[Bug c++/55159] New: pythy constexpr auto lambda pointer has no initializer

[Bug c++/55159] New: pythy constexpr auto lambda pointer has no initializer

[supercilious.dude at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55159

             Bug #: 55159
           Summary: pythy constexpr auto lambda pointer has no initializer
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: supercilious.dude@gmail.com


Pythy (http://pfultz2.github.com/Pythy/) does something akin to:

template <typename T> 
typename std::remove_reference<T>::type *addr(T &&t) 
{ 
    return &t; 
}

template <class T0, class T1>
struct min_t
{
    constexpr static auto *f =
        false ? addr([](T0 x, T1 y){ return x < y ? x : y; }) : nullptr; 
};

Which gcc (4.7.2 and 4.8.0 git master) cannot evaluate.
clang and visual studio 2012 work fine.

I know this is not strictly needed by gcc 4.8 since it has deduced auto without
the late specifier (which the other compilers lack), but its highly convenient
to have pythy work on all c++11 compilers.

[Bug c++/55159] pythy constexpr auto lambda pointer has no initializer

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55159

Daniel Krügler <daniel.kruegler at googlemail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |daniel.kruegler at
                   |                            |googlemail dot com

--- Comment #1 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2012-11-01 10:58:06 UTC ---
I really think that Pythy should fix this implementation, because it is not
supported by the C++11 standard. Any compiler accepting that is defect.
According to 9.4.2 p3:

"A static data member of literal type can be declared in the class definition
with the constexpr specifier; if so, its declaration shall specify a
brace-or-equal-initializer in which every initializer-clause that is an
assignment-expression is a constant expression."

As of 5.19 p2 the appearance of a lambda-expression prevents an expression from
satisfying the requirements of a (core) constant expression.

But changing your example to

template <typename T>
T* addr(T& t) { return &t; }

bool less(int x, int y) { return x < y ? x : y; }

const static auto* f = addr(less);

indeed points to a defect of gcc, not being able to deduce auto here. I think
this is the same bug that I can remember (but cannot find at the very moment)
which shows a similar problem during overload resolution in templates when
involving function addresses such as in

template <typename T>
T* addr(T& t) { return &t; }

bool less(int x, int y) { return x < y ? x : y; }

template<typename T>
int deduce(const T*) { return 0; }

int i = deduce(addr(less));

This example should be accepted, but gcc doesn't like it saying:

"error: no matching function for call to 'deduce(bool (*)(int, int))'|
note: candidate is:|
note: template<class T> int deduce(const T*)|
note:   template argument deduction/substitution failed:|
note:   types 'const T' and 'bool(int, int)' have incompatible cv-qualifiers"

[Bug c++/55159] pythy constexpr auto lambda pointer has no initializer

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55159

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-11-01 14:25:26 UTC ---
Daniel, you mean PR54111 maybe?

[Bug c++/55159] pythy constexpr auto lambda pointer has no initializer

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55159

--- Comment #3 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2012-11-01 15:11:51 UTC ---
(In reply to comment #2)
Hmmh, it doesn't look like that one, maybe I was wrong about an existing issue.

But it seems that gcc doesn't ignore the const (in "const T*" or "const auto*")
for functions here, which seems to be the root of the second problem (This is
*not* attempting to form a function with cv-qualifier-seq). Both

template <typename T>
T* addr(T& t) { return &t; }

bool less(int x, int y) { return x < y ? x : y; }

static auto* f = addr(less);

and

template <typename T>
T* addr(T& t) { return &t; }

bool less(int x, int y) { return x < y ? x : y; }

template<typename T>
int deduce(T*) { return 0; }

int i = deduce(addr(less));

are accepted as they should.

[Bug c++/55159] pythy constexpr auto lambda pointer has no initializer

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55159

--- Comment #4 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2012-11-02 08:10:14 UTC ---
(In reply to comment #3)
> But it seems that gcc doesn't ignore the const (in "const T*" or "const auto*")
> for functions here, which seems to be the root of the second problem (This is
> *not* attempting to form a function with cv-qualifier-seq).

I withdraw my interpretation that it is *clear* here that during template
argument deduction we can successfully match a "const T" with a function type.
This looks like a core language problem to me and I'll notify CWG in regard to
this.

[Bug c++/55159] pythy constexpr auto lambda pointer has no initializer

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55159

--- Comment #5 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2012-11-02 08:57:33 UTC ---
I need to insert another correction. My attempt to simplify the reporters bug
failed because I was mislead by the report description that "visual studio 2012
work fine" interpreting this to mean the Microsoft Visual Studio 2012 compiler.
But that one does not understand 'constexpr' therefore my reduction to 'const'
was not equivalent.

(In reply to comment #1)
> I really think that Pythy should fix this implementation, because it is not
> supported by the C++11 standard. Any compiler accepting that is defect.

This statement still holds. The original code is not conforming.

The following presents a conforming code that reproduces the problem:

template <class T0, class T1>
struct min_t
{
    static bool less(T0 x, T1 y) { return x < y ? x : y; }
    constexpr static auto* f = &less; // #5
};

min_t<int, int> mi; // #8

"8|  required from here|
5|error: declaration of 'constexpr auto* const min_t<int, int>::f' has no
initializer"

Please ignore the part about const T in template deduction. This is a different
issue and not related to this one.

[Bug c++/55159] pythy constexpr auto lambda pointer has no initializer

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55159

--- Comment #6 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-11-02 11:14:54 UTC ---
Thus, Daniel, is this invalid?

[Bug c++/55159] pythy constexpr auto lambda pointer has no initializer

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55159

--- Comment #7 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2012-11-02 11:39:07 UTC ---
(In reply to comment #6)
> Thus, Daniel, is this invalid?

I think this part of the problem is indeed valid:

template <class T0, class T1>
struct min_t
{
    static bool less(T0 x, T1 y) { return x < y ? x : y; }
    constexpr static auto* f = &less; // #5
};

min_t<int, int> mi; // #8

"8|  required from here|
5|error: declaration of 'constexpr auto* const min_t<int, int>::f' has no
initializer"

(With or without & in front of less)

According to my understanding this should be accepted.

[Bug c++/55159] pythy constexpr auto lambda pointer has no initializer

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55159

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Blocks|                            |55004

--- Comment #8 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-11-02 11:48:39 UTC ---
Ah, Ok, I was a bit lost ;) Thanks. However, my (very rough) feeling is that we
have got already quite a bit in Bugzilla in the area, constexpr, auto,
pointers...

[Bug c++/55159] pythy constexpr auto lambda pointer has no initializer

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55159

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-11-07
     Ever Confirmed|0                           |1

--- Comment #9 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-11-07 15:42:04 UTC ---
Uhm, for testcase in Comment #7 we call finish_static_data_member_decl from
instantiate_class_template_1 and we explicitly pass init == NULL_TREE, no
chances for things to work from this point on. Interestingly this is the *only*
place in pt.c where we call finish_static_data_member_decl.

[Bug c++/55159] pythy constexpr auto lambda pointer has no initializer

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55159

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
                 CC|                            |paolo.carlini at oracle dot
                   |                            |com
         Resolution|                            |DUPLICATE

--- Comment #10 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-11-07 16:11:28 UTC ---
Oh, Comment #7 is really the same as PR55003.

*** This bug has been marked as a duplicate of bug 55003 ***