Merge pull request #71 from grisumbras/feature/mp_valid_and_true

implement mp_valid_and_true

doc/mp11/utility.adoc

@@ -188,6 +188,20 @@ template<class... T> using first_or_void =
 
 Like `mp_eval_or`, but takes a quoted metafunction.
 
+## mp_valid_and_true<F, T...>
+
+    template<template<class...> class F, class... T> using mp_valid_and_true =
+        mp_eval_or<mp_false, F, T...>;
+
+`mp_valid_and_true<F, T...>` is an alias for `F<T...>` when this expression is valid, for `mp_false` otherwise.
+
+## mp_valid_and_true_q<Q, T...>
+
+    template<class Q, class... T> using mp_valid_and_true_q =
+        mp_valid_and_true<Q::template fn, T...>;
+
+Like `mp_valid_and_true`, but takes a quoted metafunction.
+
 ## mp_cond<C, T, R...>
 
     template<class C, class T, class... R> using mp_cond = /*...*/;

include/boost/mp11/utility.hpp

@@ -161,6 +161,10 @@ template<class C, class T, class Q, class... U> using mp_eval_if_not_q = mp_eval
 template<class T, template<class...> class F, class... U> using mp_eval_or = mp_eval_if_not<mp_valid<F, U...>, T, F, U...>;
 template<class T, class Q, class... U> using mp_eval_or_q = mp_eval_or<T, Q::template fn, U...>;
 
+// mp_valid_and_true
+template<template<class...> class F, class... T> using mp_valid_and_true = mp_eval_or<mp_false, F, T...>;
+template<class Q, class... T> using mp_valid_and_true_q = mp_valid_and_true<Q::template fn, T...>;
+
 // mp_cond
 
 // so elegant; so doesn't work

test/Jamfile

@@ -149,6 +149,7 @@ run mp_not_fn.cpp ;
 run mp_eval_if_not.cpp ;
 run mp_eval_or.cpp ;
 run mp_compose.cpp ;
+run mp_valid_and_true.cpp ;
 
 # integer_sequence
 run integer_sequence.cpp ;

test/mp_valid_and_true.cpp

@@ -0,0 +1,48 @@
+// Copyright 2022 Dmitry Arkhipov (grisumbras@gmail.com)
+//
+// Distributed under the Boost Software License, Version 1.0.
+//
+// See accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt
+
+
+#include <boost/mp11/utility.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+#include <cstddef>
+
+template<class T>
+using has_size_t_difference_type
+    = std::is_same<typename T::difference_type, std::size_t>;
+
+struct X
+{
+};
+
+struct Y
+{
+    using difference_type = int;
+};
+
+struct Z
+{
+    using difference_type = std::size_t;
+};
+
+int main()
+{
+    using boost::mp11::mp_valid_and_true;
+    using boost::mp11::mp_valid_and_true_q;
+    using boost::mp11::mp_quote;
+
+    BOOST_TEST_TRAIT_FALSE((mp_valid_and_true<has_size_t_difference_type, X>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid_and_true<has_size_t_difference_type, Y>));
+    BOOST_TEST_TRAIT_TRUE((mp_valid_and_true<has_size_t_difference_type, Z>));
+
+    using Q_size_t_diff = mp_quote<has_size_t_difference_type>;
+
+    BOOST_TEST_TRAIT_FALSE((mp_valid_and_true_q<Q_size_t_diff, X>));
+    BOOST_TEST_TRAIT_FALSE((mp_valid_and_true_q<Q_size_t_diff, Y>));
+    BOOST_TEST_TRAIT_TRUE((mp_valid_and_true_q<Q_size_t_diff, Z>));
+
+    return boost::report_errors();
+}