Add the -Wobjc-missing-property-synthesis flag.

build/common.gypi

 # Copyright (c) 2012 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 2535 matching lines @@
               # Clang spots more unused functions.
               '-Wno-unused-function',
               # Don't die on dtoa code that uses a char as an array index.
               '-Wno-char-subscripts',
               # Especially needed for gtest macros using enum values from Mac
               # system headers.
               # TODO(pkasting): In C++11 this is legal, so this should be
               # removed when we change to that.  (This is also why we don't
               # bother fixing all these cases today.)
               '-Wno-unnamed-type-template-args',
-              # This (rightyfully) complains about 'override', which we use
+              # This (rightfully) complains about 'override', which we use
               # heavily.
               '-Wno-c++11-extensions',
 
               # Warns on switches on enums that cover all enum values but
               # also contain a default: branch. Chrome is full of that.
               '-Wno-covered-switch-default',
             ],
             'cflags!': [
               # Clang doesn't seem to know know this flag.
               '-mfpmath=sse',
@@ skipping 512 matching lines @@
             ['chromium_mac_pch', {'GCC_PRECOMPILE_PREFIX_HEADER': 'YES'},
                                  {'GCC_PRECOMPILE_PREFIX_HEADER': 'NO'}
             ],
             # Note that the prebuilt Clang binaries should not be used for iOS
             # development except for ASan builds.
             ['clang==1', {
               'CC': '$(SOURCE_ROOT)/<(clang_dir)/clang',
               'LDPLUSPLUS': '$(SOURCE_ROOT)/<(clang_dir)/clang++',
 
               # Don't use -Wc++0x-extensions, which Xcode 4 enables by default
-              # when buliding with clang. This warning is triggered when the
+              # when building with clang. This warning is triggered when the
               # override keyword is used via the OVERRIDE macro from
               # base/compiler_specific.h.
               'CLANG_WARN_CXX0X_EXTENSIONS': 'NO',
-
+              # Warn if automatic synthesis is triggered.
+              'CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS': 'YES',

[Mark Mentovai, 2012/11/15 00:07:51]

One nit: we almost always put the -W name in a comment next to the symbolic
name, so people searching for things by the -W name that they might find in a
verbose build log have something in the .gyp files to hit on.

               'GCC_VERSION': 'com.apple.compilers.llvm.clang.1_0',
               'WARNING_CFLAGS': [
                 '-Wheader-hygiene',
                 # Don't die on dtoa code that uses a char as an array index.
                 # This is required solely for base/third_party/dmg_fp/dtoa.cc.
                 '-Wno-char-subscripts',
                 # Clang spots more unused functions.
                 '-Wno-unused-function',
                 # See comments on this flag higher up in this file.
                 '-Wno-unnamed-type-template-args',
-                # This (rightyfully) complains about 'override', which we use
+                # This (rightfully) complains about 'override', which we use
                 # heavily.
                 '-Wno-c++11-extensions',
 
                 # Warns on switches on enums that cover all enum values but
                 # also contain a default: branch. Chrome is full of that.
                 '-Wno-covered-switch-default',
               ],
             }],
             ['clang==1 and clang_use_chrome_plugins==1', {
               'OTHER_CFLAGS': [
@@ skipping 193 matching lines @@
     ['OS=="ios"', {
       'target_defaults': {
         'xcode_settings' : {
           'GCC_VERSION': 'com.apple.compilers.llvm.clang.1_0',
 
           # This next block is mostly common with the 'mac' section above,
           # but keying off (or setting) 'clang' isn't valid for iOS as it
           # also seems to mean using the custom build of clang.
 
           # Don't use -Wc++0x-extensions, which Xcode 4 enables by default
-          # when buliding with clang. This warning is triggered when the
+          # when building with clang. This warning is triggered when the
           # override keyword is used via the OVERRIDE macro from
           # base/compiler_specific.h.
           'CLANG_WARN_CXX0X_EXTENSIONS': 'NO',
+          # Warn if automatic synthesis is triggered.
+          'CLANG_WARN_OBJC_MISSING_PROPERTY_SYNTHESIS': 'YES',
           'WARNING_CFLAGS': [
             '-Wheader-hygiene',
             # Don't die on dtoa code that uses a char as an array index.
             # This is required solely for base/third_party/dmg_fp/dtoa.cc.
             '-Wno-char-subscripts',
             # Clang spots more unused functions.
             '-Wno-unused-function',
             # See comments on this flag higher up in this file.
             '-Wno-unnamed-type-template-args',
-            # This (rightyfully) complains about 'override', which we use
+            # This (rightfully) complains about 'override', which we use
             # heavily.
             '-Wno-c++11-extensions',
           ],
         },
         'target_conditions': [
           ['_type=="executable"', {
             'configurations': {
               'Release_Base': {
                 'xcode_settings': {
                   'DEPLOYMENT_POSTPROCESSING': 'YES',
@@ skipping 391 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }