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, 3 insertions, 3 deletions

diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 89bc21929..7e6620f8e 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -2862,14 +2862,14 @@ object Types {
    *
    *  @param  origin        The parameter that's tracked by the type variable.
    *  @param  creatorState  The typer state in which the variable was created.
-   *  @param  owningTree    The function part of the TypeApply tree tree that introduces
-   *                        the type variable.
+   *  @param  bindingTree   The TypeTree which introduces the type variable, or EmptyTree
+   *                        if the type variable does not correspond to a source term.
    *  @paran  owner         The current owner if the context where the variable was created.
    *
    *  `owningTree` and `owner` are used to determine whether a type-variable can be instantiated
    *  at some given point. See `Inferencing#interpolateUndetVars`.
    */
-  final class TypeVar(val origin: PolyParam, creatorState: TyperState, val owningTree: untpd.Tree, val owner: Symbol) extends CachedProxyType with ValueType {
+  final class TypeVar(val origin: PolyParam, creatorState: TyperState, val bindingTree: untpd.Tree, val owner: Symbol) extends CachedProxyType with ValueType {
 
     /** The permanent instance type of the variable, or NoType is none is given yet */
     private[core] var inst: Type = NoType