Chrome Version: N/A (Trunk source code)
OS: Ubuntu

What steps will reproduce the problem?
* Compile Firefox (which includes some Chromium headers) using clang version 5.0.0 (trunk 304217)

* OR, I expect you can trigger the same issue by building Chrome with "-Wcomma" build warning manually added wherever you add build warnings.

What is the expected result?
Successful build; expecting no Wcomma build warning for base/time/time.h

What happens instead?
This build warning (treated as an error in my build configuration):

==================
chromium/base/time/time.h:690:37: error: possible misuse of comma operator here [-Werror,-Wcomma]
          DCHECK(positive_value > 0),
                                    ^
chromium/base/time/time.h:690:11: note: cast expression to void to silence warning
          DCHECK(positive_value > 0),
          ^~~~~~~~~~~~~~~~~~~~~~~~~~
          static_cast<void>(        )
chromium/base/logging.h:702:3: note: expanded from macro 'DCHECK'
  LAZY_STREAM(LOG_STREAM(DCHECK), DCHECK_IS_ON() ? !(condition) : false) \
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
chromium/base/logging.h:364:3: note: expanded from macro 'LAZY_STREAM'
  !(condition) ? (void) 0 : ::logging::LogMessageVoidify() & (stream)
  ^
1 error generated.
==================


Please use labels and text to provide additional information.
The comma operator is tricky because it discards the result of its first operand.  Clang is wanting to be stricter & make developers opt into that behavior more clearly by wrapping the discarded operand in a "(void)" cast, if that's the behavior the developer really wants.

Here, it looks like that's the behavior that Chromium wants (at least, it looks like this code was added[1] & then rewritten[2] relatively recently, so I don't think it's a mistake).  So it would be great if the chromium source code could take clang's advice here and use a "(void)" cast, to make the discarding clearer & to suppress the build warning.  This will benefit Firefox as well, once we at Mozilla import an updated snapshot of this header down the line. :)

[1] https://chromium.googlesource.com/chromium/src/+/bad4e03c815490e4eb8ceef9b5143b58c6fefb81%5E%21/base/time/time.h
 (see "DCHECK(positive_value > 0)," at very bottom of that page)

[2] https://chromium.googlesource.com/chromium/src/+/9ca6e2937f3f435d527f601aeae2e220355d4155%5E%21/base/time/time.h