Merge pull request #5363 from dotty-staging/fix/5351

Fix #5351: Warn about implicit conversions using ImplicitConverter

Author: odersky

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

@@ -607,15 +607,24 @@ trait Checking {
   /** If `sym` is an implicit conversion, check that implicit conversions are enabled.
    *  @pre  sym.is(Implicit)
    */
-  def checkImplicitConversionDefOK(sym: Symbol)(implicit ctx: Context): Unit = sym.info.stripPoly match {
-    case mt @ MethodType(_ :: Nil)
-    if !mt.isImplicitMethod && !sym.is(Synthetic) => // it's a conversion
+  def checkImplicitConversionDefOK(sym: Symbol)(implicit ctx: Context): Unit = {
+    def check(): Unit = {
       checkFeature(
         defn.LanguageModuleClass, nme.implicitConversions,
         i"Definition of implicit conversion $sym",
         ctx.owner.topLevelClass,
         sym.pos)
-    case _ =>
+    }
+
+    sym.info.stripPoly match {
+      case mt @ MethodType(_ :: Nil)
+      if !mt.isImplicitMethod && !sym.is(Synthetic) => // it's a conversion
+        check()
+      case AppliedType(tycon, _)
+      if tycon.derivesFrom(defn.Predef_ImplicitConverter) && !sym.is(Synthetic) =>
+        check()
+      case _ =>
+    }
   }
 
   /** If `sym` is an implicit conversion, check that that implicit conversions are enabled, unless

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

@@ -1419,6 +1419,7 @@ class Typer extends Namer
   def typedValDef(vdef: untpd.ValDef, sym: Symbol)(implicit ctx: Context): Tree = track("typedValDef") {
     val ValDef(name, tpt, _) = vdef
     completeAnnotations(vdef, sym)
+    if (sym is Implicit) checkImplicitConversionDefOK(sym)
     val tpt1 = checkSimpleKinded(typedType(tpt))
     val rhs1 = vdef.rhs match {
       case rhs @ Ident(nme.WILDCARD) => rhs withType tpt1.tpe

tests/neg-custom-args/impl-conv/A.scala

@@ -3,6 +3,7 @@ package implConv
 object A {
 
   implicit def s2i(x: String): Int = Integer.parseInt(x) // error: feature
+  implicit val i2s: ImplicitConverter[Int, String] = _.toString // error: feature
 
   implicit class Foo(x: String) {
     def foo = x.length

tests/neg-custom-args/impl-conv/B.scala

@@ -6,5 +6,6 @@ object B {
   "".foo
 
   val x: Int = ""  // error: feature
+  val y: String = 1 // error: feature
 
 }