From d1cb9deb7002bdaef988989aeed00ff5c9113286 Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Thu, 26 Aug 2021 12:41:48 +0200
Subject: [PATCH] Address review comments

 - Move $throws to scala.runtime
 - Add comment
---
 compiler/src/dotty/tools/dotc/ast/Desugar.scala      |  6 ++++--
 compiler/src/dotty/tools/dotc/core/Definitions.scala |  1 +
 library/src-bootstrapped/scala/CanThrow.scala        | 10 +---------
 library/src-bootstrapped/scala/runtime/$throws.scala | 11 +++++++++++
 4 files changed, 17 insertions(+), 11 deletions(-)
 create mode 100644 library/src-bootstrapped/scala/runtime/$throws.scala

diff --git a/compiler/src/dotty/tools/dotc/ast/Desugar.scala b/compiler/src/dotty/tools/dotc/ast/Desugar.scala
index c4761ec87c13..acd81f0ac7ae 100644
--- a/compiler/src/dotty/tools/dotc/ast/Desugar.scala
+++ b/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
    *
diff --git a/compiler/src/dotty/tools/dotc/core/Definitions.scala b/compiler/src/dotty/tools/dotc/core/Definitions.scala
index e94681e7a5ef..886c9d9a5559 100644
--- a/compiler/src/dotty/tools/dotc/core/Definitions.scala
+++ b/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)
diff --git a/library/src-bootstrapped/scala/CanThrow.scala b/library/src-bootstrapped/scala/CanThrow.scala
index ce518bff19ec..4da00b8a76ac 100644
--- a/library/src-bootstrapped/scala/CanThrow.scala
+++ b/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 $throws[R, +E <: Exception] = CanThrow[E] ?=> R
-
 @experimental
 object unsafeExceptions:
   given canThrowAny: CanThrow[Exception] = ???
+
diff --git a/library/src-bootstrapped/scala/runtime/$throws.scala b/library/src-bootstrapped/scala/runtime/$throws.scala
new file mode 100644
index 000000000000..af35f7402ca3
--- /dev/null
+++ b/library/src-bootstrapped/scala/runtime/$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