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From: "yaghmour.shafik at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/62110] New: Attempting to use template conversion operator in a contextual conversion
Date: Tue, 12 Aug 2014 19:27:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62110

            Bug ID: 62110
           Summary: Attempting to use template conversion operator in a
                    contextual conversion
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yaghmour.shafik at gmail dot com

Given the following code:

class Var
{
public:

    operator int () const
    { return 0; }

    template <typename T>
    operator T () const
    { return T(); }

};

int main()
{
    Var v;
    switch (v)
    { }
}

gcc 4.9 produces the following error:

main.cpp: In function 'int main()':

main.cpp:17:14: error: default type conversion can't deduce template argument
for 'template<class T> Var::operator T() const'
     switch (v)
              ^

using the following command line options:

  -std=c++11 -Wall -Wextra -Wconversion -pedantic

Using -std=c++1y also produces the same error.

while clang 3.4 does not produce any errors. As far as I can tell clang is
correct here for C++1y and is probably correct for C++11 although that may
depend on whether you consider  N3323: "A Proposal to Tweak Certain C++
Contextual Conversions" to be a fix for C++11 or part of C++1y. 

N3323 is incorporated in N3485 which I consider to be C++11 with fixes but
perhaps that is an incorrect interpretation. Based on this assumption then if
we look at section 6.4.2 it says:

The condition shall be of integral type, enumeration type, or class type. If of
class type, the condition is contextually implicitly converted (Clause 4) to an
integral or enumeration type.

and this would force us to use section 4 paragraph 5 which does not allow or
overload resolution making int conversion the only one available for this
context.

If N3323 is not part of C++11 then it seems unclear to me whether the template
conversion function should be considered or not.

This bug report comes the following Stackoverflow question:

http://stackoverflow.com/questions/25047109/classes-with-both-template-and-non-template-conversion-operators-in-the-conditio