[Bug c++/63217] New: template conversion operator returning const reference not used for conversion in some cases

["rs2740 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63217

            Bug ID: 63217
           Summary: template conversion operator returning const reference
                    not used for conversion in some cases
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com

Minimized repro:

struct foo {
    template<class T>
    operator const T & () { static T t {}; return t;}
};

int main() {
    int t((foo()));
}

g++ complains:

prog.cc: In function 'int main()':
prog.cc:7:18: error: cannot convert 'foo' to 'int' in initialization
     int t((foo()));
                  ^

This behavior is consistently reproducible in all versions of g++ I tested.
Both Clang and MSVC 2013 compiles this code without errors.

The template `operator const T&` should have been instantiated with `T = int`
and used for the conversion. Using a non-template `operator const int &`
instead of a template makes the code compile.