Use std::void_t rather than shipping another one

stdc::void_t was a template alias of the kind that exists to make SFINAE
work, and C++17 has it in <type_traits>, which algorithms.h already
includes. The two traits that used it use the standard one now and the
alias is gone, which takes it out of the documentation as a consequence
rather than by hiding it.

Hiding it was the first attempt, and worth recording why it did not work.
It went in EXCLUDE_SYMBOLS, the containers topic page still showed it,
and the reason I did not notice is that I read the Classes list, where a
template alias is not: it was under Typedefs. Reading the list to the end
would have said so.

Windows 371, Linux 366, docs clean.

Author: SineStriker

cmake/doxygen.cmake

@@ -88,7 +88,7 @@ SORT_MEMBER_DOCS       = NO
 # is read as two patterns, and the second one matched enough to leave the index with nothing in
 # it at all. Silently, and with no warning from Doxygen. Hence *enable_if* here, which names the
 # same two specializations without a space.
-EXCLUDE_SYMBOLS        = *::detail *::detail::* stdc::str::conv* *enable_if* stdc::is_map stdc::has_key_type* stdc::has_mapped_type* stdc::void_t stdc::vlarray_base stdc::flag stdc::incompatible_flag stdc::winapi::kernel32 stdc::winapi::user32
+EXCLUDE_SYMBOLS        = *::detail *::detail::* stdc::str::conv* *enable_if* stdc::is_map stdc::has_key_type* stdc::has_mapped_type* stdc::vlarray_base stdc::flag stdc::incompatible_flag stdc::winapi::kernel32 stdc::winapi::user32
 
 # Doxygen does not see the configure step, so the switches are spelled out here. Without them
 # the export attribute reads as part of every class name, and the platform headers document

include/stdcorelib/stlextra/algorithms.h

@@ -13,20 +13,17 @@ namespace stdc {
     /// \addtogroup containers
     /// @{
 
-    template <typename...>
-    using void_t = void;
-
     template <typename, typename = void>
     struct has_key_type : std::false_type {};
 
     template <typename T>
-    struct has_key_type<T, void_t<typename T::key_type>> : std::true_type {};
+    struct has_key_type<T, std::void_t<typename T::key_type>> : std::true_type {};
 
     template <typename, typename = void>
     struct has_mapped_type : std::false_type {};
 
     template <typename T>
-    struct has_mapped_type<T, void_t<typename T::mapped_type>> : std::true_type {};
+    struct has_mapped_type<T, std::void_t<typename T::mapped_type>> : std::true_type {};
 
     template <typename T>
     struct is_map : std::conjunction<has_key_type<T>, has_mapped_type<T>> {};