[Bug libstdc++/66998] New: not_fn invocation is not SFINAE friendly

[Bug libstdc++/66998] New: not_fn invocation is not SFINAE friendly

[tomaszkam at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66998

            Bug ID: 66998
           Summary: not_fn invocation is not SFINAE friendly
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tomaszkam at gmail dot com
  Target Milestone: ---

According to the
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4529.html#func.not_fn
the following invocations not_fn(f)(args...) and !INVOKE(f, args...) (in case
when f is function object it can be simplified to !f(args)). However the in
current implementation they behavior differs in case when they are used in
SFINAE context.

For example:
#include <experimental/functional>

template<typename F, typename Arg>
auto foo1(F f, Arg arg) -> decltype(!f(arg)) { return !f(arg); }

template<typename F, typename Arg>
auto foo1(F f, Arg arg) -> decltype(!f()) { return !f(); }

template<typename F, typename Arg>
auto foo2(F f, Arg arg) -> decltype(not_fn(f)(arg)) { return not_fn(f)(arg); }

template<typename F, typename Arg>
auto foo2(F f, Arg arg) -> decltype(not_fn(f)()) { return not_fn(f)(); }

struct negator
{
    bool operator()(int) const { return false; }
    void operator()() const {}
};

int main()
{
    foo1(negator{}, 1);
//    foo2(negator{}, 1)); //Shall be equivalent to line above. Produces an
compilation error cause by instatiation error during return type deduction.
};

The problem is caused by the use of return type deduction (decltype(auto))
instead of decltype(expr) in the implementation of not_fn. Notice that such use
does not break the function precondition placed in requires clause.

[Bug libstdc++/66998] not_fn invocation is not SFINAE friendly

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66998

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-07-24
     Ever confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I'm not convinced that the wording *really* requires them to be equivalent in
all contexts, i.e. even when !INVOKE(fd, ...) is not a valid expression ... but
since you wrote the proposal I guess we can assume that's what you intended :-)

It's certainly a nice QoI improvement. I was basically lazy when I implemented
it and used decltype(auto) because it's convenient.

[Bug libstdc++/66998] not_fn invocation is not SFINAE friendly

[tomaszkam at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66998

--- Comment #2 from Tomasz Kamiński <tomaszkam at gmail dot com> ---
Sorry for missing using declaration. 

> It's certainly a nice QoI improvement. I was basically lazy when I implemented it and used decltype(auto) because it's convenient.

Yes, the decltype(auto) is convenient but the hard error produced in SFINAE
context make it acutally unusable in library. Just think of the mess that would
be caused by:
template<typename C>
delctype(auto) begin(C&& c) { return std::forward<C>(c).begin(); }

The not_fn is also not working with member pointers:
#include <experimental/functional>

using namespace std::experimental;

struct A
{
    bool mem;
};

int main()
{
    not_fn(&A::mem)(A{});
};
Should I create separate issue?
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Subject: [Bug libstdc++/66998] not_fn invocation is not SFINAE friendly
Date: Fri, 24 Jul 2015 16:41:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66998

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Tomasz Kamiński from comment #2)
> Should I create separate issue?

Oops, no we can deal with both under this PR.

If I commit my invoke() implementation then we can just use that, and it will
solve the member pointer issue.
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Subject: [Bug tree-optimization/66873] fortran variant of outer-1.c not parallelized by autopar
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idf873

--- Comment #9 from vries at gcc dot gnu.org ---
(In reply to vries from comment #7)
> Created attachment 35986 [details]
> Updated tentative patch
>
> I found that always doing graphite before parloops resulted in failures to
> parallelize reduction testcases.
>

That was due to not using -ffast-math.

By using this ( https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01863.html )
approved patch, we no longer have that problem.

However, we run into: PR66997 - outer loop reduction fails to parallelize with
graphite.

And the runtime of graphite is excessive in some cases, with prevents us from
using graphite by default, f.i. Bug 53852 - [4.9/5/6 Regression]
-ftree-loop-linear: large compile time / memory usage.

[Bug libstdc++/66998] not_fn invocation is not SFINAE friendly

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66998

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Author: redi
Date: Thu Sep  3 14:36:03 2015
New Revision: 227448

URL: https://gcc.gnu.org/viewcvs?rev=227448&root=gcc&view=rev
Log:
Make std::experimental::not_fn SFINAE-friendly.

        PR libstdc++/66998
        * include/experimental/functional (_Not_fn): Add exception
        specifications and non-deduced return types.
        (not_fn): Add exception specification and wrap pointer-to-member.
        * testsuite/experimental/functional/not_fn.cc: Test in SFINAE context
        and test pointer-to-member.

Modified:
    trunk/libstdc++-v3/ChangeLog
    trunk/libstdc++-v3/include/experimental/functional
    trunk/libstdc++-v3/testsuite/experimental/functional/not_fn.cc

[Bug libstdc++/66998] not_fn invocation is not SFINAE friendly

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66998

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Author: redi
Date: Thu Sep  3 15:10:52 2015
New Revision: 227455

URL: https://gcc.gnu.org/viewcvs?rev=227455&root=gcc&view=rev
Log:
Make std::experimental::not_fn SFINAE-friendly.

        PR libstdc++/66998
        * include/experimental/functional (_Not_fn): Add exception
        specifications and non-deduced return types.
        (not_fn): Add exception specification and wrap pointer-to-member.
        * testsuite/experimental/functional/not_fn.cc: Test in SFINAE context
        and test pointer-to-member.

Modified:
    branches/gcc-5-branch/libstdc++-v3/ChangeLog
    branches/gcc-5-branch/libstdc++-v3/include/experimental/functional
   
branches/gcc-5-branch/libstdc++-v3/testsuite/experimental/functional/not_fn.cc

[Bug libstdc++/66998] not_fn invocation is not SFINAE friendly

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66998

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |5.3

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed