shells/fish: fix the build with Clang-6.0.0

[asomers]

Summary

shells/fish: fix the build with Clang-6.0.0

Clang-6.0.0 on FreeBSD 12 doesn't define __cpp_lib_make_unique even though it
should. Patch fish to always assume that symbol is defined on FreeBSD 12.

Test Plan

Poudriere on 10.4 and 11.1. Manual build and test on 12.0.

[dim]

I'm not entirely sure that this macro is defined by any C++ standard, though there appears to be a "recommendation" note that refers to it. It would be easier to just test for the C++ version instead, I think. But if #if 1 also works, I'm fine with it.

[dim]

I don't see what the OSVERSION check is for? Since libc++ never defined __cpp_lib_make_unique, this can't have worked on earlier versions either. Why is this suddenly needed now, because we're defaulting to gnu++14? In that case you could also just crank down the standard to e.g. -std=gnu++11 or even lower.

[asomers]

In D14058#295310, @dim wrote:

I don't see what the OSVERSION check is for? Since libc++ never defined __cpp_lib_make_unique, this can't have worked on earlier versions either. Why is this suddenly needed now, because we're defaulting to gnu++14? In that case you could also just crank down the standard to e.g. -std=gnu++11 or even lower.

The problem is that while __cpp_lib_make_unique isn't defined, std::make_unique is. That causes error: call to 'make_unique' is ambiguous errors, because fish defines its own make_unique if the standard library doesn't provide one. I'm guessing that earlier versions of Clang didn't provide make_unique, at least not with our default --std option. Does libc++ have a different feature test macro for "supports all C++14 features" or something like that?

[asomers]

Using -std=c+=11 works. However, it feels wrong. We have a C++14 compiler; we should be able to use it.

[dim]

In D14058#295314, @asomers wrote:

In D14058#295310, @dim wrote:

I don't see what the OSVERSION check is for? Since libc++ never defined __cpp_lib_make_unique, this can't have worked on earlier versions either. Why is this suddenly needed now, because we're defaulting to gnu++14? In that case you could also just crank down the standard to e.g. -std=gnu++11 or even lower.

The problem is that while __cpp_lib_make_unique isn't defined, std::make_unique is. That causes error: call to 'make_unique' is ambiguous errors, because fish defines its own make_unique if the standard library doesn't provide one. I'm guessing that earlier versions of Clang didn't provide make_unique, at least not with our default --std option. Does libc++ have a different feature test macro for "supports all C++14 features" or something like that?

The compiler itself supplies the built-in macro __cplusplus for that. If the value is 201402L, we are compiling for -std=c++14 or later, and in that case std::make_unique will be available.

E.g. just #if __cplusplus >= 201402L is the correct conditional.

[asomers]

key off of __cplusplus instead of OSVERSION

[jbeich]

Can you land it?