From 726905918ace4523746fe6b90a52fbe69093b725 Mon Sep 17 00:00:00 2001
From: Guillaume Martres <smarter@ubuntu.com>
Date: Sat, 6 Apr 2019 16:33:19 +0200
Subject: [PATCH] Fix type lambda reductions involving refinements

Given:

    type F[X] = Foo[X]

We've always been able to reduce `F[_]` to `Foo[_]` if `Foo` is a class.
This commit does something similar for refinements, given:

    type G[X] = Bla { type R = X }

We can reduce `G[_]` to `Bar { type R }` (previously we reduced it to
`Bar { type R = Any }` which is incorrect).
---
 .../src/dotty/tools/dotc/core/TypeApplications.scala   |  2 ++
 tests/pos/hkRefAlias.scala                             | 10 ++++++++++
 2 files changed, 12 insertions(+)
 create mode 100644 tests/pos/hkRefAlias.scala

diff --git a/compiler/src/dotty/tools/dotc/core/TypeApplications.scala b/compiler/src/dotty/tools/dotc/core/TypeApplications.scala
index 45c5ce8586bc..85c24a7dbe10 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeApplications.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeApplications.scala
@@ -129,6 +129,8 @@ object TypeApplications {
     def apply(t: Type): Type = t match {
       case t @ AppliedType(tycon, args1) if tycon.typeSymbol.isClass =>
         t.derivedAppliedType(apply(tycon), args1.mapConserve(applyArg))
+      case t @ RefinedType(parent, name, TypeAlias(info)) =>
+        t.derivedRefinedType(apply(parent), name, applyArg(info).bounds)
       case p: TypeParamRef if p.binder == tycon =>
         args(p.paramNum) match {
           case TypeBounds(lo, hi) =>
diff --git a/tests/pos/hkRefAlias.scala b/tests/pos/hkRefAlias.scala
new file mode 100644
index 000000000000..58ec787def1c
--- /dev/null
+++ b/tests/pos/hkRefAlias.scala
@@ -0,0 +1,10 @@
+class Bar
+class X
+class Y extends X
+
+object Test {
+  type G[X] = Bar { type R = X }
+
+  implicitly[G[_] =:= (Bar { type R })]
+  implicitly[G[_ >: Y <: X] =:= (Bar { type R >: Y <: X })]
+}