Re: [PATCH] libstdc++: Diagnose visitors with different return types [PR95904]

[Jonathan Wakely via Gcc-patches]

On 29/09/20 19:35 +0300, Ville Voutilainen via Libstdc++ wrote:

On Tue, 29 Sep 2020 at 14:20, Jonathan Wakely <jwak...@redhat.com> wrote:

I think this is what we want:

   template<typename _Tp, typename... _Types>
     constexpr inline __same_types = (is_same_v<_Tp, _Types> && ...);

is_same_v is very cheap, it uses the built-in directly, so you don't
need to instantiate any class templates at all.

>+
>+  template <unsigned long int _Idx, class _Visitor, class _Variant>

typename not class please.

>+    decltype(auto) __check_visitor_result(_Visitor&& __vis,

New line after the decltype(auto) please, not in the middle of the
parameter list.

Aye.

diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index dd8847cf829..6f647d622c4 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -182,7 +182,7 @@ namespace __variant
  // used for raw visitation with indices passed in
  struct __variant_idx_cookie { using type = __variant_idx_cookie; };
  // Used to enable deduction (and same-type checking) for std::visit:
-  template<typename> struct __deduce_visit_result { };
+  template<typename _Tp> struct __deduce_visit_result { using type = _Tp; };

  // Visit variants that might be valueless.
  template<typename _Visitor, typename... _Variants>
@@ -1017,7 +1017,22 @@ namespace __variant

      static constexpr auto
      _S_apply()
-      { return _Array_type{&__visit_invoke}; }
+      {
+       constexpr bool __visit_ret_type_mismatch =
+         _Array_type::__result_is_deduced::value
+         && !is_same_v<typename _Result_type::type,
+                       decltype(__visit_invoke(std::declval<_Visitor>(),
+                                               std::declval<_Variants>()...))>;
+       if constexpr (__visit_ret_type_mismatch)
+         {
+           static_assert(!__visit_ret_type_mismatch,
+                         "std::visit requires the visitor to have the same "
+                         "return type for all alternatives of a variant");
+           return __nonesuch{};
+         }
+       else
+         return _Array_type{&__visit_invoke};
+      }
    };

  template<typename _Result_type, typename _Visitor, typename... _Variants>
@@ -1692,6 +1707,26 @@ namespace __variant
                           std::forward<_Variants>(__variants)...);
    }

+  template<typename _Tp, typename... _Types>
+     constexpr inline bool __same_types = (is_same_v<_Tp, _Types> && ...);
+
+  template <unsigned long int _Idx, typename _Visitor, typename _Variant>
+    decltype(auto)
+    __check_visitor_result(_Visitor&& __vis, _Variant&& __variant)
+    {
+      return std::forward<_Visitor>(__vis)(
+        std::get<_Idx>(std::forward<_Variant>(__variant)));

Looks good, the new error is nice.

git apply warns about some whitespace errors:

/dev/shm/pr95904.diff:51: indent with spaces.
        std::get<_Idx>(std::forward<_Variant>(__variant)));
/dev/shm/pr95904.diff:73: indent with spaces.
        {
/dev/shm/pr95904.diff:77: indent with spaces.
                std::variant_size<remove_reference_t<_Variants>...>::value>());
/dev/shm/pr95904.diff:92: indent with spaces.
          std::forward<_Visitor>(__visitor),
warning: 4 lines add whitespace errors.

OK for trunk with those leading spaces switched to tab.

Thanks!