[Bug c++/53660] New: function pointer conversion function template with nested-name-specifier ignored

[Bug c++/53660] New: function pointer conversion function template with nested-name-specifier ignored

[potswa at mac dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53660

             Bug #: 53660
           Summary: function pointer conversion function template with
                    nested-name-specifier ignored
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: potswa@mac.com


template< bool >
struct enable_if { typedef void (*t)(); };

struct c {
    template< bool t = true >
    operator typename enable_if< t >::t () { return []{}; }
};

int main() {
    c()(); // error: no match for call to ‘(c) ()’
}

http://stackoverflow.com/questions/11018810

In C++03 it seems that it was impossible to define a conversion function
template that would render its class Callable. Yet there is a function
add_template_conv_candidate (cp/call.c) which seems to be used only for this
purpose. It was added back in 1997 and it's not clear that it's been tested in
a while. Now with default function template arguments the above construct is
possible, and it should be called but isn't.

The problem is that the test for function pointer type (etc.) is applied before
instantiation in build_op_call_1. The test probably belongs in
add_conv_candidate.

[Bug c++/53660] function pointer conversion function template with nested-name-specifier ignored

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53660

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I don't think this is a bug.

GCC, ICC and clang all reject it the same way.
MSV rejects it with:
<source>(10): error C2783: 'c::operator enable_if<__formal>::t(void)': could
not deduce template argument for 't'

[Bug c++/53660] function pointer conversion function template with nested-name-specifier ignored

[potswa at mac dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53660

--- Comment #2 from David Krauss <potswa at mac dot com> ---
(In reply to Andrew Pinski from comment #1)
> I don't think this is a bug.

Right, the behavior looks correct.

As for a nested-name-specifier working in this context, it works if it's not
dependent. As for the declaration being a function template, it works as long
as the conversion-type-id is not dependent.

Oddly, a dependent conversion-type-id *is* allowed when the function is not
templated.

Anyway, the standard says to inspect the conversion function declarations and
find the ones referring to function (-pointer, -reference) types. That doesn't
admit application of template default arguments, as implicit or explicit
conversion does.

[Bug c++/53660] function pointer conversion function template with nested-name-specifier ignored

[potswa at mac dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53660

David Krauss <potswa at mac dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #3 from David Krauss <potswa at mac dot com> ---
(The standardese mentions "for each … where the conversion-type-id denotes a
type" which excludes the case of being a dependent type.)