add a CI job for explicit conversions on every os

.travis.yml

@@ -145,6 +145,11 @@ matrix:
   - os: osx
     osx_image: xcode12
 
+  - os: osx
+    osx_image: xcode12
+    env:
+      - IMPLICIT_CONVERSIONS=OFF
+
   # Linux / GCC
 
   - os: linux
@@ -203,6 +208,16 @@ matrix:
         sources: ['ubuntu-toolchain-r-test']
         packages: ['g++-9', 'ninja-build']
 
+  - os: linux
+    compiler: gcc
+    env:
+      - COMPILER=g++-9
+      - IMPLICIT_CONVERSIONS=OFF
+    addons:
+      apt:
+        sources: ['ubuntu-toolchain-r-test']
+        packages: ['g++-9', 'ninja-build']
+
   - os: linux
     compiler: gcc
     env:
@@ -315,10 +330,12 @@ script:
   - if [[ "${COMPILER}" != "" ]]; then export CXX=${COMPILER}; fi
   # by default, use the single-header version
   - if [[ "${MULTIPLE_HEADERS}" == "" ]]; then export MULTIPLE_HEADERS=OFF; fi
+  # by default, use implicit conversions
+  - if [[ "${IMPLICIT_CONVERSIONS}" == "" ]]; then export IMPLICIT_CONVERSIONS=ON; fi
 
   # compile and execute unit tests
   - mkdir -p build && cd build
-  - cmake .. ${CMAKE_OPTIONS} -DJSON_MultipleHeaders=${MULTIPLE_HEADERS} -DJSON_BuildTests=On -GNinja && cmake --build . --config Release
+  - cmake .. ${CMAKE_OPTIONS} -DJSON_MultipleHeaders=${MULTIPLE_HEADERS} -DJSON_ImplicitConversions=${IMPLICIT_CONVERSIONS} -DJSON_BuildTests=On -GNinja && cmake --build . --config Release
   - ctest -C Release --timeout 2700 -V -j
   - cd ..