From b159489576fc7afdee5b2d93e9465dbc87f8069e Mon Sep 17 00:00:00 2001
From: Guillaume Martres <smarter@ubuntu.com>
Date: Sat, 8 Apr 2017 22:51:16 +0200
Subject: Fix #2201: Less aggressive type application reduction for better
 inference

Previously we believed that reducing type applications did not affect
type inference as long as the reduced type constructor had the same
arity as the unreduced one, for example reducing `Foo[X, Y]` is fine
when `Foo` is defined as:

type Foo[A, B] = Bar[A, B]

but not when it's defined as:

type Foo[A] = Bar[A, A]

But this is not a sufficient condition: the bounds of the type
constructor arguments also matter for type inference, so we need to be
more strict and disallow reductions in cases like:

type Foo[A, B] = Bar[B, A]

and:

type Foo[A, B] = Bar[A, Int]
---
 tests/pos/i2201a.scala |  8 ++++++++
 tests/pos/i2201b.scala | 14 ++++++++++++++
 tests/pos/i2201c.scala | 11 +++++++++++
 3 files changed, 33 insertions(+)
 create mode 100644 tests/pos/i2201a.scala
 create mode 100644 tests/pos/i2201b.scala
 create mode 100644 tests/pos/i2201c.scala

(limited to 'tests')

diff --git a/tests/pos/i2201a.scala b/tests/pos/i2201a.scala
new file mode 100644
index 000000000..165f0a76e
--- /dev/null
+++ b/tests/pos/i2201a.scala
@@ -0,0 +1,8 @@
+class Foo[T]
+
+class Fix[F[_]](unfix: F[Fix[F]])
+object DocTree {
+  type Const[T] = Foo[Int]
+  type FixConst = Fix[Const]
+  def docTree(s: Const[FixConst]): FixConst = new Fix(s)
+}
diff --git a/tests/pos/i2201b.scala b/tests/pos/i2201b.scala
new file mode 100644
index 000000000..4aafc0d28
--- /dev/null
+++ b/tests/pos/i2201b.scala
@@ -0,0 +1,14 @@
+trait X
+trait Y
+
+object Test {
+  type One[A <: X, B <: Y]
+
+  type Two[TA <: Y, TB <: X] = One[TB, TA]
+
+  def foo[M[_ <: Y, _ <: X]](x: M[_ <: Y, _ <: X]) = x
+
+  val a: Two[Y, X] = ???
+
+  foo(a)
+}
diff --git a/tests/pos/i2201c.scala b/tests/pos/i2201c.scala
new file mode 100644
index 000000000..91f2529d9
--- /dev/null
+++ b/tests/pos/i2201c.scala
@@ -0,0 +1,11 @@
+object Test {
+  implicit val theAnswer: Int = 42
+
+  type Swap[A, B] = (B, A)
+
+  def foo[M[_, _], T, S](x: M[T, S])(implicit ev: T) = ev
+
+  val a: Swap[Int, String] = ("hi", 1)
+
+  foo(a)
+}
-- 
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