Ticket #2793: arg_list.diff

boost/parameter/aux_/arg_list.hpp

@@ -25 +25 @@
 #include <boost/preprocessor/repetition/enum_params.hpp>
 #include <boost/preprocessor/repetition/enum_binary_params.hpp>
 #include <boost/preprocessor/facilities/intercept.hpp>
+#include <boost/utility/enable_if.hpp>
 
 namespace boost { namespace parameter {
 
@@ -187 +188 @@
       , typename TaggedArg::value_type
     >::type value_type;
 
+private:
+    // This metafunction is used to make a type dependent on a template
+    // parameter. This is required below to delay the instantiation of the
+    // return type of functions.
+    // Since `arg_list` is used for named template parameters, the `reference`
+    // type can't always be expected to be a type valid as a return type
+    // (it could legitimately be a function type or an array type). Unless
+    // delayed, the function signatures are instantiated even when the
+    // functions are not actually used (in the case of named template
+    // parameters) because of how the `arg_list` is implemented; by means
+    // of inheritance from `Next`.
+    //
+    // Note: In the code below, we sometimes use enable_if with is_same
+    //       when the signature is not really dependent on anything but
+    //       we still need to introduce a template parameter to have a
+    //       dependent return type.
+    template <class Dummy, class Type>
+    struct make_dependent_on {
+        typedef Type type;
+    };
+
+public:
+
     TaggedArg arg;      // Stores the argument
 
     // Store the arguments in successive nodes of this list
@@ -255 +279 @@
     // Helpers that handle the case when TaggedArg is
     // empty<T>.
     template <class D>
-    reference get_default(D const&, mpl::false_) const
+    typename make_dependent_on<D, reference>::type
+    get_default(D const&, mpl::false_) const
     {
         return arg.value;
     }
 
     template <class D>
-    reference get_default(D const& d, mpl::true_) const
+    typename make_dependent_on<D, reference>::type
+    get_default(D const& d, mpl::true_) const
     {
         return arg.value ? arg.value.get() : arg.value.construct(d.value);
     }
@@ -323 +349 @@
     // reached, indicating no matching argument was passed, the
     // default is returned, or if no default_ or lazy_default was
     // passed, compilation fails.
-    reference get(keyword<key_type> const&) const
+
+    // Note: See the comment @make_dependent_on to know the rationale behind
+    //       the use of enable_if with is_same instead of a straight overload.
+    template <class KeyType>
+    typename enable_if<is_same<KeyType, key_type>,
+    reference>::type get(keyword<KeyType> const&) const
     {
         BOOST_MPL_ASSERT_NOT((holds_maybe));
         return arg.value;
     }
 
     template <class Default>
-    reference get(default_<key_type,Default> const& d) const
+    typename make_dependent_on<Default, reference>::type
+    get(default_<key_type,Default> const& d) const
     {
         return get_default(d, holds_maybe());
     }
 
     template <class Default>
-    reference get(lazy_default<key_type, Default>) const
+    typename make_dependent_on<Default, reference>::type
+    get(lazy_default<key_type, Default>) const
     {
         return arg.value;
     }
 
 #else
 
-    reference operator[](keyword<key_type> const&) const
+    // Note: See the comment @make_dependent_on to know the rationale behind
+    //       the use of enable_if with is_same instead of a straight overload.
+    template <class KeyType>
+    typename enable_if<is_same<KeyType, key_type>,
+    reference>::type operator[](keyword<KeyType> const&) const
     {
         BOOST_MPL_ASSERT_NOT((holds_maybe));
         return arg.value;
     }
 
     template <class Default>
-    reference operator[](default_<key_type, Default> const& d) const
+    typename make_dependent_on<Default, reference>::type
+    operator[](default_<key_type, Default> const& d) const
     {
         return get_default(d, holds_maybe());
     }
 
     template <class Default>
-    reference operator[](lazy_default<key_type, Default>) const
+    typename make_dependent_on<Default, reference>::type
+    operator[](lazy_default<key_type, Default>) const
     {
         return arg.value;
     }