Fix #1757: Be more careful about positions of type variable binders

We interpolate a type variable if the current tree contains the type variables
binding tree. Previously, this was the application owning the variable. However,
sometimes this tree is transformed so that the containment test fails, and
type variables are instantiated too late (in the case of #1757 this was never).

We fix this by

 - setting the binding tree to the type tree that first contains the type variable
 - making sure that tree is never copied literally anywhere else.

It's a tricky dance, but I believe we got it right now.

1 files changed, 2 insertions, 2 deletions

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 4ec1c460f..f3417ceac 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -1962,12 +1962,12 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
           if (pt.isInstanceOf[PolyProto]) tree
           else {
             var typeArgs = tree match {
-              case Select(qual, nme.CONSTRUCTOR) => qual.tpe.widenDealias.argTypesLo
+              case Select(qual, nme.CONSTRUCTOR) => qual.tpe.widenDealias.argTypesLo.map(TypeTree)
               case _ => Nil
             }
             if (typeArgs.isEmpty) typeArgs = constrained(poly, tree)._2
             convertNewGenericArray(
-              adaptInterpolated(tree.appliedToTypes(typeArgs), pt, original))
+              adaptInterpolated(tree.appliedToTypeTrees(typeArgs), pt, original))
           }
         case wtp =>
           pt match {