Keep a warning's problems when it is combined with a failure

modules/core/src/main/scala/result.scala

@@ -404,7 +404,7 @@ trait ResultInstances extends ResultInstances0 {
             case Result.Warning(ps, a) =>
               ff match {
                 case err @ Result.InternalError(_) => err
-                case fail @ Result.Failure(_) => fail
+                case Result.Failure(ps0) => Result.Failure(ps0 ++ ps)
                 case Result.Success(f) => Result.Warning(ps, f(a))
                 case Result.Warning(ps0, f) => Result.Warning(ps0 ++ ps, f(a))
               }

modules/core/src/test/scala/laws/ResultSuite.scala

@@ -20,6 +20,7 @@ import cats.data.NonEmptyChain
 import cats.kernel.laws.discipline.{EqTests, SemigroupTests}
 import cats.laws.discipline.{ApplicativeTests, MonadErrorTests, ParallelTests, TraverseTests}
 import cats.laws.discipline.arbitrary._
+import cats.syntax.parallel._
 import munit.DisciplineSuite
 import org.scalacheck.{Arbitrary, Cogen, Gen}
 import org.scalacheck.Arbitrary.{arbitrary => getArbitrary}
@@ -71,4 +72,17 @@ class ResultSuite extends DisciplineSuite {
   checkAll("Eq[Result[Int]]", EqTests[Result[Int]].eqv)
 
   checkAll("Applicative[ResultT] @ Int", ApplicativeTests[Result].applicative[Int, Int, Int])
+
+  // A `Warning` carries problems as well as a value, so combining one with a `Failure` must
+  // retain both sets whichever side the warning is on.
+  test("Parallel accumulates a warning's problems on either side of a failure") {
+    val warning: Result[Int] = Result.Warning(NonEmptyChain(Problem("w")), 1)
+    val failure: Result[Int] = Result.Failure(NonEmptyChain(Problem("f")))
+
+    def messages(r: Result[(Int, Int)]): List[String] =
+      r.toProblems.toList.map(_.message)
+
+    assertEquals(messages((warning, failure).parTupled), List("w", "f"))
+    assertEquals(messages((failure, warning).parTupled), List("f", "w"))
+  }
 }