Require GCC 5 and C++14

at the last developer meeting we decided that we would raise the minimum compiler requirement of DUNE 2.6 to GCC 5.

This merge request states that change and also raises the minimum required C++ level to -std=c++14.

added 1 commit

• d382c820 - [ci] Stop testing with Debian 8

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This requirement seems to be fulfilled by any major distribution (in it's current stable release). Hence, I fully support this.

added 2 commits

• 864144b5 - [cmake] Make sure that compilers support relaxed constexpr in C++14 mode
• e354de05 - [cmake] Improve formatting of error message when compiler fails compatibility check.

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added 1 commit

• e491b92f - [cmake] Fix C++14 compatibility test

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dune-devel discussion

User poll on the Dune list.

Until recently I was using gcc 4.9 and in general I'm not in favour of using to many new features because it usually hurts portability.

@robert.kloefkorn Can you give a little more details: Which portability is hurt by new features?

• Porting to other distributions should not be effected, if the compiler is supported.
• Porting forward to newer compiler versions should not be a problem at all. If it is, that's in fact good, because it indicates that we misuse a new language feature and rely on a broken implementation in the supported compilers.
• Porting to older compiler versions is surely neither supported nor intended. If there really are important target systems still using gcc 4.9 that's a show stopper. But since the last debian release I'm not aware of any such system.

FWIW, dune-grid now requires gcc 5+ (dune-grid!213 (merged))

@ansgar: Yes, I mentioned that in the corresponding MR. GCC-4.9 compatibility would be simple: Just don't use std::is_trivially_copyable (with configure check) to protect the code. However, from this discussion I gathered it would not be worth the effort.

I think this comes down to: Is there someone who is really stuck with GCC 4.9 and can't upgrade? Otherwise, I really think Debian stable is something we can expect people to be compatible with (and Stretch has been out for quite some time already).

No user answered so far from our users and no developer alleged an used system failing the required compiler. Can we motivate more core developers vote on this?

Or maybe people just don't care because it's not a problem for them?

By requiring cmake 3.1 debian 8 is out of the game anyway. I cannot see a reason for not merging this. However, we should take Roberts concerns seriously. @robert.kloefkorn can you provide more details, or is merging OK for you?

mentioned in merge request !323 (merged)

Go ahead. I just wanted to point out that cutting edge is not always what one wants here.

merged

mentioned in commit f4291c38

mentioned in issue #91 (closed)

mentioned in merge request staging/dune-uggrid!132 (merged)