From 8d7a795e1e3200cdf8d1df492337527df963150a Mon Sep 17 00:00:00 2001
From: satorg <satorg@users.noreply.github.com>
Date: Tue, 5 May 2026 23:42:27 -0700
Subject: [PATCH] re-use `Apply.map2Eval` implementation in `FlatMap`

---
 core/src/main/scala/cats/FlatMap.scala         |  6 ++++--
 .../test/scala/cats/tests/OptionTSuite.scala   | 18 ++++++++++++++++--
 2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/core/src/main/scala/cats/FlatMap.scala b/core/src/main/scala/cats/FlatMap.scala
index cd1b4356ce..0249d3cd8d 100644
--- a/core/src/main/scala/cats/FlatMap.scala
+++ b/core/src/main/scala/cats/FlatMap.scala
@@ -113,8 +113,10 @@ trait FlatMap[F[_]] extends Apply[F] with FlatMapArityFunctions[F] {
   override def map2[A, B, Z](fa: F[A], fb: F[B])(f: (A, B) => Z): F[Z] =
     flatMap(fa)(a => map(fb)(b => f(a, b)))
 
-  override def map2Eval[A, B, Z](fa: F[A], fb: Eval[F[B]])(f: (A, B) => Z): Eval[F[Z]] =
-    Eval.now(flatMap(fa)(a => map(fb.value)(b => f(a, b))))
+  override def map2Eval[A, B, Z](fa: F[A], efb: Eval[F[B]])(f: (A, B) => Z): Eval[F[Z]] =
+    // Can't simply call `super.map2Eval(...)` because of MiMa errors.
+    // Neither can simly remove this override for the same reason.
+    efb.map(fb => map2(fa, fb)(f))
 
   override def productR[A, B](fa: F[A])(fb: F[B]): F[B] =
     flatMap(fa)(_ => fb)
diff --git a/tests/shared/src/test/scala/cats/tests/OptionTSuite.scala b/tests/shared/src/test/scala/cats/tests/OptionTSuite.scala
index 1fe21fa7ea..e9ec8800e2 100644
--- a/tests/shared/src/test/scala/cats/tests/OptionTSuite.scala
+++ b/tests/shared/src/test/scala/cats/tests/OptionTSuite.scala
@@ -25,15 +25,16 @@ import cats.*
 import cats.data.{Const, IdT, OptionT, State}
 import cats.kernel.laws.discipline.{EqTests, MonoidTests, OrderTests, PartialOrderTests, SemigroupTests}
 import cats.laws.discipline.*
+import cats.laws.discipline.SemigroupalTests.Isomorphisms
 import cats.laws.discipline.arbitrary.*
 import cats.laws.discipline.eq.*
-import cats.laws.discipline.SemigroupalTests.Isomorphisms
 import cats.syntax.applicative.*
+import cats.syntax.apply.*
 import cats.syntax.either.*
+import cats.syntax.eq.*
 import cats.syntax.flatMap.*
 import cats.syntax.functor.*
 import cats.syntax.monadError.*
-import cats.syntax.eq.*
 import org.scalacheck.Prop.*
 
 class OptionTSuite extends CatsSuite {
@@ -583,6 +584,19 @@ class OptionTSuite extends CatsSuite {
     }
   }
 
+  test("Apply.map2Eval is stack safe for OptionT via Foldable.foldRight") {
+    val G = Applicative[OptionT[Id, *]]
+    val obtained =
+      Foldable[List]
+        .foldRight(List.range(0, 12345), Eval.now(OptionT.pure[Id](0L))) { (a, accEvalG) =>
+          G.pure(a).map2Eval(accEvalG) { _ + _ }
+        }
+        .value
+        .value
+
+    assertEquals(obtained, Some(76193340L))
+  }
+
   /**
    * Testing that implicit resolution works. If it compiles, the "test" passes.
    */