Report warnings as errors on Travis and AppVeyor

.appveyor.yml

@@ -28,6 +28,6 @@ install:
     7z x 3.2.9.zip -y > $null
     $env:CMAKE_INCLUDE_PATH = "eigen-eigen-dc6cfdf9bcec"
 build_script:
-- cmake -A "%CMAKE_ARCH%"
+- cmake -A "%CMAKE_ARCH%" -DPYBIND11_WERROR=ON
 - set MSBuildLogger="C:\Program Files\AppVeyor\BuildAgent\Appveyor.MSBuildLogger.dll"
 - cmake --build . --config Release --target check -- /v:m /logger:%MSBuildLogger%

.travis.yml

@@ -86,7 +86,10 @@ install:
     export CMAKE_EXTRA_ARGS="${CMAKE_EXTRA_ARGS} -DCMAKE_INCLUDE_PATH=eigen-eigen-dc6cfdf9bcec"
   fi
 script:
-- $SCRIPT_RUN_PREFIX cmake ${CMAKE_EXTRA_ARGS} -DPYBIND11_PYTHON_VERSION=$PYTHON -DPYBIND11_CPP_STANDARD=-std=c++$CPP
+- $SCRIPT_RUN_PREFIX cmake ${CMAKE_EXTRA_ARGS}
+    -DPYBIND11_PYTHON_VERSION=$PYTHON
+    -DPYBIND11_CPP_STANDARD=-std=c++$CPP
+    -DPYBIND11_WERROR=ON
 - $SCRIPT_RUN_PREFIX make CTEST_OUTPUT_ON_FAILURE=TRUE check -j 2
 after_script:
 - if [ -n "$DOCKER" ]; then docker stop "$containerid"; docker rm "$containerid"; fi

CMakeLists.txt

@@ -17,6 +17,7 @@ endif()
 
 option(PYBIND11_INSTALL "Install pybind11 header files?" ${PYBIND11_MASTER_PROJECT})
 option(PYBIND11_TEST    "Build pybind11 test suite?"     ${PYBIND11_MASTER_PROJECT})
+option(PYBIND11_WERROR  "Report all warnings as errors"  OFF)
 
 # Add a CMake parameter for choosing a desired Python version
 set(PYBIND11_PYTHON_VERSION "" CACHE STRING "Python version to use for compiling the example application")
@@ -142,6 +143,14 @@ function(pybind11_enable_warnings target_name)
   else()
     target_compile_options(${target_name} PRIVATE -Wall -Wextra -Wconversion)
   endif()
+
+  if(PYBIND11_WERROR)
+    if(MSVC)
+        target_compile_options(${target_name} PRIVATE /WX)
+    else()
+        target_compile_options(${target_name} PRIVATE -Werror)
+    endif()
+  endif()
 endfunction()
 
 if (PYBIND11_TEST)