From 3cea1dc6758709e9fd1d87e03e7baee023a1dc72 Mon Sep 17 00:00:00 2001
From: Tooru Fujisawa <arai_a@mac.com>
Date: Fri, 3 Nov 2023 08:13:19 +0000
Subject: [PATCH] Bug 1850723 - Make mozilla::Result constructor take another
 result with convertible value. r=emilio,glandium

Differential Revision: https://phabricator.services.mozilla.com/D191538
---
 mfbt/Result.h             | 12 +++++-----
 mfbt/tests/TestResult.cpp | 47 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+), 5 deletions(-)

diff --git a/mfbt/Result.h b/mfbt/Result.h
index c3c96d1fbfb3..052920fdbf17 100644
--- a/mfbt/Result.h
+++ b/mfbt/Result.h
@@ -561,11 +561,13 @@ class [[nodiscard]] Result final {
   }
 
   /**
-   * Create a (success/error) result from another (success/error) result with a
-   * different but convertible error type. */
-  template <typename E2,
-            typename = std::enable_if_t<std::is_convertible_v<E2, E>>>
-  MOZ_IMPLICIT constexpr Result(Result<V, E2>&& aOther)
+   * Create a (success/error) result from another (success/error) result with
+   * different but convertible value and error types.
+   */
+  template <typename V2, typename E2,
+            typename = std::enable_if_t<std::is_convertible_v<V2, V> &&
+                                        std::is_convertible_v<E2, E>>>
+  MOZ_IMPLICIT constexpr Result(Result<V2, E2>&& aOther)
       : mImpl(aOther.isOk() ? Impl{aOther.unwrap()}
                             : Impl{aOther.unwrapErr()}) {}
 
diff --git a/mfbt/tests/TestResult.cpp b/mfbt/tests/TestResult.cpp
index dd9c5f695e75..a2e10640c5d9 100644
--- a/mfbt/tests/TestResult.cpp
+++ b/mfbt/tests/TestResult.cpp
@@ -806,6 +806,52 @@ static void ZeroIsEmptyErrorTest() {
   }
 }
 
+class Foo {};
+
+class C1 {};
+class C2 : public C1 {};
+
+class E1 {};
+class E2 : public E1 {};
+
+void UpcastTest() {
+  {
+    C2 c2;
+
+    mozilla::Result<C2*, Failed> result(&c2);
+    mozilla::Result<C1*, Failed> copied(std::move(result));
+
+    MOZ_RELEASE_ASSERT(copied.inspect() == &c2);
+  }
+
+  {
+    E2 e2;
+
+    mozilla::Result<Foo, E2*> result(Err(&e2));
+    mozilla::Result<Foo, E1*> copied(std::move(result));
+
+    MOZ_RELEASE_ASSERT(copied.inspectErr() == &e2);
+  }
+
+  {
+    C2 c2;
+
+    mozilla::Result<C2*, E2*> result(&c2);
+    mozilla::Result<C1*, E1*> copied(std::move(result));
+
+    MOZ_RELEASE_ASSERT(copied.inspect() == &c2);
+  }
+
+  {
+    E2 e2;
+
+    mozilla::Result<C2*, E2*> result(Err(&e2));
+    mozilla::Result<C1*, E1*> copied(std::move(result));
+
+    MOZ_RELEASE_ASSERT(copied.inspectErr() == &e2);
+  }
+}
+
 /* * */
 
 int main() {
@@ -819,5 +865,6 @@ int main() {
   AndThenTest();
   UniquePtrTest();
   ZeroIsEmptyErrorTest();
+  UpcastTest();
   return 0;
 }