[C++] DCHECK custom messages are unreachable in release

[asfimport]

For release builds, DCHECK( x ) << y << z; currently expands to

((void)(x)); 
while (false) ::arrow::util::ArrowLogBase() << y << z;

This is unreachable which is an error using clang-7

Reporter: Ben Kietzman / @bkietz
Assignee: Ben Kietzman / @bkietz

Related issues:

• [C++] Do not set -Werror when using BUILD_WARNING_LEVEL=CHECKIN in release mode (is related to)

PRs and other links:

• GitHub Pull Request #3599
• GitHub Pull Request #4027

_{Note: This issue was originally created as ARROW-4520. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 3599
#3599

[asfimport]

Ben Kietzman / @bkietz:
After #3854 clang is complaining that custom messages are unreachable again; arguments to the DCHECK macros need to be explicitly ignored and the NullLog needs to be voidified

[asfimport]

Wes McKinney / @wesm:
This issue only arises with -DBUILD_WARNING_LEVEL=CHECKIN which enables a bunch of warnings in addition to -Werror. We have not strived for a warning-free build in release mode so I think we should close this and perhaps raise a CMake error if the user tries to use Release and CHECKIN together

[asfimport]

Wes McKinney / @wesm:
I'll submit a PR to improve the developer documentation about this