While building device/bluetooth/bluetooth_adapter_mac.mm with the 10.13 SDK (from Xcode 9b3 9M174d), -Wunguarded-availability-new produces warnings (errors with -Werror) about enum constants from CoreBluetooth.framework being unavailable before 10.13. The errors are reproduced below.

The enum constants are CBManagerStateUnsupported and CBManagerStatePoweredOn. These are members of the CBManagerState enum, which is defined in <CoreBluetooth/CBManager.h> as

typedef NS_ENUM(NSInteger, CBManagerState) {
        CBManagerStateUnknown = 0,
        CBManagerStateResetting,
        CBManagerStateUnsupported,
        CBManagerStateUnauthorized,
        CBManagerStatePoweredOff,
        CBManagerStatePoweredOn,
} NS_ENUM_AVAILABLE(10_13, 10_0);

which does state that these are available beginning in 10.13. Indeed, this isn’t in earlier SDKs. However, in previous SDKs, the CBCentralManagerState enum is available in <CoreBluetooth/CBCentralManager.h>. This has been deprecated in 10.13 with the recommendation to use CBManager instead, but the names and values are equivalent except for the presence/absence of “Central”. These older names can be used with older SDKs.

The problem is that -Wunguarded-availability-new makes it difficult or impossible to write code that is compatible with the 10.13 SDK and older SDKs (such as 10.10, which Chrome currently uses for official builds).

device/bluetooth/bluetooth_adapter_mac.h attempts to allow the new names to be used with older SDKs by providing local definitions of the new names in terms of the old when building with pre-10.13 SDKs (https://chromium.googlesource.com/chromium/src/+/724d9901b9f8f44e67a9b9d6da0722bb443e5d0c/device/bluetooth/bluetooth_adapter_mac.h#32, https://chromium.googlesource.com/chromium/src/+/724d9901b9f8f44e67a9b9d6da0722bb443e5d0c). This worked until -Wunguarded-availability-new.

We’re now in a situation where we can’t use the old names with the 10.13 SDK without triggering the errors in  bug 729906 , and we can’t use the new names with the 10.13 SDK because Clang doesn’t realize that we have our own definitions of them when building with older SDKs.

The errors:

../../device/bluetooth/bluetooth_adapter_mac.mm:162:33: error: 'CBManagerStateUnsupported' is only available on macOS 10_13 or newer [-Werror,-Wunguarded-availability-new]
                                CBManagerStateUnsupported);
                                ^~~~~~~~~~~~~~~~~~~~~~~~~
../../../../../Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/System/Library/Frameworks/CoreBluetooth.framework/Headers/CBManager.h:31:28: note: 'CBManagerState' has been explicitly marked partial here
typedef NS_ENUM(NSInteger, CBManagerState) {
                           ^
../../device/bluetooth/bluetooth_adapter_mac.mm:162:33: note: enclose 'CBManagerStateUnsupported' in an @available check to silence this warning
                                CBManagerStateUnsupported);
                                ^~~~~~~~~~~~~~~~~~~~~~~~~
../../device/bluetooth/bluetooth_adapter_mac.mm:171:33: error: 'CBManagerStatePoweredOn' is only available on macOS 10_13 or newer [-Werror,-Wunguarded-availability-new]
                                CBManagerStatePoweredOn);
                                ^~~~~~~~~~~~~~~~~~~~~~~
../../../../../Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/System/Library/Frameworks/CoreBluetooth.framework/Headers/CBManager.h:31:28: note: 'CBManagerState' has been explicitly marked partial here
typedef NS_ENUM(NSInteger, CBManagerState) {
                           ^
../../device/bluetooth/bluetooth_adapter_mac.mm:171:33: note: enclose 'CBManagerStatePoweredOn' in an @available check to silence this warning
                                CBManagerStatePoweredOn);
                                ^~~~~~~~~~~~~~~~~~~~~~~
../../device/bluetooth/bluetooth_adapter_mac.mm:605:45: error: 'CBManagerStatePoweredOn' is only available on macOS 10_13 or newer [-Werror,-Wunguarded-availability-new]
  if ([low_energy_central_manager_ state] < CBManagerStatePoweredOn) {
                                            ^~~~~~~~~~~~~~~~~~~~~~~
../../../../../Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.13.sdk/System/Library/Frameworks/CoreBluetooth.framework/Headers/CBManager.h:31:28: note: 'CBManagerState' has been explicitly marked partial here
typedef NS_ENUM(NSInteger, CBManagerState) {
                           ^
../../device/bluetooth/bluetooth_adapter_mac.mm:605:45: note: enclose 'CBManagerStatePoweredOn' in an @available check to silence this warning
  if ([low_energy_central_manager_ state] < CBManagerStatePoweredOn) {
                                            ^~~~~~~~~~~~~~~~~~~~~~~
3 errors generated.