Handle unpatched VS xtree header

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 if (current_cpu == "arm") {
   import("//build/config/arm.gni")
 }
 if (current_cpu == "mipsel" || current_cpu == "mips64el") {
   import("//build/config/mips.gni")
@@ skipping 30 matching lines @@
   # address space, and it doesn't support cross-compiling).
   use_gold = is_linux && current_cpu == "x64"
 
   # use_debug_fission: whether to use split DWARF debug info
   # files. This can reduce link time significantly, but is incompatible
   # with some utilities such as icecc and ccache. Requires gold and
   # gcc >= 4.8 or clang.
   # http://gcc.gnu.org/wiki/DebugFission
   use_debug_fission =
       !is_win && use_gold && linux_use_bundled_binutils && !use_ccache
+
+  # Whether the VS xtree header has been patched to disable warning 4702. If
+  # it has, then we don't need to disable 4702 (unreachable code warning).
+  # The patch is preapplied to the internal toolchain and hence all bots.
+  msvs_xtree_patched = false

[brettw, 2015/03/02 19:14:26]

Can you wrap this comment + arg in an is_win conditional so people on other
platforms don't see this variable in their gn args? You need to put the comment
inside the conditional for "gn args" to associated it with the parameter
properly.

if (is_win) {
  # Whether the ...
  msvs_xtree_patch = false
}

 }
 
 # default_include_dirs ---------------------------------------------------------
 #
 # This is a separate config so that third_party code (which would not use the
 # source root and might have conflicting versions of some headers) can remove
 # this and specify their own include paths.
 config("default_include_dirs") {
   include_dirs = [
     "//",
@@ skipping 707 matching lines @@
       "/wd4100",  # Unreferenced formal function parameter.
       "/wd4121",  # Alignment of a member was sensitive to packing.
       "/wd4244",  # Conversion: possible loss of data.
       "/wd4481",  # Nonstandard extension: override specifier.
       "/wd4505",  # Unreferenced local function has been removed.
       "/wd4510",  # Default constructor could not be generated.
       "/wd4512",  # Assignment operator could not be generated.
       "/wd4610",  # Class can never be instantiated, constructor required.
       "/wd4996",  # Deprecated function warning.
     ]
+
+    # VS xtree header file needs to be patched or 4702 (unreachable code
+    # warning) is reported if _HAS_EXCEPTIONS=0. Disable the warning if xtree is
+    # not patched.
+    if (!msvs_xtree_patched) {
+      if (exec_script("../../win_is_xtree_patched.py", [], "value") == 1) {
+        msvs_xtree_patched = true
+      } else {
+        cflags += [ "/wd4702" ]  # Unreachable code.
+      }
+    }
   } else {
     # Common GCC warning setup.
     cflags = [
       # Enables.
       "-Wendif-labels",  # Weird old-style text after an #endif.
       "-Werror",  # Warnings as errors.
 
       # Disables.
       "-Wno-missing-field-initializers",  # "struct foo f = {0};"
       "-Wno-unused-parameter",  # Unused function parameters.
@@ skipping 293 matching lines @@
       cflags += [ "-gsplit-dwarf" ]
     }
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }