HPX does not compile with G++ version prior to 9.3.

[JiakunYan]

Expected Behavior

Compile successfully for any compilers supporting c++17.

Actual Behavior

G++ 8.5 gave me an error error: expected unqualified-id before '[' token

Steps to Reproduce the Problem

Just compile it with G++ 8.5.

Specifications

• HPX Version: the master branch ad3ac76e159692355d4715f8b275c0a062b859af
• Platform (compiler, OS): G++ 8.5. This should apply to any version prior to g++9.3.

Comment

This is not a fault of HPX, but G++, but this ticket can at least serve as an explanation for future users encountering this error. This is caused by a bug in G++ prior to 9.3. G++ doesn't parse correctly the [[maybe_unused]] attribute for the constructor's first argument. However, HPX uses it in libs/core/executors/include/hpx/executors/service_executors.hpp line 49.

explicit service_executor([[maybe_unused]] hpx::util::io_service_pool* pool) noexcept

Reference: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81429.

[hkaiser]

We can make this attribute conditional, though.

[Johan511]

Do we look into replacing maybe_unused with a #define, possibly something like HPX_UNUSED which falls back to attribute((unused)) for older GCC versions?

[hkaiser]

We do have HPX_UNUSED, however its use is different from [[maybe_unused]]. So instead of writing:

void foo([[maybe_unused]] int bar) {}

one would have to write:

void foo(int bar) { HPX_UNUSED(bar); }

We should make those changes only singular places when necessary, though.

[Johan511]

So everywhere we use [[maybe_unused]] should be replaced with HPX_UNUSED

or we could have another #define for [[maybe_unused]] which falls back to __attribute((unused))

Any comments on what you believe is better?

[hkaiser]

So everywhere we use [[maybe_unused]] should be replaced with HPX_UNUSED

No! Everywhere possible where we currently use HPX_UNUSED we should use [[maybe_unused]] instead.

or we could have another #define for [[maybe_unused]] which falls back to __attribute((unused))

How many spots would that be used in? If it's just a handful, I think it's not worth the trouble.