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, 7 insertions, 4 deletions

diff --git a/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala b/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala
index 9a20a452e..ed6b95c3b 100644
--- a/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala
+++ b/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala
@@ -353,20 +353,23 @@ object ProtoTypes {
    *  Also, if `owningTree` is non-empty, add a type variable for each parameter.
    *  @return  The added polytype, and the list of created type variables.
    */
-  def constrained(pt: PolyType, owningTree: untpd.Tree)(implicit ctx: Context): (PolyType, List[TypeVar]) = {
+  def constrained(pt: PolyType, owningTree: untpd.Tree)(implicit ctx: Context): (PolyType, List[TypeTree]) = {
     val state = ctx.typerState
     assert(!(ctx.typerState.isCommittable && owningTree.isEmpty),
       s"inconsistent: no typevars were added to committable constraint ${state.constraint}")
 
-    def newTypeVars(pt: PolyType): List[TypeVar] =
+    def newTypeVars(pt: PolyType): List[TypeTree] =
       for (n <- (0 until pt.paramNames.length).toList)
-      yield new TypeVar(PolyParam(pt, n), state, owningTree, ctx.owner)
+      yield {
+        val tt = new TypeTree().withPos(owningTree.pos)
+        tt.withType(new TypeVar(PolyParam(pt, n), state, tt, ctx.owner))
+      }
 
     val added =
       if (state.constraint contains pt) pt.newLikeThis(pt.paramNames, pt.paramBounds, pt.resultType)
       else pt
     val tvars = if (owningTree.isEmpty) Nil else newTypeVars(added)
-    ctx.typeComparer.addToConstraint(added, tvars)
+    ctx.typeComparer.addToConstraint(added, tvars.tpes.asInstanceOf[List[TypeVar]])
     (added, tvars)
   }