Full Bifunctor widening

typelevel · May 27, 2020 · 974a335 · 974a335
1 parent d546614
commit 974a335
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Showing 2 changed files with 8 additions and 5 deletions.
diff --git a/core/src/main/scala/cats/AutoVariance.scala b/core/src/main/scala/cats/AutoVariance.scala
@@ -1,10 +1,13 @@
 package cats
 
-trait AutoVariance {
+trait AutoVariance extends AutoVarianceLowPriority {
   implicit def autoWidenFunctor[F[_]: Functor, A, B >: A](fa: F[A]): F[B] = Functor[F].widen(fa)
 
   implicit def autoNarrowContravariant[F[_]: Contravariant, A, B <: A](fa: F[A]): F[B] = Contravariant[F].narrow(fa)
 
-  implicit def autoLeftWidenBifunctor[F[_, _]: Bifunctor, A, B >: A, C](fac: F[A, C]): F[B, C] =
-    Bifunctor[F].leftWiden(fac)
+}
+
+trait AutoVarianceLowPriority {
+  implicit def autoWidenBifunctor[F[_, _]: Bifunctor, A, B >: A, C, D >: C](fac: F[A, C]): F[B, D] =
+    Bifunctor[F].leftWiden(Bifunctor[F].rightFunctor.widen(fac))
 }
diff --git a/tests/src/test/scala/cats/tests/VarianceSuite.scala b/tests/src/test/scala/cats/tests/VarianceSuite.scala
@@ -24,9 +24,9 @@ class VarianceSuite extends CatsSuite {
 
   test("Auto-variance should left widen a bifunctor automatically") {
     def shouldInfer[F[_, _]: Bifunctor](fi: F[Int, Int]) =
-      fi.leftMap(i => if (true) Left(Bar(i)) else Right(Baz))
+      fi.bimap(i => if (true) Left(Bar(i)) else Right(Baz), i => if (true) Left(Bar(i)) else Right(Baz))
 
-    def inferred[F[_, _]: Bifunctor](fi: F[Int, Int]): F[Either[Foo, Foo], Int] = shouldInfer[F](fi)
+    def inferred[F[_, _]: Bifunctor](fi: F[Int, Int]): F[Either[Foo, Foo], Either[Foo, Foo]] = shouldInfer[F](fi)
   }
 
 }