tagged<user-defined type> does not work

[CaseyCarter]

Since tagged getters use qualified std::get to access the fields of Base and users are forbidden to overload std::get. A possible solution is to use ADL to find get function templates in the user's namespace(s) as implemented in range-v3's tagged.

Proposed Resolution

This issue is resolved by the PR for #363.

[CaseyCarter]

SUPERSEDED

(This PR also resolves #418.)

Modify [taggedtup.tagged]/4 as follows:

 A tagged getter class with DerivedCharacteristic D, ElementIndex N, and ElementName "name" shall
 provide the following interface:

   struct __TAGGED_GETTER {
-    constexpr decltype(auto) name() & { return get<N>(static_cast<D&>(*this)); }
+    constexpr decltype(auto) name() &
+      noexcept(noexcept(ADL::GET<N>(static_cast<D&>(*this))))
+    {
+      return ADL::GET<N>(static_cast<D&>(*this));
+    }
-    constexpr decltype(auto) name() const & { return get<N>(static_cast<const D&>(*this)); }
+    constexpr decltype(auto) name() const &
+      noexcept(noexcept(ADL::GET<N>(static_cast<const D&>(*this))))
+    {
+      return ADL::GET<N>(static_cast<const D&>(*this));
+    }
-    constexpr decltype(auto) name() && { return get<N>(static_cast<D&&>(*this)); }
+    constexpr decltype(auto) name() &&
+      noexcept(noexcept(ADL::GET<N>(static_cast<D&&>(*this))))
+    {
+      return ADL::GET<N>(static_cast<D&&>(*this));
+    }
+    constexpr decltype(auto) name() const &&
+      noexcept(noexcept(ADL::GET<N>(static_cast<const D&&>(*this))))
+    {
+      return ADL::GET<N>(static_cast<const D&&>(*this));
+    }
   };

+Where ADL::GET refers to the exposition-only function template defined in an 
+exposition-only sub-namespace:
+
+  namespace ADL {
+    using std::get;
+
+    template<size_t I, class T>
+    constexpr decltype(auto) GET(T&& t)
+      noexcept(noexcept(get<N>(std::forward<T>(t))))
+   {
+      return get<N>(std::forward<T>(t));
+   }
+ }

Also strike the redundant and now incorrect example [alg.tagspec]/3:

-3 [Example: tag::in is a type such that TAGGET(D, tag::in, N) names a type [...]

[ericniebler]

Looks good.

[CaseyCarter]

LEWG has killed tagged for C++20 - closing this as NAD.