Add tests for into_err and fix initializer call.

src/maybe/result.hpp

@@ -419,7 +419,7 @@ namespace maybe {
         constexpr static result<void, E> default_ok() noexcept
         {
             return std::experimental::constexpr_move(
-                result<void, E>(void(), internal::placeholder{}));
+                result<void, E>(internal::placeholder{}));
         }
 
         /**

tests/CMakeLists.txt

@@ -8,6 +8,7 @@ add_executable(${TARGET}
         result_tests.cpp
         result_map_tests.cpp
         result_map_err_tests.cpp
+        result_into_err_tests.cpp
         result_and_then_tests.cpp
         example_test.cpp)
 

tests/result_into_err_tests.cpp

@@ -0,0 +1,35 @@
+#include "catch.hpp"
+
+#include <maybe/result.hpp>
+
+class A final {
+public:
+    A(std::string value) : value(value)
+    {
+    }
+    std::string value;
+};
+
+class B final {
+};
+
+using maybe::result;
+
+TEST_CASE("result_into_err")
+{
+    SECTION("converts result<A, E>::err to result<B, E>::err")
+    {
+        auto a = result<A, int>::err(42);
+        auto b = a.into_err<result<B, int>>();
+        REQUIRE(!b);
+        REQUIRE(42 == b.err_value());
+    }
+
+    SECTION("converts result<A, E>::err to result<void, E>::err")
+    {
+        auto a = result<A, int>::err(42);
+        auto b = a.into_err<result<void, int>>();
+        REQUIRE(!b);
+        REQUIRE(42 == b.err_value());
+    }
+}