[Bug c++/50306] New: g++ accepts code with ambiguous, templated conversion operators

[Bug c++/50306] New: g++ accepts code with ambiguous, templated conversion operators

[malaperle at omnialabs dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50306

             Bug #: 50306
           Summary: g++ accepts code with ambiguous, templated conversion
                    operators
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: malaperle@omnialabs.net


This code does not generate an error or warning:

class A{};
class B : public A {};

template <class T> class SmartPtr{
public:
    template<typename OtherT> operator const SmartPtr<OtherT>&() const;

    template<typename OtherT> operator SmartPtr<OtherT>() const;
};

void func(SmartPtr<A>) {
}

int main() {
    SmartPtr<B> b;
    func(b); // this should be ambiguous?
}

But it is an error with MSVC 10 and Clang 2.8.

If the templates on the operators are removed, it also becomes ambiguous with
gcc:

class A {};
class B : public A {};

template <class T> class SmartPtr {
public:
    operator const SmartPtr<A>&() const;
    operator SmartPtr<A>() const;
};

void func(SmartPtr<A>) {
}

int main() {
    SmartPtr<B> b;
    func(b); // func is ambiguous
}

[Bug c++/50306] ambiguous templated conversion operators accepted

[malaperle at omnialabs dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50306

--- Comment #1 from Marc-Andre Laperle <malaperle at omnialabs dot net> 2011-11-17 18:00:27 UTC ---
I have never worked with the GCC source code before, any pointers to where I
should start to look to fix this issue? Thanks!

[Bug c++/50306] ambiguous templated conversion operators accepted

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50306

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |manu at gcc dot gnu.org

--- Comment #2 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2011-11-17 18:27:00 UTC ---
(In reply to comment #1)
> I have never worked with the GCC source code before, any pointers to where I
> should start to look to fix this issue? Thanks!

http://gcc.gnu.org/wiki/DebuggingGCC

You could put a breakpoint for the second testcase in error(). Find out why the
error triggers, and then find out what is the difference with the first
testcase. In any case, the relevant code is in gcc/cp/. I hope you don't get
scared by the looks. It ain't pretty.

However, this seems to be fixed in trunk (at least revision 180166):

manuel@gcc12:~$ ~/trunk/180166/build/gcc/cc1plus pr50306.cc
 void func(SmartPtr<A>) int main()
pr50306.cc:15:9: error: conversion from ‘SmartPtr<B>’ to ‘SmartPtr<A>’ is
ambiguous
pr50306.cc:14:15: note: candidates are:
pr50306.cc:7:3: note: SmartPtr<T>::operator SmartPtr<A>() const [with T = B]
pr50306.cc:6:3: note: SmartPtr<T>::operator const SmartPtr<A>&() const [with T
= B]
pr50306.cc:10:6: error:   initializing argument 1 of ‘void func(SmartPtr<A>)’

Maybe you should try 4.6.2?

[Bug c++/50306] ambiguous templated conversion operators accepted

[malaperle at omnialabs dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50306

--- Comment #3 from Marc-Andre Laperle <malaperle at omnialabs dot net> 2011-11-19 05:38:32 UTC ---
I tried 4.6.2, r181503 and r180166 and they all compiled the sample code
without error. Do you have any local changes or special parameters that made it
not compile?

[Bug c++/50306] ambiguous templated conversion operators accepted

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50306

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> 2011-11-19 12:11:05 UTC ---
Yes, I confirm the issue doesn't seem fixed neither mainline nor 4_6-branch.

[Bug c++/50306] ambiguous templated conversion operators accepted

[manu at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50306

Manuel López-Ibáñez <manu at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011-11-19
     Ever Confirmed|0                           |1

--- Comment #5 from Manuel López-Ibáñez <manu at gcc dot gnu.org> 2011-11-19 12:46:13 UTC ---
You are right, it seems I tested the second example by mistake.