This is stupid, but the GCC maintainers refused to change it:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425

For some reason, KJ's uses of (void) did not warn with GCC 5 but do warn with GCC 7. Supposedly, GCC *never* supported silencing with (void), so there must have been some other bug that caused GCC to fail to trigger the warning previously -- maybe related to the fact that the values being returned are non-trivial types?

C++17 introduces `[[nodiscard]]` which is defined as being squelchable using `(void)`, but we're still on C++14, and KJ_UNUSED_RESULT is a post-declaration attribute so can't be defined in terms of the new C++17 attribute even if the compiler supports it. Sigh.