[Bug c++/64455] New: A constexpr variable template can't be used with enable_if

[Bug c++/64455] New: A constexpr variable template can't be used with enable_if

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64455

            Bug ID: 64455
           Summary: A constexpr variable template can't be used with
                    enable_if
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ville.voutilainen at gmail dot com
                CC: jason at redhat dot com

#include <type_traits>

template<typename Type>
constexpr bool IsType = true;

template<class T>
struct X {
    typedef typename std::enable_if<IsType<T>,T>::type type;
};

int main()
{
   X<int>::type t;
}

dionne.cpp:8:37: error: the value of ‘IsType<T>’ is not usable in a constant
expression
     typedef typename std::enable_if<IsType<T>,T>::type type;
                                     ^
dionne.cpp:4:16: note: ‘IsType<T>’ used in its own initializer
 constexpr bool IsType = true;
                ^
dionne.cpp:8:48: error: the value of ‘IsType<T>’ is not usable in a constant
expression
     typedef typename std::enable_if<IsType<T>,T>::type type;
                                                ^
dionne.cpp:4:16: note: ‘IsType<T>’ used in its own initializer
 constexpr bool IsType = true;
                ^
dionne.cpp:8:48: note: in template argument for type ‘bool’ 
     typedef typename std::enable_if<IsType<T>,T>::type type;
                                                ^

Clang accepts the code.
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From: "belagod at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/64456] New: [AArch64] pr64252.c fail on aarch64_be
Date: Wed, 31 Dec 2014 13:43:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idd456

            Bug ID: 64456
           Summary: [AArch64] pr64252.c fail on aarch64_be
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: belagod at gcc dot gnu.org

This new test fails for BE:

FAIL: gcc.dg/vect/pr64252.c -flto -ffat-lto-objects execution test
FAIL: gcc.dg/vect/pr64252.c execution test

but it is fixed by this patch series:

https://gcc.gnu.org/ml/gcc-patches/2014-11/msg01431.html
https://gcc.gnu.org/ml/gcc-patches/2014-10/msg01099.html
https://gcc.gnu.org/ml/gcc-patches/2014-11/msg02797.html
https://gcc.gnu.org/ml/gcc-patches/2014-12/msg01087.html

[Bug c++/64455] A constexpr variable template can't be used with enable_if

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64455

--- Comment #1 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Btw, this problem prevents using the C++14 trait aliases with the forthcoming
trait variable templates as they were intended, because

template <class T> enable_if_t<is_same_v<T, int>> f() {}

is also erroneously rejected with the diagnostic that is_same_v<T, int>
is not usable in a constant expression.

[Bug c++/64455] A constexpr variable template can't be used with enable_if

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64455

--- Comment #2 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Reduced to avoid using type_traits:

template<typename Type>
constexpr bool IsType = true;

template <bool b, class T> struct Test
{
};

template <class T>
struct Test<true, T>
{
        typedef T type;
};

template<class T>
struct X {
    typedef typename Test<IsType<T>,T>::type type;
};

int main()
{
   X<int>::type t;
}

[Bug c++/64455] A constexpr variable template can't be used with enable_if

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64455

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
Author: jason
Date: Tue Jan  6 20:44:51 2015
New Revision: 219268

URL: https://gcc.gnu.org/viewcvs?rev=219268&root=gcc&view=rev
Log:
    PR c++/64455
    * pt.c (type_dependent_expression_p): Handle variable templates.
    * constexpr.c (potential_constant_expression_1): Use it.

Added:
    trunk/gcc/testsuite/g++.dg/cpp1y/var-templ21.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/constexpr.c
    trunk/gcc/cp/pt.c

[Bug c++/64455] A constexpr variable template can't be used with enable_if

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64455

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |jason at gcc dot gnu.org
         Resolution|---                         |FIXED
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> ---
Fixed.