SI-9074 Fix generic substitution with package objects, overloading

Takes a leaf out of dotty's book [1] and makes `asSeenFrom`
tranaparently 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.

It also lets us remove the fix for SI-6225.

[1] scala/scala3#282

Author: retronym

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

@@ -807,12 +807,7 @@ 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/reflect/scala/reflect/internal/tpe/TypeMaps.scala

@@ -433,8 +433,10 @@ private[internal] trait TypeMaps {
     (pre eq NoType) || (pre eq NoPrefix) || !isPossiblePrefix(clazz)
     )
 
-  def newAsSeenFromMap(pre: Type, clazz: Symbol): AsSeenFromMap =
-    new AsSeenFromMap(pre, clazz)
+  def newAsSeenFromMap(pre: Type, clazz: Symbol): AsSeenFromMap = {
+    val pre1 = if (pre.termSymbolDirect.hasPackageFlag && !clazz.hasPackageFlag) pre.packageObject.typeOfThis else pre
+    new AsSeenFromMap(pre1, clazz)
+  }
 
   /** A map to compute the asSeenFrom method.
     */

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
+}