given canThrowAny: CanThrow[Exception] = ???
+

library/src-bootstrapped/scala/runtime/$throws.scala

@@ -0,0 +1,11 @@
+package scala.runtime
+import annotation.experimental
+
+/** A helper type to allow syntax like
+ *
+ *    def f(): T throws Ex1 | Ex2
+ *
+ *  Used in desugar.throws.
+ */
+@experimental
+infix type $throws[R, +E <: Exception] = CanThrow[E] ?=> R