SI-9074 Fix generic substitution with package objects, overloading

Takes a leaf out of dotty's book [1] and makes `asSeenFrom`
transparently change the prefix from the package class to the
package object when needed.

This fixes generic subsitution during overload resolution, as
reported in SI-9074.

This subsumes the former fix for SI-6225, which is removed here.

[1] scala/scala3#282

Author: retronym

src/compiler/scala/tools/nsc/typechecker/Contexts.scala

@@ -810,12 +810,7 @@ trait Contexts { self: Analyzer =>
       val qual = imp.qual
 
       val qualSym = qual.tpe.typeSymbol
-      val pre =
-        if (qualSym.isPackageClass)
-          // SI-6225 important if the imported symbol is inherited by the the package object.
-          qualSym.packageObject.typeOfThis
-        else
-          qual.tpe
+      val pre = qual.tpe
       def collect(sels: List[ImportSelector]): List[ImplicitInfo] = sels match {
         case List() =>
           List()

src/compiler/scala/tools/nsc/typechecker/Infer.scala

@@ -1447,7 +1447,7 @@ trait Infer extends Checkable {
           log(s"Attaching AntiPolyType-carrying overloaded type to $sym")
           // Multiple alternatives which are within bounds; spin up an
           // overloaded type which carries an "AntiPolyType" as a prefix.
-          val tparams = newAsSeenFromMap(pre, hd.owner) mapOver hd.typeParams
+          val tparams = new AsSeenFromMap(pre, hd.owner) mapOver hd.typeParams
           val bounds  = tparams map (_.tpeHK) // see e.g., #1236
           val tpe     = PolyType(tparams, OverloadedType(AntiPolyType(pre, bounds), alts))
           finish(sym setInfo tpe, tpe)

src/interactive/scala/tools/nsc/interactive/Global.scala

@@ -159,18 +159,6 @@ class Global(settings: Settings, _reporter: Reporter, projectName: String = "")
   override def forInteractive = true
   override protected def synchronizeNames = true
 
-  override def newAsSeenFromMap(pre: Type, clazz: Symbol): AsSeenFromMap =
-    new InteractiveAsSeenFromMap(pre, clazz)
-
-  class InteractiveAsSeenFromMap(pre: Type, clazz: Symbol) extends AsSeenFromMap(pre, clazz) {
-    /** The method formerly known as 'instParamsRelaxed' goes here if it's still necessary,
-     *  which it is currently supposed it is not.
-     *
-     *  If it is, change AsSeenFromMap method correspondingTypeArgument to call an overridable
-     *  method rather than aborting in the failure case.
-     */
-  }
-
   /** A map of all loaded files to the rich compilation units that correspond to them.
    */
   val unitOfFile = mapAsScalaMapConverter(new ConcurrentHashMap[AbstractFile, RichCompilationUnit] {

src/reflect/scala/reflect/internal/Types.scala

@@ -666,7 +666,7 @@ trait Types
         )
         if (trivial) this
         else {
-          val m     = newAsSeenFromMap(pre.normalize, clazz)
+          val m     = new AsSeenFromMap(pre.normalize, clazz)
           val tp    = m(this)
           val tp1   = existentialAbstraction(m.capturedParams, tp)
 

src/reflect/scala/reflect/internal/tpe/TypeMaps.scala

@@ -449,12 +449,15 @@ private[internal] trait TypeMaps {
     (pre eq NoType) || (pre eq NoPrefix) || !isPossiblePrefix(clazz)
     )
 
-  def newAsSeenFromMap(pre: Type, clazz: Symbol): AsSeenFromMap =
-    new AsSeenFromMap(pre, clazz)
+  @deprecated("Use new AsSeenFromMap instead", "2.12.0")
+  final def newAsSeenFromMap(pre: Type, clazz: Symbol): AsSeenFromMap = new AsSeenFromMap(pre, clazz)
 
   /** A map to compute the asSeenFrom method.
     */
-  class AsSeenFromMap(seenFromPrefix: Type, seenFromClass: Symbol) extends TypeMap with KeepOnlyTypeConstraints {
+  class AsSeenFromMap(seenFromPrefix0: Type, seenFromClass: Symbol) extends TypeMap with KeepOnlyTypeConstraints {
+    private val seenFromPrefix: Type = if (seenFromPrefix0.typeSymbolDirect.hasPackageFlag && !seenFromClass.hasPackageFlag)
+      seenFromPrefix0.packageObject.typeOfThis
+    else seenFromPrefix0
     // Some example source constructs relevant in asSeenFrom:
     //
     // object CaptureThis {

test/files/pos/t9074.scala

@@ -0,0 +1,24 @@
+package blam {
+
+  package foo {
+
+    trait F[T] {
+      def f(d: Double, t: T): T = ???
+      def f(d: Int, t: T): T = ???
+      def f(d: String, t: T): T = ???
+
+      def g[A](a: T): T = ???
+      def g(a: Int) = ???
+    }
+  }
+
+  package object foo extends foo.F[Double] {
+    override def f(d: Double, t: Double): Double = ???
+  }
+}
+
+object Test {
+  import blam._
+  foo.f("3", 4.0)
+  foo.g[Any](1d) : Double
+}

test/files/pos/t9074b.scala

@@ -0,0 +1,15 @@
+trait Echo [T] {
+  def echo(t: T): Unit
+}
+
+trait IntEcho extends Echo[Int] {
+  def echo(t: Int) = println(t)
+}
+
+object echo extends IntEcho
+package object echo1  extends IntEcho
+
+object App extends App {
+  echo.echo(1)
+  echo1.echo(1)
+}