Fix #33

tests/issues.cpp

@@ -73,3 +73,10 @@ TEST_CASE("Issue 31", "[issues.31]") {
     tl::expected< void, std::string > result2 = result;
     result2 = result;
 }
+
+TEST_CASE("Issue 33", "[issues.33]") {
+    tl::expected<void, int> res {tl::unexpect, 0};
+    REQUIRE(!res);    
+    res = res.map_error([](auto i) { return 42; });
+    REQUIRE(res.error() == 42);
+}

tl/expected.hpp

@@ -2053,6 +2053,7 @@ auto expected_map_impl(Exp &&exp, F &&f) -> expected<void, err_t<Exp>> {
 #if defined(TL_EXPECTED_CXX14) && !defined(TL_EXPECTED_GCC49) &&               \
     !defined(TL_EXPECTED_GCC54) && !defined(TL_EXPECTED_GCC55)
 template <class Exp, class F,
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,          
           class Ret = decltype(detail::invoke(std::declval<F>(),
                                               std::declval<Exp>().error())),
           detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
@@ -2064,6 +2065,7 @@ constexpr auto map_error_impl(Exp &&exp, F &&f) {
                                                std::forward<Exp>(exp).error()));
 }
 template <class Exp, class F,
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,                    
           class Ret = decltype(detail::invoke(std::declval<F>(),
                                               std::declval<Exp>().error())),
           detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
@@ -2076,8 +2078,35 @@ auto map_error_impl(Exp &&exp, F &&f) {
   detail::invoke(std::forward<F>(f), std::forward<Exp>(exp).error());
   return result(unexpect, monostate{});
 }
+template <class Exp, class F,
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr,          
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+constexpr auto map_error_impl(Exp &&exp, F &&f) {
+  using result = expected<exp_t<Exp>, detail::decay_t<Ret>>;
+  return exp.has_value()
+             ? result()
+             : result(unexpect, detail::invoke(std::forward<F>(f),
+                                               std::forward<Exp>(exp).error()));
+}
+template <class Exp, class F,
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr,                    
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+auto map_error_impl(Exp &&exp, F &&f) {
+  using result = expected<exp_t<Exp>, monostate>;
+  if (exp.has_value()) {
+    return result();
+  }
+
+  detail::invoke(std::forward<F>(f), std::forward<Exp>(exp).error());
+  return result(unexpect, monostate{});
+}    
 #else
 template <class Exp, class F,
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,                              
           class Ret = decltype(detail::invoke(std::declval<F>(),
                                               std::declval<Exp>().error())),
           detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
@@ -2092,6 +2121,7 @@ constexpr auto map_error_impl(Exp &&exp, F &&f)
 }
 
 template <class Exp, class F,
+          detail::enable_if_t<!std::is_void<exp_t<Exp>>::value> * = nullptr,                                        
           class Ret = decltype(detail::invoke(std::declval<F>(),
                                               std::declval<Exp>().error())),
           detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
@@ -2104,6 +2134,36 @@ auto map_error_impl(Exp &&exp, F &&f) -> expected<exp_t<Exp>, monostate> {
   detail::invoke(std::forward<F>(f), std::forward<Exp>(exp).error());
   return result(unexpect, monostate{});
 }
+
+template <class Exp, class F,
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr,                              
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<!std::is_void<Ret>::value> * = nullptr>
+constexpr auto map_error_impl(Exp &&exp, F &&f)
+    -> expected<exp_t<Exp>, detail::decay_t<Ret>> {
+  using result = expected<exp_t<Exp>, detail::decay_t<Ret>>;
+
+  return exp.has_value()
+             ? result()
+             : result(unexpect, detail::invoke(std::forward<F>(f),
+                                               std::forward<Exp>(exp).error()));
+}
+
+template <class Exp, class F,
+          detail::enable_if_t<std::is_void<exp_t<Exp>>::value> * = nullptr,                                        
+          class Ret = decltype(detail::invoke(std::declval<F>(),
+                                              std::declval<Exp>().error())),
+          detail::enable_if_t<std::is_void<Ret>::value> * = nullptr>
+auto map_error_impl(Exp &&exp, F &&f) -> expected<exp_t<Exp>, monostate> {
+  using result = expected<exp_t<Exp>, monostate>;
+  if (exp.has_value()) {
+    return result();
+  }
+
+  detail::invoke(std::forward<F>(f), std::forward<Exp>(exp).error());
+  return result(unexpect, monostate{});
+}    
 #endif
 
 #ifdef TL_EXPECTED_CXX14