c++/6451: function template argument deduction wrong

[Jens.Maurer@gmx.net]

>Number:         6451
>Category:       c++
>Synopsis:       function template argument deduction wrong
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          rejects-legal
>Submitter-Id:   net
>Arrival-Date:   Thu Apr 25 07:26:00 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     Jens Maurer
>Release:        3.0.4 and 3.2 20020423 (experimental)
>Organization:
>Environment:
Linux 2.4.17
>Description:
The attached program gives errors with gcc 3.0.4, although
according to ISO C++ section 14.8.2p2, only type deduction
should fail, so the other overload should be taken.

[...]
template-deduct-fail-datamember.cc: In instantiation of `has_int_data_x<X1>':
template-deduct-fail-datamember.cc:38:   instantiated from here
template-deduct-fail-datamember.cc:17: invalid use of undefined type `struct 
   has_int_data_x<X1>'
template-deduct-fail-datamember.cc:8: forward declaration of `struct 
   has_int_data_x<X1>'
[...]

It even produces an internal error with 3.2 20020423 (as 
a regression from 3.0.4):

Please submit a full bug report,
with preprocessed source if appropriate.
See <URL:http://www.gnu.org/software/gcc/bugs.html> for instructions.
jmaurer@gigue:~/C++/ISO/Curacao/htdocs/demo > /opt/exp/gcc/bin/g++ template-deduct-fail-datamember.cc
>How-To-Repeat:
#include <iostream>

struct small_t { int m[1]; };
struct large_t { int m[20]; };

// find if a class has a data member (of a given type)

template<class T> struct has_int_data_x {
    private:
        template<class U> struct helper {
            template<int U::*> struct inner { };
        };
        template<class U> static small_t check(typename helper<U>::template
inner<&U::x>*);
        template<class U> static large_t check(...);
    public:
        static const bool value = sizeof(check<T>(0)) == sizeof(small_t);
};

template<class T> const bool has_int_data_x<T>::value;

struct X1
{
  int x;
};

struct X2
{
  double x;
};

struct X3
{
  int y;
};

int main() {
    std::cout << has_int_data_x<X1>::value
              << has_int_data_x<X2>::value
              << has_int_data_x<X3>::value << std::endl;
    return 0;
}

>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted: