Further minor tweaks to result<>'s failure type

Signed-off-by: Christian Parpart <christian@parpart.family>

src/crispy/result.h

@@ -19,7 +19,7 @@ class failure
     constexpr failure(const failure&) = default;
     constexpr failure(failure&&) noexcept = default;
 
-    template <typename Err = E>
+    template <typename Err = E, typename = std::enable_if_t<std::is_constructible_v<E, Err&&>>>
     constexpr explicit failure(Err&& e): _value { std::forward<Err>(e) }
     {
     }

src/crispy/result_test.cpp

@@ -114,6 +114,20 @@ TEST_CASE("result.or_else")
     // test const lvalue reference
     auto const c = b.or_else([](error_code e) -> result { return failure { error_code(int(e) + 1) }; });
     REQUIRE(c.error() == error_code::E3);
+
+    // ensure chaining works over rvalues
+    auto const b2 = result { failure { error_code::E1 } } // E1
+                        .or_else([](error_code) { return result { 12 }; })
+                        .or_else([](error_code) { return result { 13 }; });
+    REQUIRE(b2.value() == 12);
+
+    // ensure chaining works over lvalues
+    auto const someError = result { failure { error_code::E1 } };
+    auto const c2 = someError // E1
+                        .or_else([](error_code) { return result { 14 }; })
+                        .or_else([](error_code) { return result { 15 }; })
+                        .or_else([](error_code) { return result { 16 }; });
+    REQUIRE(c2.value() == 14);
 }
 
 TEST_CASE("result.emplace")