Re: [v3 PATCH] Implement C++17 GB50 resolution

[Jonathan Wakely <jwakely at redhat dot com>]

Hi, Dinka, thanks for the patch.

On 14/02/17 21:22 +0000, Dinka Ranns wrote:
> diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
> index ceae7f8..6a6995c 100644
> --- a/libstdc++-v3/include/std/chrono
> +++ b/libstdc++-v3/include/std/chrono
> @@ -349,50 +349,50 @@ _GLIBCXX_END_NAMESPACE_VERSION
> 	operator-() const
> 	{ return duration(-__r); }
>
> -	duration&
> +	constexpr duration&

This needs to use _GLIBCXX17_CONSTEXPR instead of 'constexpr'

These functions aren't constexpr in C++11 and C++14, and the standard
(annoyingly) forbids us from adding constexpr anywhere it isn't
present in the standard.

The macro _GLIBCXX17_CONSTEXPR expands to 'constexpr' if __cplusplus >
201402L, and expands to nothing otherwise.

Each new 'constexpr' you've added needs to use that macro.

> diff --git a/libstdc++-v3/testsuite/20_util/duration/arithmetic/constexpr.cc b/libstdc++-v3/testsuite/20_util/duration/arithmetic/constexpr.cc
> index 285f941..1128a52 100644
> --- a/libstdc++-v3/testsuite/20_util/duration/arithmetic/constexpr.cc
> +++ b/libstdc++-v3/testsuite/20_util/duration/arithmetic/constexpr.cc
> @@ -19,11 +19,31 @@
>
> #include <chrono>
> #include <testsuite_common_types.h>

There should be a blank line before and after this function, however
...

> +constexpr auto test_operators()
> +{
> +  std::chrono::nanoseconds d1 { };
> +  d1++;

This new function uses C++14 return type deduction, so will fail if
the test is run in C++11 mode (the default is C++14, but it can be
overridden on the command-line).

> +  ++d1;
> +  d1--;
> +  --d1;

Also once you change the new 'constexpr' specifiers to use the
_GLIBCXX17_CONSTEXPR macro this test will fail in C++14 mode. I think
this new function needs to be moved to a new test file, such as
testsuite/20_util/duration/arithmetic/constexpr_c++17.cc

That should contain just your new test_operators() function, because
the rest of the class will be tested by the existing constexpr.cc test
file. So something like:

// { dg-options "-std=gnu++17" }
// { dg-do compile { target c++1z } }

// Copyright etc. etc.
// ...

#include <chrono>

constexpr auto test_operators()
{
 // ...
}

constexpr auto d4 = test_operators();


Note that the "dg-do compile" line should use the c++1z target instead
of c++11, and needs to override the default dialect with a dg-options
line.

This test doesn't need a "main" function because it's a "dg-do
compile" test, so isn't linked. (The existing test that you modified
didn't need one either, but it doesn't do any harm leaving it there).


> diff --git a/libstdc++-v3/testsuite/20_util/time_point/arithmetic/constexpr.cc b/libstdc++-v3/testsuite/20_util/time_point/arithmetic/constexpr.cc
> new file mode 100644
> index 0000000..e87a226
> --- /dev/null
> +++ b/libstdc++-v3/testsuite/20_util/time_point/arithmetic/constexpr.cc
> @@ -0,0 +1,39 @@
> +// { dg-do compile { target c++11 } }

Since the time_point member functions will only be constexpr in C++17
this test also needs to use c++1z instead of c++11, and needs to
override the default C++14 dialect, i.e.

// { dg-options "-std=gnu++17" }
// { dg-do compile { target c++1z } }