Feature/maybe unused replace dune unused

added 3 commits

• adbb5ad1 - Replace DUNE_UNUSED by C++17's [[maybe_unused]]
• 1a21b6f6 - Use [[maybe_unused] instead of wrongly used DUNE_UNUSED_PARAMETER
• a5b4f7ca - Deprecate DUNE_UNUSED and DUNE_UNUSED_PARAMTER

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I want to get rid of our own macros that suppress unused variable and unused parameter warnings. C++17 provides the attribute [[maybe_unused]] and it can be used for both of our DUNE_UNUSED and DUNE_UNUSED_PARAM in a standard fashion without relying on compiler extensions or CMake checks.
But there is a catch: GCC < 9.3 has a bug that marking the first argument of a constructor as maybe unused leads to a crash (bug 81429). Within the core modules, UG grid, typetree, and functions this constellation occurred only once.

How should I proceed?

1. Working around the bug as in dune-geometry / dune-geometry!158 (merged). I would add this piece of information to the changelog. It would make the deprecation of the whole header smooth.

2. Only deprecate DUNE_UNUSED and leave DUNE_UNUSED_PARAM as it is. In this case we cannot have deprecation warnings (possible for headers, not for macros).

We can get rid of the CMake checks after Dune 2.8 either way.

What do you think?

Either option is fine with me.

What do you think?

How about

3. Simply replace the unused macros where possible, but don't deprecate macros until we drop them in one go?

I want to get rid of the CMake check. Just this single occurrence of the GCC bug prevents me from doing so. Waiting until we drop support of GCC 9 will take quite a bit…
But you are right, it is a valid option.

added 33 commits

• a5b4f7ca...cdf32144 - 31 commits from branch master
• 5ee4bc18 - Replace DUNE_UNUSED by C++17's [[maybe_unused]]
• f3941816 - Use [[maybe_unused] instead of wrongly used DUNE_UNUSED_PARAMETER

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marked this merge request as ready

added 2 commits

• de6204ba - Replace DUNE_UNUSED by C++17's [[maybe_unused]]
• 25ef0618 - Use [[maybe_unused] instead of wrongly used DUNE_UNUSED_PARAMETER

Compare with previous version

I removed the deprecation of dune/common/deprecated.hh, as I got no consent on doing so as GCC < 9.3 has issues with [[maybe_unused]] for the first argument in a constructor.

If nobody objects, I am going to merge the other parts.

added 1 commit

• 1c1849e3 - Deprecate DUNE_UNUSED

Compare with previous version

Thinking further about a smooth transition path for DUNE_UNUSED I propose the following (extension of option 1):

• Deprecate the macro DUNE_UNUSED, no warning will be emitted.
• After Dune 2.8 we remove the CMake test, thus HAS_ATTRIBUTE_UNUSED will be always undefined and DUNE_UNUSED has no effect.
• The used will get a warning for an unused variable and can investigate the problem. There is no guarantee that the user will act, but it increases the chances.
• After Dune 2.9 we remove the macro.

Any objections?

added 1 commit

• 5de7f0fc - Add deprecation warning for DUNE_UNUSED

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added 8 commits

• 5de7f0fc...0cab3f48 - 4 commits from branch master
• c4d37fa5 - Replace DUNE_UNUSED by C++17's [[maybe_unused]]
• acc3cf1a - Use [[maybe_unused] instead of wrongly used DUNE_UNUSED_PARAMETER
• 505db0ab - Deprecate DUNE_UNUSED
• e2a1d550 - Add deprecation warning for DUNE_UNUSED

Compare with previous version

merged

mentioned in commit ca9389e1

mentioned in merge request !959 (merged)

mentioned in commit 0291dd6d