From 7e62f9005bffb19733d6fafee8380ee486ee1337 Mon Sep 17 00:00:00 2001
From: eido79 <eido79@gmail.com>
Date: Thu, 3 Nov 2022 19:06:32 +0100
Subject: [PATCH] [WIP] is_substitute for variant types

Fix #721, #722.
Experimental: use optional<variant>
---
 .../spirit/home/x3/operator/alternative.hpp   |  9 +++--
 .../home/x3/support/traits/is_substitute.hpp  | 16 +++++----
 .../traits/variant_find_substitute.hpp        |  2 +-
 .../support/traits/variant_has_substitute.hpp |  2 +-
 test/x3/alternative.cpp                       | 36 ++++++++++++++++---
 5 files changed, 49 insertions(+), 16 deletions(-)
 mode change 100644 => 100755 include/boost/spirit/home/x3/support/traits/is_substitute.hpp
 mode change 100644 => 100755 include/boost/spirit/home/x3/support/traits/variant_find_substitute.hpp
 mode change 100644 => 100755 include/boost/spirit/home/x3/support/traits/variant_has_substitute.hpp

diff --git a/include/boost/spirit/home/x3/operator/alternative.hpp b/include/boost/spirit/home/x3/operator/alternative.hpp
index 8d3ef26612..8cbc7b3a57 100644
--- a/include/boost/spirit/home/x3/operator/alternative.hpp
+++ b/include/boost/spirit/home/x3/operator/alternative.hpp
@@ -128,10 +128,13 @@ namespace boost { namespace spirit { namespace x3 { namespace detail
         >;
     };
 
-    template <template <typename...> typename A, template <typename...> typename Seq, typename T>
-    struct type_sequence_to_alternative_attribute<A, Seq<unused_type, T>>
+    template <template <typename...> typename A, template <typename...> typename Seq, typename T, typename... Ts>
+    struct type_sequence_to_alternative_attribute<A, Seq<unused_type, T, Ts...>>
     {
-        using type = boost::optional<T>;
+       using type = conditional_t<sizeof...(Ts) == 0,
+           boost::optional<T>,
+           boost::optional<A<T, Ts...>>
+       >;
     };
 
     template <template <typename...> class A, typename P, typename C>
diff --git a/include/boost/spirit/home/x3/support/traits/is_substitute.hpp b/include/boost/spirit/home/x3/support/traits/is_substitute.hpp
old mode 100644
new mode 100755
index 8f3da4d48a..3b8872ddec
--- a/include/boost/spirit/home/x3/support/traits/is_substitute.hpp
+++ b/include/boost/spirit/home/x3/support/traits/is_substitute.hpp
@@ -70,13 +70,17 @@ namespace boost { namespace spirit { namespace x3 { namespace traits
             >::type>
           : mpl::true_ {};
 
+        // Can T substitute a Variant<...>?
         template <typename T, typename Attribute>
-        struct is_substitute_impl<T, Attribute,
-            typename enable_if<
-                is_variant<Attribute>
-            >::type>
-          : variant_has_substitute<Attribute, T>
-        {};
+        struct is_substitute_impl<T, Attribute, typename enable_if<is_variant<Attribute>>::type>
+        {
+            // That is:
+            // boost::variant<X, Y, Z> t;
+            // boost::variant<Y, X> attribute;
+            // t = attribute;
+            // or better: move_to(attribute, t) should work
+            using type = mpl::bool_<std::is_convertible_v<Attribute, T>>;
+        };
     }
 
