Fix problem with extension methods in givens

For these we need an eligibility pre-check. That pre-check
did not skip leading implicit parameters, so failed to recognize
eligible extension methods.

Author: odersky

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

@@ -1363,7 +1363,10 @@ trait Applications extends Compatibility {
    */
   def hasExtensionMethodNamed(tp: Type, xname: TermName, argType: Type, resultType: Type)(using Context) = {
     def qualifies(mbr: Denotation) =
-      mbr.exists && isApplicableType(tp.select(xname, mbr), argType :: Nil, resultType)
+      mbr.exists
+      && isApplicableType(
+            normalize(tp.select(xname, mbr), WildcardType),
+            argType :: Nil, resultType)
     tp.memberBasedOnFlags(xname, required = ExtensionMethod) match {
       case mbr: SingleDenotation => qualifies(mbr)
       case mbr => mbr.hasAltWith(qualifies(_))

tests/pos/i9530.scala

@@ -0,0 +1,37 @@
+trait Food
+case class Banana(color: String) extends Food
+
+trait Diet[A <: Animal]:
+  type F <: Food
+  def food: Seq[F]
+
+trait Animal
+object Animal:
+  extension [A <: Animal](using diet: Diet[A])(animal: A) def food1 = diet.food
+  extension [A <: Animal](animal: A)(using diet: Diet[A]) def food2 = diet.food
+
+extension [A <: Animal](using diet: Diet[A])(animal: A) def food3 = diet.food
+extension [A <: Animal](animal: A)(using diet: Diet[A]) def food4 = diet.food
+
+trait Monkey extends Animal
+
+given Diet[Monkey] with
+  type F = Banana
+  def food: Seq[Banana] = Seq(new Banana("yellow"), Banana("green"))
+
+trait FoodOps
+given FoodOps with
+  extension [A <: Animal](using diet: Diet[A])(animal: A) def food5 = diet.food
+  extension [A <: Animal](animal: A)(using diet: Diet[A]) def food6 = diet.food
+
+
+val monkey = new Monkey {}
+
+val foods = Seq(
+  monkey.food1, // doesn't compile
+  monkey.food2, // compiles
+  monkey.food3, // compiles
+  monkey.food4, // compiles
+  monkey.food5, // doesn't compile
+  monkey.food6, // compiles
+)