Enable implicit fallthrough warning and fix examples of (google#747)

Looking at the changes that were needed, I am pretty sure this warning
caught a real bug in the code.

Fixes google#746

CMakeLists.txt

@@ -33,7 +33,15 @@ option(SHADERC_ENABLE_NV_EXTENSIONS "Enables support of Nvidia-specific extensio
 include(GNUInstallDirs)
 include(cmake/setup_build.cmake)
 include(cmake/utils.cmake)
-
+include(CheckCXXCompilerFlag)
+
+# This flag is not supported on Windows and some older version of GCC
+# that our bots use.
+unset(COMPILER_SUPPORTS_FALLTHROUGH_WARNING CACHE)
+check_cxx_compiler_flag(-Wimplicit-fallthrough COMPILER_SUPPORTS_FALLTHROUGH_WARNING)
+if (COMPILER_SUPPORTS_FALLTHROUGH_WARNING)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wimplicit-fallthrough")
+endif()
 
 if(SHADERC_ENABLE_NV_EXTENSIONS)
     add_definitions(-DNV_EXTENSIONS)

libshaderc_spvc/src/spvc.cc

@@ -173,8 +173,10 @@ void shaderc_spvc_compile_options_set_msl_platform(
   switch (platform) {
     case shaderc_spvc_msl_platform_ios:
       options->msl.platform = spirv_cross::CompilerMSL::Options::iOS;
+    break;
     case shaderc_spvc_msl_platform_macos:
       options->msl.platform = spirv_cross::CompilerMSL::Options::macOS;
+    break;
   }
 }
 

spvc/test/known_failures

@@ -29,20 +29,12 @@ shaders-msl/frag/image-query-lod.msl22.frag,False
 shaders-msl/frag/image-query-lod.msl22.frag,True
 shaders-msl/frag/nonuniform-qualifier.msl2.frag,False
 shaders-msl/frag/nonuniform-qualifier.msl2.frag,True
-shaders-msl/frag/sampler-compare-cascade-gradient.ios.frag,False
-shaders-msl/frag/sampler-compare-cascade-gradient.ios.frag,True
 shaders-msl/frag/shadow-compare-global-alias.invalid.frag,False
 shaders-msl/frag/shadow-compare-global-alias.invalid.frag,True
 shaders-msl/frag/subgroup-builtins.msl22.frag,False
 shaders-msl/frag/subgroup-builtins.msl22.frag,True
-shaders-msl/vert/resource-arrays-leaf.ios.vert,False
-shaders-msl/vert/resource-arrays-leaf.ios.vert,True
-shaders-msl/vert/resource-arrays.ios.vert,False
-shaders-msl/vert/resource-arrays.ios.vert,True
 shaders-msl/vert/texture_buffer.texture-buffer-native.msl21.vert,False
 shaders-msl/vert/texture_buffer.texture-buffer-native.msl21.vert,True
-shaders-msl/vulkan/comp/subgroups.nocompat.invalid.vk.msl21.ios.comp,False
-shaders-msl/vulkan/comp/subgroups.nocompat.invalid.vk.msl21.ios.comp,True
 shaders-reflection/asm/aliased-entry-point-names.asm.multi,False
 shaders-reflection/asm/op-source-glsl-ssbo-1.asm.comp,False
 shaders-reflection/asm/op-source-glsl-ssbo-2.asm.comp,False