🐛 fix for #586

Also added a Travis builder with -std=c++1z

.travis.yml

@@ -177,6 +177,16 @@ matrix:
         sources: ['ubuntu-toolchain-r-test']
         packages: g++-6
 
+  - os: linux
+    compiler: gcc
+    env:
+      - COMPILER=g++-6
+      - CXXFLAGS=-std=c++1z
+    addons:
+      apt:
+        sources: ['ubuntu-toolchain-r-test']
+        packages: g++-6
+
   # Linux / Clang
 
   - os: linux

src/json.hpp

@@ -3742,7 +3742,7 @@ class basic_json
 #ifndef _MSC_VER  // fix for issue #167 operator<< ambiguity under VS2015
                    and not std::is_same<ValueType, std::initializer_list<typename string_t::value_type>>::value
 #endif
-#if defined(_MSC_VER) && _MSC_VER >1900 && defined(_HAS_CXX17) && _HAS_CXX17 == 1 // fix for issue #464
+#if (defined(__cplusplus) && __cplusplus == 201703L) || (defined(_MSC_VER) && _MSC_VER >1900 && defined(_HAS_CXX17) && _HAS_CXX17 == 1) // fix for issue #464
                    and not std::is_same<ValueType, typename std::string_view>::value
 #endif
                    , int >::type = 0 >