Allow selectDynamic and applyDynamic to be extension methods (#17106)

Allow selectDynamic and applyDynamic to be extension methods when
dispatching structurally.

Fixes #17100

Author: odersky

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

@@ -181,12 +181,12 @@ trait Dynamic {
     val vargss = termArgss(tree)
 
     def structuralCall(selectorName: TermName, classOfs: => List[Tree]) = {
-      val selectable = adapt(qual, defn.SelectableClass.typeRef)
+      val selectable = adapt(qual, defn.SelectableClass.typeRef | defn.DynamicClass.typeRef)
 
       // ($qual: Selectable).$selectorName("$name")
       val base =
         untpd.Apply(
-          untpd.TypedSplice(selectable.select(selectorName)).withSpan(fun.span),
+          untpd.Select(untpd.TypedSplice(selectable), selectorName).withSpan(fun.span),
           (Literal(Constant(name.encode.toString)) :: Nil).map(untpd.TypedSplice(_)))
 
       val scall =
@@ -219,19 +219,19 @@ trait Dynamic {
     extension (tree: Tree)
       /** The implementations of `selectDynamic` and `applyDynamic` in `scala.reflect.SelectDynamic` have no information about the expected return type of a value/method which was declared in the refinement,
        *  only the JVM type after erasure can be obtained through reflection, e.g.
-       *  
+       *
        *  class Foo(val i: Int) extends AnyVal
        *  class Reflective extends reflect.Selectable
        *  val reflective = new Reflective {
        *    def foo = Foo(1) // Foo at compile time, java.lang.Integer in reflection
        *  }
-       * 
+       *
        *  Because of that reflective access cannot be implemented properly in `scala.reflect.SelectDynamic` itself
        *  because it's not known there if the value should be wrapped in a value class constructor call or not.
        *  Hence the logic of wrapping is performed here, relying on the fact that the implementations of `selectDynamic` and `applyDynamic` in `scala.reflect.SelectDynamic` are final.
        */
       def maybeBoxingCast(tpe: Type) =
-        val maybeBoxed = 
+        val maybeBoxed =
           if ValueClasses.isDerivedValueClass(tpe.classSymbol) && qual.tpe <:< defn.ReflectSelectableTypeRef then
             val genericUnderlying = ValueClasses.valueClassUnbox(tpe.classSymbol.asClass)
             val underlying = tpe.select(genericUnderlying).widen.resultType

tests/pos/i17100.scala

@@ -0,0 +1,14 @@
+trait Sel extends Selectable
+
+extension (s: Sel)
+  def selectDynamic(name: String) = ???
+  def applyDynamic(name: String)(x: Int) = ???
+  def applyDynamic(name: String)() = ???
+
+val sel = (new Sel {}).asInstanceOf[Sel{ def foo: String; def bar(x: Int): Int; def baz(): Int }]
+val foo = sel.selectDynamic("foo")
+val foo2 = sel.foo
+val foo3 = sel.bar(2)
+val foo4 = sel.baz()
+
+

tests/pos/i17100a.scala

@@ -0,0 +1,12 @@
+
+import scala.language.dynamics
+trait Sel extends Dynamic
+
+extension (s: Sel)
+  def selectDynamic(name: String) = ???
+
+val sel = new Sel {}
+val foo = sel.foo
+val sel2 = (new Sel {}).asInstanceOf[Sel{ def foo: String }]
+val foo2 = sel2.foo
+

tests/semanticdb/expect/Advanced.expect.scala

@@ -25,11 +25,11 @@ class Wildcards/*<-advanced::Wildcards#*/ {
 object Test/*<-advanced::Test.*/ {
   val s/*<-advanced::Test.s.*/ = new Structural/*->advanced::Structural#*/
   val s1/*<-advanced::Test.s1.*/ = s/*->advanced::Test.s.*/.s1/*->advanced::Structural#s1().*/
-  val s1x/*<-advanced::Test.s1x.*/ = s/*->advanced::Test.s.*/.s1/*->advanced::Structural#s1().*/.x
+  val s1x/*<-advanced::Test.s1x.*/ = s/*->advanced::Test.s.*/.s1/*->advanced::Structural#s1().*/.x/*->scala::reflect::Selectable#selectDynamic().*/
   val s2/*<-advanced::Test.s2.*/ = s/*->advanced::Test.s.*/.s2/*->advanced::Structural#s2().*/
-  val s2x/*<-advanced::Test.s2x.*/ = s/*->advanced::Test.s.*/.s2/*->advanced::Structural#s2().*/.x
+  val s2x/*<-advanced::Test.s2x.*/ = s/*->advanced::Test.s.*/.s2/*->advanced::Structural#s2().*/.x/*->scala::reflect::Selectable#selectDynamic().*/
   val s3/*<-advanced::Test.s3.*/ = s/*->advanced::Test.s.*/.s3/*->advanced::Structural#s3().*/
-  val s3x/*<-advanced::Test.s3x.*/ = s/*->advanced::Test.s.*/.s3/*->advanced::Structural#s3().*/.m(???/*->scala::Predef.`???`().*/)
+  val s3x/*<-advanced::Test.s3x.*/ = s/*->advanced::Test.s.*/.s3/*->advanced::Structural#s3().*/.m/*->scala::reflect::Selectable#applyDynamic().*/(???/*->scala::Predef.`???`().*/)
 
