Fix setting the C++ standard for Travis builds

.travis.yml

@@ -206,7 +206,7 @@ matrix:
     compiler: gcc
     env:
       - COMPILER=g++-8
-      - CXXFLAGS=-std=c++17
+      - CMAKE_OPTIONS=-DJSON_ForceCpp17=ON
     addons:
       apt:
         sources: ['ubuntu-toolchain-r-test']
@@ -290,7 +290,7 @@ matrix:
     compiler: clang
     env:
       - COMPILER=clang++-7
-      - CXXFLAGS=-std=c++1z
+      - CMAKE_OPTIONS=-DJSON_ForceCpp17=ON
     addons:
       apt:
         sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-7']

include/nlohmann/json.hpp

@@ -72,6 +72,10 @@ SOFTWARE.
 #include <nlohmann/detail/value_t.hpp>
 #include <nlohmann/json_fwd.hpp>
 
+#if defined(JSON_HAS_CPP_17)
+    #include <string_view>
+#endif
+
 /*!
 @brief namespace for Niels Lohmann
 @see https://github.com/nlohmann
@@ -2863,9 +2867,9 @@ 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
-#if defined(JSON_HAS_CPP_17) && (defined(__GNUC__) || (defined(_MSC_VER) and _MSC_VER <= 1914))
-                   and not std::is_same<ValueType, typename std::string_view>::value
 #endif
+#if defined(JSON_HAS_CPP_17)
+                   and not std::is_same<ValueType, typename std::string_view>::value
 #endif
                    and detail::is_detected<detail::get_template_function, const basic_json_t&, ValueType>::value
                    , int >::type = 0 >

single_include/nlohmann/json.hpp

@@ -12841,6 +12841,10 @@ class serializer
 // #include <nlohmann/json_fwd.hpp>
 
 
+#if defined(JSON_HAS_CPP_17)
+    #include <string_view>
+#endif
+
 /*!
 @brief namespace for Niels Lohmann
 @see https://github.com/nlohmann
@@ -15632,9 +15636,9 @@ 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
-#if defined(JSON_HAS_CPP_17) && (defined(__GNUC__) || (defined(_MSC_VER) and _MSC_VER <= 1914))
-                   and not std::is_same<ValueType, typename std::string_view>::value
 #endif
+#if defined(JSON_HAS_CPP_17)
+                   and not std::is_same<ValueType, typename std::string_view>::value
 #endif
                    and detail::is_detected<detail::get_template_function, const basic_json_t&, ValueType>::value
                    , int >::type = 0 >

test/CMakeLists.txt

@@ -2,6 +2,7 @@ option(JSON_Sanitizer "Build test suite with Clang sanitizer" OFF)
 option(JSON_Valgrind "Execute test suite with Valgrind" OFF)
 option(JSON_NoExceptions "Build test suite without exceptions" OFF)
 option(JSON_Coverage "Build test suite with coverage information" OFF)
+option(JSON_ForceCpp17 "Use C++17 standard" OFF)
 
 if(JSON_Sanitizer)
     message(STATUS "Building test suite with Clang sanitizer")
@@ -85,7 +86,7 @@ if(MSVC)
 	# Disable warning C4566: character represented by universal-character-name '\uFF01' cannot be represented in the current code page (1252)
 	# Disable warning C4996: 'nlohmann::basic_json<std::map,std::vector,std::string,bool,int64_t,uint64_t,double,std::allocator,nlohmann::adl_serializer>::operator <<': was declared deprecated
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /wd4389 /wd4309 /wd4566 /wd4996")
-	
+
 	# https://github.com/nlohmann/json/issues/1114
 	set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} /bigobj")
 endif()
@@ -104,6 +105,15 @@ foreach(file ${files})
       CATCH_CONFIG_FAST_COMPILE
       $<$<CXX_COMPILER_ID:MSVC>:_SCL_SECURE_NO_WARNINGS>
     )
+    if (${CMAKE_VERSION} VERSION_LESS "3.8.0")
+        if (JSON_ForceCpp17)
+            message(FATAL_ERROR "Cannot force C++17 with this CMake version, you need at least version 3.8")
+        endif()
+    else()
+        if (JSON_ForceCpp17)
+            target_compile_features(${testcase} PRIVATE cxx_std_17)
+        endif()
+    endif()
     target_compile_options(${testcase} PRIVATE
         $<$<CXX_COMPILER_ID:MSVC>:/EHsc;$<$<CONFIG:Release>:/Od>>
         $<$<NOT:$<CXX_COMPILER_ID:MSVC>>:-Wno-deprecated;-Wno-float-equal>

test/src/unit-conversions.cpp

@@ -51,6 +51,20 @@ using nlohmann::json;
     #include <string_view>
 #endif
 
+template<typename From, typename To>
+struct is_implicitly_convertible
+{
+  private:
+    template <typename C>
+    static std::true_type test(decltype(&C::operator To));
+
+    template <typename C>
+    static std::false_type test(...);
+
+  public:
+    static constexpr bool value = std::is_same<std::true_type, decltype(test<From>(nullptr))>::value;
+};
+
 TEST_CASE("value conversion")
 {
     SECTION("get an object (explicit)")
@@ -480,8 +494,7 @@ TEST_CASE("value conversion")
 #if defined(JSON_HAS_CPP_17)
         SECTION("std::string_view")
         {
-            std::string_view s = j;
-            CHECK(json(s) == j);
+            static_assert(!is_implicitly_convertible<decltype(j), std::string_view>::value);
         }
 #endif