Review/7629 deprecate view bounds

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -2258,7 +2258,15 @@ self =>
         if (contextBoundBuf ne null) {
           while (in.token == VIEWBOUND) {
             contextBoundBuf += atPos(in.skipToken()) {
-              makeFunctionTypeTree(List(Ident(pname)), typ())
+              val t = typ()
+              def discourage() = {
+                val msg = "Use an implicit parameter instead."
+                val eg = s"Instead of `def $owner[$pname <% $t](p: $pname)`, use `def $owner(p: $pname)(implicit ev: $pname => $t)`."
+                if (settings.future) syntaxError(in.offset, f"View bounds have been removed. $msg%n$eg")
+                else deprecationWarning(in.offset, f"View bounds are deprecated. $msg%n$eg")
+              }
+              discourage()
+              makeFunctionTypeTree(List(Ident(pname)), t)
             }
           }
           while (in.token == COLON) {
@@ -2690,7 +2698,8 @@ self =>
           classContextBounds = contextBoundBuf.toList
           val tstart = (in.offset :: classContextBounds.map(_.pos.startOrPoint)).min
           if (!classContextBounds.isEmpty && mods.isTrait) {
-            syntaxError("traits cannot have type parameters with context bounds `: ...' nor view bounds `<% ...'", skipIt = false)
+            val viewBoundsExist = if (settings.future) "" else " nor view bounds `<% ...'"
+              syntaxError(s"traits cannot have type parameters with context bounds `: ...'$viewBoundsExist", skipIt = false)
             classContextBounds = List()
           }
           val constrAnnots = if (!mods.isTrait) constructorAnnotations() else Nil

test/files/run/iterator-from.check

@@ -0,0 +1,8 @@
+iterator-from.scala:14: warning: View bounds are deprecated. Use an implicit parameter instead.
+Instead of `def testSet[A <% Ordered[A]](p: A)`, use `def testSet(p: A)(implicit ev: A => Ordered[A])`.
+  def testSet[A <% Ordered[A]](set: SortedSet[A], list: List[A]) {
+                             ^
+iterator-from.scala:27: warning: View bounds are deprecated. Use an implicit parameter instead.
+Instead of `def testMap[A <% Ordered[A]](p: A)`, use `def testMap(p: A)(implicit ev: A => Ordered[A])`.
+  def testMap[A <% Ordered[A], B](map: SortedMap[A, B], list: List[(A, B)]) {
+                             ^

test/files/run/iterator-from.flags

@@ -0,0 +1 @@
+-deprecation

test/files/run/macro-expand-implicit-macro-is-view.check

@@ -1 +1,5 @@
+Macros_Test_2.scala:10: warning: View bounds are deprecated. Use an implicit parameter instead.
+Instead of `def bar[T <% Option[Int]](p: T)`, use `def bar(p: T)(implicit ev: T => Option[Int])`.
+  def bar[T <% Option[Int]](x: T) = println(x)
+                          ^
 2

test/files/run/macro-expand-implicit-macro-is-view.flags

@@ -0,0 +1 @@
+-deprecation