DCHECK_NOTNULL generates "expression result unused" errors with clang in strict compilation mode

[joydeep-b]

Synopsis:
When compiling a program in release mode, treating warnings as errors (-DNDEBUG -Wall -Werror), the use of DCHECK_NOTNULL results in "expression result unused" errors when compiling with clang.

Steps to reproduce:

1. Enter the following contents in a file named test.cc:

#include "glog/logging.h"

void Foo(int* x) {
  DCHECK_NOTNULL(x);
  *x = 42;
}

int main() {
  int a = 0;
  Foo(&a);
}

2. Compile the program with:

clang++ -Werror -Wall test.cc -lglog -o test -DNDEBUG

This will result in the following output:

test.cc:4:18: error: expression result unused [-Werror,-Wunused-value]
  DCHECK_NOTNULL(x);
                 ^
/usr/include/glog/logging.h:1042:30: note: expanded from macro 'DCHECK_NOTNULL'
#define DCHECK_NOTNULL(val) (val)
                             ^~~
1 error generated.

[chen3feng]

I fixed this issue by this change in our internal codebase:

// A small helper for DCHECK_NOTNULL().
template <typename T>
T* NotCheckNotNull(T* t) {
  return t;
}

#define DCHECK_NOTNULL(val) (::google::NotCheckNotNull(val))

FYI.