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
+
+  if (is_win) {
+    # 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
+  }
 }
 
 # 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 &&
+        exec_script("../../win_is_xtree_patched.py", [], "value") == 0) {
+      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" ]
   }
 }