Fix #122 by adding an extra compile check in universal ctor (via @lig…

…htmare) + test case

Makefile

@@ -61,7 +61,7 @@ out/%.o: test/t/%.cpp Makefile $(ALL_HEADERS)
 	mkdir -p ./out
 	$(CXX) -c -o $@ $< -Iinclude -Itest/include $(DEBUG_FLAGS) $(COMMON_FLAGS) $(CXXFLAGS)
 
-out/unit: out/unit.o out/binary_visitor_1.o out/binary_visitor_2.o out/binary_visitor_3.o out/binary_visitor_4.o out/binary_visitor_5.o out/binary_visitor_6.o out/issue21.o out/mutating_visitor.o out/optional.o out/recursive_wrapper.o out/sizeof.o out/unary_visitor.o out/variant.o
+out/unit: out/unit.o out/binary_visitor_1.o out/binary_visitor_2.o out/binary_visitor_3.o out/binary_visitor_4.o out/binary_visitor_5.o out/binary_visitor_6.o out/issue21.o out/issue122.o out/mutating_visitor.o out/optional.o out/recursive_wrapper.o out/sizeof.o out/unary_visitor.o out/variant.o
 	mkdir -p ./out
 	$(CXX) -o $@ $^ $(LDFLAGS)
 

include/mapbox/variant.hpp

@@ -570,7 +570,7 @@ class variant
 
     // http://isocpp.org/blog/2012/11/universal-references-in-c11-scott-meyers
     template <typename T, typename Traits = detail::value_traits<T, Types...>,
-              typename Enable = typename std::enable_if<Traits::is_valid>::type>
+              typename Enable = typename std::enable_if<Traits::is_valid && !std::is_same<variant<Types...>, typename Traits::value_type>::value>::type >
     VARIANT_INLINE variant(T&& val) noexcept(std::is_nothrow_constructible<typename Traits::target_type, T&&>::value)
         : type_index(Traits::index)
     {

test/t/issue122.cpp

@@ -0,0 +1,20 @@
+#include "catch.hpp"
+
+#include <mapbox/variant.hpp>
+#include <mapbox/variant_io.hpp>
+
+// https://github.com/mapbox/variant/issues/122
+
+struct X
+{
+    template <typename ValueType>
+    X(const ValueType&)  {}
+};
+
+
+TEST_CASE("Correctly choose appropriate constructor", "[variant]")
+{
+    mapbox::util::variant<X, int> a{123};
+    decltype(a) b(a);
+    REQUIRE(a.which() == b.which());
+}