[Bug c++/58820] New: lambda multiple inheritance operator() not ambiguous

[Bug c++/58820] New: lambda multiple inheritance operator() not ambiguous

[sutambe at yahoo dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58820

            Bug ID: 58820
           Summary: lambda multiple inheritance operator() not ambiguous
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sutambe at yahoo dot com

The following lambda overloads are not ambiguous but they are reported as such.

template <class... Fs> 
struct overload_set : Fs...
{
  overload_set(Fs... f)
    : Fs(f)...
  {}  
};

template <class... Fs> 
overload_set<Fs...> overload(Fs... x)
{
  return overload_set<Fs...>(x...);
}

int main(void)
{
  double d = 10; 
  overload(
      [](int i)  { },
      [](double d)  { },
      [](char c) { } 
    )(d);
}

[Bug c++/58820] lambda multiple inheritance operator() not ambiguous

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58820

--- Comment #1 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
This looks like a more fundamental name lookup problem of gcc to me. It can be
reproduced with function object types that are no lambda closures:

template <class... Fs> 
struct overload_set : Fs...
{
  overload_set(Fs... f)
    : Fs(f)...
  {}  
};

template <class... Fs> 
overload_set<Fs...> overload(Fs... x)
{
  return overload_set<Fs...>(x...);
}

template<class T>
struct Closure 
{
  void operator()(T) const {}
  using FPtr = void(*)(T);
  operator FPtr() const;
};

int main()
{
  double d = 10; 
  overload(
      Closure<int>(),
      Closure<double>(),
      Closure<char>() 
    )(d);
}
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From: "paolo.carlini at oracle dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/58820] lambda multiple inheritance operator() not ambiguous
Date: Mon, 21 Oct 2013 12:22:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idX820

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-10-21
             Blocks|54367                       |
     Ever confirmed|0                           |1

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Thanks Daniel. Could even be a duplicate then.

[Bug c++/58820] lambda multiple inheritance operator() not ambiguous

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58820

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Blocks|54367                       |
             Status|NEW                         |RESOLVED

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Unrelated to lambdas really.  Here is a C++98 code which shows the same issue
with clang:
template<class t>
struct i
{
    t operator()(t i);
};
struct overload_set1 : i<int>, i<double>
{
  overload_set1() {}
};
int main(void)
{
  double d1 = 10;
  overload_set1 tt;
  tt(d1);
}


To make the original code work (you need C++17 really):
Adding
  using Fs::operator()...;
allows GCC (and ICC) to work.

So this is a bug in clang.
I don't think we need a testcase as we must likely already have a C++98 one
even.


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54367
[Bug 54367] [meta-bug] lambda expressions