Provide a -Werror mode and ensure xtensor passes with it

Conditionally comment out the few test that raise a warning by design,
explaining why in each case.

Author: serge-sans-paille

test/CMakeLists.txt

@@ -64,6 +64,8 @@ if(NOT _cxx_std_flag)
   message(FATAL_ERROR "xtensor-blas needs a C++14-compliant compiler.")
 endif()
 
+OPTION(XTENSOR_ENABLE_WERROR "Turn on -Werror" OFF)
+
 if(CMAKE_CXX_COMPILER_ID MATCHES "GNU" OR (CMAKE_CXX_COMPILER_ID MATCHES "Intel" AND NOT WIN32))
   CHECK_CXX_COMPILER_FLAG(-march=native arch_native_supported)
   if(arch_native_supported AND NOT CMAKE_CXX_FLAGS MATCHES "-march")
@@ -74,6 +76,9 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "GNU" OR (CMAKE_CXX_COMPILER_ID MATCHES "Intel"
   if (XTENSOR_DISABLE_EXCEPTIONS)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-exceptions")
   endif()
+  if (XTENSOR_ENABLE_WERROR)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror -DSKIP_ON_WERROR")
+  endif()
 elseif(CMAKE_CXX_COMPILER_ID MATCHES "MSVC")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${_cxx_std_flag} /MP /bigobj")
   set(CMAKE_EXE_LINKER_FLAGS ${CMAKE_EXE_LINKER_FLAGS} /MANIFEST:NO)
@@ -95,6 +100,9 @@ elseif(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
     if (XTENSOR_DISABLE_EXCEPTIONS)
       set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-exceptions")
     endif()
+    if (XTENSOR_ENABLE_WERROR)
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror -DSKIP_ON_WERROR")
+    endif()
   else() # We are using clang-cl
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${_cxx_std_flag} /bigobj")
     set(CMAKE_EXE_LINKER_FLAGS ${CMAKE_EXE_LINKER_FLAGS} /MANIFEST:NO)

test/test_xnan_functions.cpp

@@ -268,15 +268,21 @@ namespace xt
         /**********************
          * long long *
          **********************/
+#ifndef SKIP_ON_WERROR
+        // intermediate computation done in double may imply precision loss
         CHECK_RESULT_TYPE_FOR_ALL(along, signed long long);
+#endif
         CHECK_TEMPLATED_RESULT_TYPE_FOR_ALL(along, int, signed long long, double);
         CHECK_TEMPLATED_RESULT_TYPE_FOR_ALL(along, long, signed long long, double);
         CHECK_TEMPLATED_RESULT_TYPE_FOR_ALL(along, signed long long, signed long long, double);
 
         /**********************
          * unsigned long long *
          **********************/
+#ifndef SKIP_ON_WERROR
+        // intermediate computation done in double may imply precision loss
         CHECK_RESULT_TYPE_FOR_ALL(aulong, unsigned long long);
+#endif
         CHECK_TEMPLATED_RESULT_TYPE_FOR_ALL(aulong, unsigned int, unsigned long long, double);
         CHECK_TEMPLATED_RESULT_TYPE_FOR_ALL(aulong, unsigned long, unsigned long long, double);
         CHECK_TEMPLATED_RESULT_TYPE_FOR_ALL(aulong, unsigned long long, unsigned long long, double);
@@ -285,7 +291,10 @@ namespace xt
          * float *
          *********/
         CHECK_RESULT_TYPE_FOR_ALL(afloat, float);
+#ifndef SKIP_ON_WERROR
+        // final conversion to int may imply conversion loss
         CHECK_TEMPLATED_RESULT_TYPE_FOR_ALL(afloat, int, float, float);
+#endif
         CHECK_TEMPLATED_RESULT_TYPE_FOR_ALL(afloat, float, float, float);
         CHECK_TEMPLATED_RESULT_TYPE_FOR_ALL(afloat, double, double, double);
 

test/test_xreducer.cpp

@@ -544,8 +544,12 @@ namespace xt
         EXPECT_TRUE(all(equal(mean0, expect0)));
         EXPECT_TRUE(all(equal(mean1, expect1)));
 
+#ifndef SKIP_ON_WERROR
+        // may loose precision because uint8_t is casted to long for intermediate
+        // computations and then divided by a double for mean
         xarray<uint8_t> c = {1, 2};
         EXPECT_EQ(mean(c)(), 1.5);
+#endif
 
         const auto rvalue_xarray = [] () { return xtensor<double, 1>({1, 2}); };
         EXPECT_EQ(mean(rvalue_xarray(), {0})(), 1.5);