[Bug c++/52892] New: Function pointer loses constexpr qualification

[Bug c++/52892] New: Function pointer loses constexpr qualification

[EricMCornelius at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52892

             Bug #: 52892
           Summary: Function pointer loses constexpr qualification
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: EricMCornelius@gmail.com


The following test case is failing:

#include <cstdio>

constexpr std::size_t fibonacci(std::size_t val) {
  return (val <= 2) ? 1 : fibonacci(val - 1) + fibonacci(val - 2);
}

template <typename Function>
struct Defer {
  constexpr Defer(const Function func_) : func(func_) { }

  const Function func;

  template <typename... Args>
  constexpr auto operator () (const Args&... args) -> decltype(func(args...)) {
    return func(args...);
  }
};

template <typename Function>
constexpr Defer<Function> make_deferred(const Function f) {
  return Defer<Function>(f);
}

int main(int argc, char* argv[]) {
  constexpr auto deferred = make_deferred(&fibonacci);
  static_assert(deferred(25) == 75025, "Static fibonacci call failed");
}

src/main.cpp: In function 'int main(int, char**)':
src/main.cpp:151:3: error: non-constant condition for static assertion
src/main.cpp:151:28:   in constexpr expansion of
'deferred.Defer<Function>::operator()<{int}>((* &25))'
src/main.cpp:140:24: error: expression 'fibonacci' does not designate a
constexpr function
test.make:129: recipe for target `obj/Debug/main.o' failed
make[1]: *** [obj/Debug/main.o] Error 1
makefile:16: recipe for target `test' failed
make: *** [test] Error 2

Based on my reading of the standard, this should be allowed behavior, and works
as expected with clang 3.1 (152539).

Note that the following behavior also fails similarly:

int main(int argc, char* argv[]) {
  constexpr auto deferred = make_deferred(&fibonacci);
  constexpr auto func = deferred.func;
  constexpr auto val = func(25);
}

src/main.cpp: In function 'int main(int, char**)':
src/main.cpp:152:31: error: expression 'fibonacci' does not designate a
constexpr function

Whereas this succeeds:

int main(int argc, char* argv[]) {
  constexpr auto func = &fibonacci;
  static_assert(func(25) == 75025, "Static fibonacci call failed");
}

[Bug c++/52892] Function pointer loses constexpr qualification

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52892

--- Comment #1 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2012-04-07 13:15:13 UTC ---
(In reply to comment #0)
[..]
> Based on my reading of the standard, this should be allowed behavior, and 
> works as expected with clang 3.1 (152539).

I agree that this should work, this was the clear intention for the core
language defect

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1193 

I tried to break down the example to understand what's going wrong and here is
one simpler example:

//---------------
constexpr bool is_negative(int x) { return x < 0; }

struct Defer
{
#if 0
  typedef bool (*Function)(int);
  Function func;
  constexpr Defer(Function func) : func(func) {}
#else
  bool (*func)(int);
  constexpr Defer(bool (* func)(int)) : func(func) {}
#endif
  template<class... Args>
  constexpr auto operator()(const Args&... args) -> decltype(func(args...))
  {
    return func(args...);
  }
};

template<class Function>
constexpr Defer make_deferred(Function func)
{
  return Defer(func);
}

int main()
{
  constexpr Defer deferred(make_deferred(is_negative));
  static_assert(deferred(-2), "Error");
}
//---------------

As written, this example is well-formed. But once we change the pre-processor
directive "#if 0" to "#if 1", we have a similar error. It seems that after
introduction of the typedef for the function pointer type gcc no longer
attempts to consider the track the constness.

It is possible to construct an even simpler example. Consider the code example
from CWG defect 1193 again:

constexpr bool is_negative(int x) { return x < 0; }
constexpr bool check(int x, bool (*p)(int)) { return p(x); }
static_assert(check(-2, is_negative), "Error");

gcc accepts it as it should. Now lets introduce a typedef for the function
pointer used in check:

constexpr bool is_negative(int x) { return x < 0; }
typedef bool (*Function)(int);
constexpr bool check(int x, Function p) { return p(x); }
static_assert(check(-2, is_negative), "Error");

Now we get a similar error as in your example:

"4|error: non-constant condition for static assertion|
 4|  in constexpr expansion of 'check(-2, is_negative)'|
 3|error: expression 'is_negative' does not designate a constexpr function"

The template parameter in your example has similar effects as a typedef. Both
use cases should not invalidate the constexpr character.

[Bug c++/52892] Function pointer loses constexpr qualification

[adrien at guinet dot me]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52892

Adrien Guinet <adrien at guinet dot me> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |adrien at guinet dot me

--- Comment #2 from Adrien Guinet <adrien at guinet dot me> 2012-05-29 09:26:29 UTC ---
Hello everyone,

Im ay have an issue related to this issue, if that can help:

#include <iostream>

struct A
{
    void f(int i) { std::cout << "f " << i << " " << _i << std::endl; }
    void f2(int i) { std::cout << "f2 " << i << " " << _i << std::endl; }
    int _i; 
};

template <typename F, F f>
struct class_f
{
    typedef F f_type;
    static constexpr f_type f_value = f;
    static constexpr f_type get() { return f; }
};


int main()
{
    typedef class_f<decltype(&A::f), &A::f> ff_t;

    // This does not compile
    class_f<ff_t::f_type, ff_t::f_value> ff; 

    // This does not compile either
    class_f<ff_t::f_type, static_cast<ff_t::f_type>(&A::f)> ff2;

    // This does
    class_f<ff_t::f_type, &A::f> ff_works;
}

It looks like using a function pointer that has been "instantiated" as an
"f_type" makes the compilation fails. here is the output of g++-4.7 with the
first "failing" tests :

$ g++-4.7 -std=c++0x ftempl.cpp
ftempl.cpp: In function ‘int main()’:
ftempl.cpp:24:37: error: could not convert template argument ‘class_f<void
(A::*)(int), &A::f>::f_value’ to ‘void (A::*)(int)’
ftempl.cpp:24:41: error: invalid type in declaration before ‘;’ token


$ g++-4.7 --version
g++-4.7 (Debian 4.7.0-8) 4.7.0

[Bug c++/52892] Function pointer loses constexpr qualification

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52892

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-05-02
             Blocks|                            |55004
     Ever Confirmed|0                           |1

[Bug c++/52892] Function pointer loses constexpr qualification

[paolo at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52892

--- Comment #3 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> ---
Author: paolo
Date: Wed Aug 27 17:03:34 2014
New Revision: 214579

URL: https://gcc.gnu.org/viewcvs?rev=214579&root=gcc&view=rev
Log:
/cp
2014-08-27  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/52892
    * semantics.c (cxx_eval_call_expression): Use STRIP_NOPS on the
    result of cxx_eval_constant_expression.

/testsuite
2014-08-27  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/52892
    * g++.dg/cpp0x/constexpr-52892-1.C: New.
    * g++.dg/cpp0x/constexpr-52892-2.C: Likewise.
    * g++.dg/cpp0x/constexpr-52282-1.C: Likewise.


Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-52282-1.C
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-52892-1.C
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-52892-2.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/semantics.c
    trunk/gcc/testsuite/ChangeLog

[Bug c++/52892] Function pointer loses constexpr qualification

[paolo.carlini at oracle dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52892

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |5.0

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Fixed.