   val e/*<-advanced::Test.e.*/ = new Wildcards/*->advanced::Wildcards#*/
   val e1/*<-advanced::Test.e1.*/ = e/*->advanced::Test.e.*/.e1/*->advanced::Wildcards#e1().*/
@@ -45,7 +45,7 @@ object Test/*<-advanced::Test.*/ {
 
   // see: https://github.com/lampepfl/dotty/pull/14608#discussion_r835642563
   lazy val foo/*<-advanced::Test.foo.*/: (reflect.Selectable/*->scala::reflect::Selectable#*/ { type A/*<-local16*/ = Int/*->scala::Int#*/ }) &/*->scala::`&`#*/ (reflect.Selectable/*->scala::reflect::Selectable#*/ { type A/*<-local17*/ = Int/*->scala::Int#*/; val a/*<-local18*/: A/*->local17*/ }) = ???/*->scala::Predef.`???`().*/
-  def bar/*<-advanced::Test.bar().*/: foo/*->advanced::Test.foo.*/.A/*->local17*/ = foo/*->advanced::Test.foo.*/.a
+  def bar/*<-advanced::Test.bar().*/: foo/*->advanced::Test.foo.*/.A/*->local17*/ = foo/*->advanced::Test.foo.*/.a/*->scala::reflect::Selectable#selectDynamic().*/
 }
 
 

tests/semanticdb/metac.expect

@@ -49,7 +49,7 @@ Uri => Advanced.scala
 Text => empty
 Language => Scala
 Symbols => 60 entries
-Occurrences => 134 entries
+Occurrences => 138 entries
 Synthetics => 3 entries
 
 Symbols:
@@ -187,18 +187,21 @@ Occurrences:
 [27:6..27:9): s1x <- advanced/Test.s1x.
 [27:12..27:13): s -> advanced/Test.s.
 [27:14..27:16): s1 -> advanced/Structural#s1().
+[27:16..27:18): .x -> scala/reflect/Selectable#selectDynamic().
 [28:6..28:8): s2 <- advanced/Test.s2.
 [28:11..28:12): s -> advanced/Test.s.
 [28:13..28:15): s2 -> advanced/Structural#s2().
 [29:6..29:9): s2x <- advanced/Test.s2x.
 [29:12..29:13): s -> advanced/Test.s.
 [29:14..29:16): s2 -> advanced/Structural#s2().
+[29:16..29:18): .x -> scala/reflect/Selectable#selectDynamic().
 [30:6..30:8): s3 <- advanced/Test.s3.
 [30:11..30:12): s -> advanced/Test.s.
 [30:13..30:15): s3 -> advanced/Structural#s3().
 [31:6..31:9): s3x <- advanced/Test.s3x.
 [31:12..31:13): s -> advanced/Test.s.
 [31:14..31:16): s3 -> advanced/Structural#s3().
+[31:16..31:18): .m -> scala/reflect/Selectable#applyDynamic().
 [31:19..31:22): ??? -> scala/Predef.`???`().
 [33:6..33:7): e <- advanced/Test.e.
 [33:14..33:23): Wildcards -> advanced/Wildcards#
@@ -233,6 +236,7 @@ Occurrences:
 [47:11..47:14): foo -> advanced/Test.foo.
 [47:15..47:16): A -> local17
 [47:19..47:22): foo -> advanced/Test.foo.
+[47:22..47:24): .a -> scala/reflect/Selectable#selectDynamic().
 [52:6..52:13): HKClass <- advanced/HKClass#
 [52:14..52:15): F <- advanced/HKClass#[F]
 [52:20..52:21): T <- advanced/HKClass#`<init>`().[F][T]