Merge pull request #2540 from karzhenkov/disrupt-cxx17-tests

Travis doesn't run any tests in C++17 mode

.travis.yml

@@ -210,7 +210,7 @@ matrix:
     compiler: gcc
     env:
       - COMPILER=g++-9
-      - CXXFLAGS=-std=c++2a
+      - CXX_STANDARD=17
     addons:
       apt:
         sources: ['ubuntu-toolchain-r-test']
@@ -294,11 +294,11 @@ matrix:
     compiler: clang
     env:
       - COMPILER=clang++-7
-      - CXXFLAGS=-std=c++1z
+      - CXX_STANDARD=17
     addons:
       apt:
         sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-trusty-7']
-        packages: ['g++-6', 'clang-7', 'ninja-build']
+        packages: ['g++-7', 'clang-7', 'ninja-build']
 
 ################
 # build script #
@@ -321,6 +321,9 @@ script:
   # by default, use implicit conversions
   - if [[ "${IMPLICIT_CONVERSIONS}" == "" ]]; then export IMPLICIT_CONVERSIONS=ON; fi
 
+  # append CXX_STANDARD to CMAKE_OPTIONS if required
+  - CMAKE_OPTIONS+=${CXX_STANDARD:+ -DCMAKE_CXX_STANDARD=$CXX_STANDARD -DCMAKE_CXX_STANDARD_REQUIRED=ON}
+
   # compile and execute unit tests
   - mkdir -p build && cd build
   - cmake .. ${CMAKE_OPTIONS} -DJSON_MultipleHeaders=${MULTIPLE_HEADERS} -DJSON_ImplicitConversions=${IMPLICIT_CONVERSIONS} -DJSON_BuildTests=On -GNinja && cmake --build . --config Release

include/nlohmann/json.hpp

@@ -73,6 +73,10 @@ SOFTWARE.
 #include <nlohmann/json_fwd.hpp>
 #include <nlohmann/ordered_map.hpp>
 
+#if defined(JSON_HAS_CPP_17)
+    #include <string_view>
+#endif
+
 /*!
 @brief namespace for Niels Lohmann
 @see https://github.com/nlohmann

single_include/nlohmann/json.hpp

@@ -16663,6 +16663,10 @@ template <class Key, class T, class IgnoredLess = std::less<Key>,
 }  // namespace nlohmann
 
 
+#if defined(JSON_HAS_CPP_17)
+    #include <string_view>
+#endif
+
 /*!
 @brief namespace for Niels Lohmann
 @see https://github.com/nlohmann

test/src/unit-conversions.cpp

@@ -48,10 +48,6 @@ using nlohmann::json;
     #define JSON_HAS_CPP_14
 #endif
 
-#if defined(JSON_HAS_CPP_17)
-    #include <string_view>
-#endif
-
 TEST_CASE("value conversion")
 {
     SECTION("get an object (explicit)")