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