Remove empty argument lists for classes with only context bounds

Closes #21418

compiler/src/dotty/tools/dotc/typer/Migrations.scala

@@ -113,6 +113,12 @@ trait Migrations:
         em"""Context bounds will map to context parameters.
             |A `using` clause is needed to pass explicit arguments to them.$rewriteMsg""",
         tree.srcPos, mversion)
+      tree match
+        case Apply(ta @ TypeApply(Select(New(_), _), _), Nil) =>
+          // Remove empty arguments for calls to new that may precede the context bound.
+          // They are no longer necessary.
+          patch(Span(ta.span.end, pt.args.head.span.start - 1), "")
+        case _ => ()
       if mversion.needsPatch && pt.args.nonEmpty then
         patch(Span(pt.args.head.span.start), "using ")
   end contextBoundParams

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -77,6 +77,7 @@ class CompilationTests {
       compileFile("tests/rewrites/i17399.scala", unindentOptions.and("-rewrite")),
       compileFile("tests/rewrites/i20002.scala", defaultOptions.and("-indent", "-rewrite")),
       compileDir("tests/rewrites/annotation-named-pararamters", defaultOptions.and("-rewrite", "-source:3.6-migration")),
+      compileFile("tests/rewrites/i21418.scala", unindentOptions.and("-rewrite", "-source:3.5-migration")),
     ).checkRewrites()
   }
 

tests/rewrites/i21418.check

@@ -0,0 +1,23 @@
+trait Effect[F[_]]
+class Countdown[F[_]: Effect]
+class Countdown1[F[_]: Effect](howMany: Int)
+class Countdown2[F[_]: Effect, F2[_]: Effect]
+
+def foo[F[_]: Effect]() =
+  "foo"
+
+@main def Test = {
+  val a = new Countdown[Option](using ???)
+  val b = Countdown[Option](using ???)
+  new Countdown[Option](using ???)
+  val c = Countdown[List](using ???)
+  new Countdown2[List, Option](using ???, ???)
+  Countdown2[List, Option](using ???, ???)
+  new Countdown1[Option](10)(using ???)
+  new Array[Int](10)
+  new scala.collection.immutable.HashSet[Int]
+  new scala.collection.immutable.HashSet[Int]()
+  new scala.collection.immutable.HashSet[Int] ()
+  foo()(using ???)
+  foo() (using ???)
+}

tests/rewrites/i21418.scala

@@ -0,0 +1,23 @@
+trait Effect[F[_]]
+class Countdown[F[_]: Effect]
+class Countdown1[F[_]: Effect](howMany: Int)
+class Countdown2[F[_]: Effect, F2[_]: Effect]
+
+def foo[F[_]: Effect]() =
+  "foo"
+
+@main def Test = {
+  val a = new Countdown[Option]()(???)
+  val b = Countdown[Option]()(???)
+  new Countdown[Option] ()(???)
+  val c = Countdown[List] () (???)
+  new Countdown2[List, Option] ()   (???, ???)
+  Countdown2[List, Option] ()   (???, ???)
+  new Countdown1[Option](10)(???)
+  new Array[Int](10)
+  new scala.collection.immutable.HashSet[Int]
+  new scala.collection.immutable.HashSet[Int]()
+  new scala.collection.immutable.HashSet[Int] ()
+  foo()(???)
+  foo() (???)
+}