     template <typename T, typename Attribute, typename Enable /*= void*/>
diff --git a/include/boost/spirit/home/x3/support/traits/variant_find_substitute.hpp b/include/boost/spirit/home/x3/support/traits/variant_find_substitute.hpp
old mode 100644
new mode 100755
index e95ba0e655..69f0c8de29
--- a/include/boost/spirit/home/x3/support/traits/variant_find_substitute.hpp
+++ b/include/boost/spirit/home/x3/support/traits/variant_find_substitute.hpp
@@ -28,7 +28,7 @@ namespace boost { namespace spirit { namespace x3 { namespace traits
         typedef typename
             mpl::eval_if<
                 is_same<iter_1, end>,
-                mpl::find_if<types, traits::is_substitute<T, mpl::_1> >,
+                mpl::find_if<types, traits::is_substitute<mpl::_1, T>>,
                 mpl::identity<iter_1>
             >::type
         iter;
diff --git a/include/boost/spirit/home/x3/support/traits/variant_has_substitute.hpp b/include/boost/spirit/home/x3/support/traits/variant_has_substitute.hpp
old mode 100644
new mode 100755
index 440947b520..3b1995f6a5
--- a/include/boost/spirit/home/x3/support/traits/variant_has_substitute.hpp
+++ b/include/boost/spirit/home/x3/support/traits/variant_has_substitute.hpp
@@ -28,7 +28,7 @@ namespace boost { namespace spirit { namespace x3 { namespace traits
         typedef typename
             mpl::eval_if<
                 is_same<iter_1, end>,
-                mpl::find_if<types, traits::is_substitute<T, mpl::_1>>,
+                mpl::find_if<types, traits::is_substitute<mpl::_1, T>>,
                 mpl::identity<iter_1>
             >::type
         iter;
diff --git a/test/x3/alternative.cpp b/test/x3/alternative.cpp
index be1af4faf1..13c0a0ddfd 100644
--- a/test/x3/alternative.cpp
+++ b/test/x3/alternative.cpp
@@ -10,6 +10,7 @@
 #include <boost/variant.hpp>
 #include <boost/fusion/include/vector.hpp>
 #include <boost/fusion/include/at.hpp>
+#include <boost/fusion/include/std_pair.hpp>
 
 #include <string>
 #include <iostream>
@@ -304,14 +305,14 @@ main()
         BOOST_TEST(test_attr("abaabb", +('a' >> attr(Foo{}) | 'b' >> attr(int{})), x));
     }
 
-    {   // compile checks
+    { // Compile time attribute checks
         using namespace boost::spirit::x3;
 
         test_attribute_of_alternative<boost::variant<char, int>>(char_ | int_);
         test_attribute_of_alternative<boost::variant<char, int, double>>(char_ | int_ | double_);
-        test_attribute_of_alternative<boost::variant<unused_type, char, int, double>>(char_ | int_ | double_ | eps);
-        test_attribute_of_alternative<boost::variant<unused_type, char, int, double>>(char_ | int_ | double_ | eps | int_);
-        test_attribute_of_alternative<boost::variant<unused_type, char, int, double>>(char_ | int_ | double_ | eps | int_ | eps);
+        test_attribute_of_alternative<boost::optional<boost::variant<char, int, double>>>(char_ | int_ | double_ | eps);
+        test_attribute_of_alternative<boost::optional<boost::variant<char, int, double>>>(char_ | int_ | double_ | eps | int_);
+        test_attribute_of_alternative<boost::optional<boost::variant<char, int, double>>>(char_ | int_ | double_ | eps | int_ | eps);
         test_attribute_of_alternative<unused_type>(eps);
         test_attribute_of_alternative<boost::optional<int>>(eps | int_);
         test_attribute_of_alternative<boost::optional<int>>(eps | int_);
@@ -320,9 +321,34 @@ main()
         test_attribute_of_alternative<int>(int_);
 
         test_attribute_of_alternative<boost::variant<boost::fusion::deque<int, int>, char>>((int_ >> int_) | char_);
-        test_attribute_of_alternative<boost::variant<unused_type, char, boost::fusion::deque<int, int>>>(char_ | (int_ >> int_) | eps);
+        test_attribute_of_alternative<boost::optional<boost::variant<char, boost::fusion::deque<int, int>>>>(char_ | (int_ >> int_) | eps);
         test_attribute_of_alternative<boost::optional<boost::fusion::deque<int, int>>>(eps | (int_ >> int_));
     }
 
+    { // boost variant order of parameters relevant in 1.78.0 ? #721
+        struct A {};
+        struct B {};
+        struct C {};
+
+        using namespace boost::spirit::x3;
+        char const* s = "";
+        boost::variant<std::pair<int, boost::variant<B, A>>, C> target;
+        auto expr = attr(int{}) >> (attr(A{}) | attr(B{})) | attr(C{});
+        BOOST_TEST(test_attr(s, expr, target));
+    }
+
+    { // A | A not being simplified to A in 1.78.0 #722
+        struct A {};
+        struct B {};
+        struct C {};
+
+        using namespace boost::spirit::x3;
+        char const* s = "";
+        using X = boost::variant<B, A>;
+        boost::variant<std::pair<A, X>, C> target;
+        auto expr = ((attr(A{}) | attr(A{})) >> lit("=") >> (attr(A{}) | attr(B{}))) | attr(C{});
+        BOOST_TEST(test_attr(s, expr, target));
+    }
+
     return boost::report_errors();
 }