Insert conversions also on selections wrapped in type applications

In i12708.scala, the problematic function was a selection `qual.m[tvs]` that was already
applied to type variables. In that case we need to backtrack, forget the type variables
and try to insert a conversion or extension method on `qual`.

Fixes #12708

Author: odersky

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

@@ -3045,12 +3045,12 @@ class Typer extends Namer
     }
   }
 
-  /** If this tree is a select node `qual.name` that does not conform to `pt`,
-   *  try to insert an implicit conversion `c` around `qual` so that
-   *  `c(qual).name` conforms to `pt`.
+  /** If this tree is a select node `qual.name` (possibly applied to type variables)
+   *  that does not conform to `pt`, try to insert an implicit conversion `c` around
+   *  `qual` so that `c(qual).name` conforms to `pt`.
    */
   def tryInsertImplicitOnQualifier(tree: Tree, pt: Type, locked: TypeVars)(using Context): Option[Tree] = trace(i"try insert impl on qualifier $tree $pt") {
-    tree match {
+    tree match
       case tree @ Select(qual, name) if name != nme.CONSTRUCTOR =>
         val selProto = SelectionProto(name, pt, NoViewsAllowed, privateOK = false)
         if selProto.isMatchedBy(qual.tpe) then None
@@ -3061,8 +3061,9 @@ class Typer extends Namer
             else Some(adapt(tree1, pt, locked))
           } { (_, _) => None
           }
+      case TypeApply(fn, args) if args.forall(_.isInstanceOf[TypeVarBinder[_]]) =>
+        tryInsertImplicitOnQualifier(fn, pt, locked)
       case _ => None
-    }
   }
 
   /** Given a selection `qual.name`, try to convert to an extension method

tests/pos/i12708.scala

@@ -0,0 +1,37 @@
+import language.implicitConversions
+
+trait AdditiveSemigroup[A]
+
+final class AdditiveSemigroupOps[A](lhs: A)(implicit as: AdditiveSemigroup[A]) {
+  def +(rhs: A): A = ???
+  def ^(rhs: A): A = ???
+}
+
+trait AdditiveSemigroupSyntax {
+  implicit def additiveSemigroupOps[A: AdditiveSemigroup](a: A): AdditiveSemigroupOps[A] =
+     new AdditiveSemigroupOps(a)
+}
+
+object syntax {
+  object additiveSemigroup extends AdditiveSemigroupSyntax
+}
+
+object App {
+
+  def main(args: Array[String]): Unit = {
+    import syntax.additiveSemigroup._
+
+    implicit def IntAlgebra[A]: AdditiveSemigroup[Map[Int, A]] = ???
+
+    def res[A]: Map[Int, A] = {
+      val a: Map[Int, A] = Map.empty
+      val b: Map[Int, A] = Map.empty
+      // Calls the operator on AdditiveSemigroupOps
+      a ^ b
+      // Calls the operator + on AdditiveSemigroupOps only in Scala 2
+      // In Scala 3 tries to call `+` on Map
+      a + b
+    }
+  }
+
+}