Merge branch 'develop' of https://github.com/nlohmann/json into feature/optional

 Conflicts:

	test/src/unit-conversions.cpp

.travis.yml

@@ -124,18 +124,6 @@ matrix:
 
   # OSX / Clang
 
-  - os: osx
-    osx_image: xcode9.3
-
-  - os: osx
-    osx_image: xcode9.4
-
-  - os: osx
-    osx_image: xcode10
-
-  - os: osx
-    osx_image: xcode10.1
-
   - os: osx
     osx_image: xcode10.2
 
@@ -222,7 +210,7 @@ matrix:
     compiler: gcc
     env:
       - COMPILER=g++-9
-      - CXXFLAGS=-std=c++2a
+      - CXX_STANDARD=17
     addons:
       apt:
         sources: ['ubuntu-toolchain-r-test']
@@ -306,11 +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 #
@@ -334,6 +322,9 @@ 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