Reject structural applications of polymorphic methods

Fixes #15065

We always intended to reject them, but crashed before we got to that point.

Author: odersky

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -578,12 +578,12 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
             pre
           case _ =>
             tree1
-            
+
         val countsAsPure =
           if dropOp(tree1).symbol.isInlineVal
           then isIdempotentExpr(tree1)
           else isPureExpr(tree1)
-          
+
         if countsAsPure then Literal(value).withSpan(tree.span)
         else
           val pre = dropOp(tree1)
@@ -879,14 +879,15 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
       }
       !tree.symbol.exists && tree.isTerm && hasRefinement(tree.qualifier.tpe)
     }
-    def loop(tree: Tree): Boolean = tree match {
+    def loop(tree: Tree): Boolean = tree match
+      case TypeApply(fun, _) =>
+        loop(fun)
       case Apply(fun, _) =>
         loop(fun)
       case tree: Select =>
         isStructuralTermSelect(tree)
       case _ =>
         false
-    }
     loop(tree)
   }
 

tests/pos/i15065.scala

@@ -0,0 +1,8 @@
+trait Foo extends reflect.Selectable
+
+val foo = new Foo:
+  def bar[A](): Int = ???
+
+val x = foo.bar()      // error: Structural access not allowed on method bar because it is polymorphic
+
+val y = foo.bar[Int]() // error: Structural access not allowed on method bar because it is polymorphic