Fix #9408: Warn on implicit view resolved with -source:3.0-migration

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

@@ -779,7 +779,27 @@ trait Implicits:
           SelectionProto(name, memberProto, compat, privateOK = false)
         case tp => tp
       }
-      try inferImplicit(adjust(to), from, from.span)
+
+      def isOldStyleFunctionConversion(tpe: Type): Boolean =
+        tpe match {
+          case PolyType(_, resType) => isOldStyleFunctionConversion(resType)
+          case _ => tpe.derivesFrom(defn.FunctionClass(1)) && !tpe.derivesFrom(defn.ConversionClass) && !tpe.derivesFrom(defn.SubTypeClass)
+        }
+
+      try
+        val inferred = inferImplicit(adjust(to), from, from.span)
+
+        inferred match {
+          case SearchSuccess(_, ref, _) =>
+            if isOldStyleFunctionConversion(ref.underlying) then
+              report.migrationWarning(
+                i"The conversion ${ref} will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.",
+                from
+              )
+          case _ =>
+        }
+
+        inferred
       catch {
         case ex: AssertionError =>
           implicits.println(s"view $from ==> $to")

tests/neg-custom-args/fatal-warnings/i9408a.check

@@ -0,0 +1,24 @@
+-- Error: tests/neg-custom-args/fatal-warnings/i9408a.scala:16:20 ------------------------------------------------------
+16 |  val length: Int = "qwerty" // error
+   |                    ^^^^^^^^
+   |The conversion (Test3.implicitLength : String => Int) will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.
+-- Error: tests/neg-custom-args/fatal-warnings/i9408a.scala:21:20 ------------------------------------------------------
+21 |  val length: Int = "qwerty" // error
+   |                    ^^^^^^^^
+   |The conversion (Test4.implicitLength : => String => Int) will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.
+-- Error: tests/neg-custom-args/fatal-warnings/i9408a.scala:26:20 ------------------------------------------------------
+26 |  val length: Int = "qwerty" // error
+   |                    ^^^^^^^^
+   |The conversion (Test5.implicitLength : [A] => String => Int) will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.
+-- Error: tests/neg-custom-args/fatal-warnings/i9408a.scala:31:20 ------------------------------------------------------
+31 |  val length: Int = "qwerty" // error
+   |                    ^^^^^^^^
+   |The conversion (Test6.implicitLength : Map[String, Int]) will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.
+-- Error: tests/neg-custom-args/fatal-warnings/i9408a.scala:35:60 ------------------------------------------------------
+35 |  implicit def a2int[A](a: A)(implicit ev: A => Int): Int = a // error
+   |                                                            ^
+   |The conversion (ev : A => Int) will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.
+-- Error: tests/neg-custom-args/fatal-warnings/i9408a.scala:59:2 -------------------------------------------------------
+59 |  123.foo // error
+   |  ^^^
+   |The conversion (Test11.a2foo : [A] => A => Test11.Foo) will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.

tests/neg-custom-args/fatal-warnings/i9408a.scala

@@ -0,0 +1,86 @@
+import language.`3.0-migration`
+import scala.language.implicitConversions
+
+object Test1 {
+  implicit def implicitLength(s: String): Int = s.length
+  val length: Int = "qwerty" // ok
+}
+
+object Test2 {
+  implicit val implicitLength: Conversion[String, Int] = _.length
+  val length: Int = "qwerty" // ok
+}
+
+object Test3 {
+  implicit val implicitLength: String => Int = _.length
+  val length: Int = "qwerty" // error
+}
+
+object Test4 {
+  implicit def implicitLength: String => Int = _.length
+  val length: Int = "qwerty" // error
+}
+
+object Test5 {
+  implicit def implicitLength[A]: String => Int = _.length
+  val length: Int = "qwerty" // error
+}
+
+object Test6 {
+  implicit val implicitLength: Map[String, Int] = Map("qwerty" -> 6)
+  val length: Int = "qwerty" // error
+}
+
+object Test7 {
+  implicit def a2int[A](a: A)(implicit ev: A => Int): Int = a // error
+}
+
+object Test8 {
+  implicit def a2int[A](a: A)(implicit ev: A => Int): Int = ev(a) // ok
+}
+
+object Test9 {
+  implicit def a2int[A](a: A)(implicit ev: A <:< Int): Int = a // ok
+}
+
+object Test10 {
+  trait Foo {
+    def foo = "foo"
+  }
+  implicit def a2foo[A](a: A): Foo = new Foo {}
+  123.foo // ok
+}
+
+object Test11 {
+  trait Foo {
+    def foo = "foo"
+  }
+  implicit def a2foo[A]: A => Foo = _ => new Foo {}
+  123.foo // error
+}
+
+object Test12 {
+  implicit class FooOps(a: Any) {
+    def foo = "foo"
+  }
+  123.foo // ok
+}
+
+object Test13 {
+  def foo()(implicit x: String => Int) = ???
+  implicit val f: String => Int = _.size
+  foo()  // ok
+}
+
+object Test14 {
+  case class MySeq[A](underlying: Seq[A])
+
+  implicit def mySeq2seq[A](mySeq: MySeq[A]): Seq[A] = mySeq.underlying
+  val s: Seq[Int] = MySeq(Seq(1, 2, 3)) // ok
+}
+
+object Test15 {
+  implicit def implicitSeq[A]: Seq[A] = ???
+  def foo(implicit ev: Seq[Int]): Unit = ???
+  foo
+}

tests/neg-custom-args/fatal-warnings/i9408b.check

@@ -0,0 +1,5 @@
+
+-- Error: tests/neg-custom-args/fatal-warnings/i9408b/Test_2.scala:6:20 ------------------------------------------------
+6 |  val length: Int = "abc" // error
+  |                    ^^^^^
+  |The conversion (test.conversions.Conv.implicitLength : String => Int) will not be applied implicitly here in Scala 3 because only implicit methods and instances of Conversion class will continue to work as implicit views.

tests/neg-custom-args/fatal-warnings/i9408b/Conv_1.scala

@@ -0,0 +1,5 @@
+package test.conversions
+
+object Conv {
+  implicit val implicitLength: String => Int = _.length
+}

tests/neg-custom-args/fatal-warnings/i9408b/Test_2.scala

@@ -0,0 +1,7 @@
+import language.`3.0-migration`
+import scala.language.implicitConversions
+
+object Test {
+  import test.conversions.Conv._
+  val length: Int = "abc" // error
+}