[Bug c++/51066] New: [C++0x] warning about binding an rvalue-reference to an implicit conversion result

[Bug c++/51066] New: [C++0x] warning about binding an rvalue-reference to an implicit conversion result

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51066

             Bug #: 51066
           Summary: [C++0x] warning about binding an rvalue-reference to
                    an implicit conversion result
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: redi@gcc.gnu.org


Inspired by the thread beginning with c++std-core-20672, G++ should warn about
this code:

struct A { };
struct B { B(A) {} };

A a;
B&& b = a;

warning: 'b' will bind to a temporary of type 'B' created by implicit
conversion from 'a'


This should only give a warning for a local variable of rvalue reference type
that is initialized by a non-reference-related object so that an implicit
conversion occurs.

There should be no warning when the conversion initializes a function parameter
e.g.

void f(B&&);
A a;
f(a);  // ok

[Bug c++/51066] [C++0x] warning about binding an rvalue-reference to an implicit conversion result

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51066

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-01-19 19:42:33 UTC ---
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1414 is related

[Bug c++/51066] [C++0x] warning about binding an rvalue-reference to an implicit conversion result

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51066

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This warning would also help with this horror:
https://twitter.com/zygoloid/status/1367301323838812160

#include <stdio.h>
void f(float&&) { puts("float"); }
void f(int&&) { puts("int"); }
template<typename T> void g(T x) { f(x); }
int main() {
  g(1.0f);
  g(2);
}

Prints "int\nfloat\n" because f(x) cannot bind directly to an rvalue reference
so there is an implicit conversion.

[Bug c++/51066] [C++0x] warning about binding an rvalue-reference to an implicit conversion result

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51066

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And this one https://twitter.com/wakomeup/status/1274778577087627267

struct B { };
struct D : B { };
B b;
D&& d(b);              // binds to implicit conversion result
D&& dd(std::move(b));  // binds to implicit conversion result

[Bug c++/51066] [C++0x] warning about binding an rvalue-reference to an implicit conversion result

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51066

--- Comment #5 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #3)
> This warning would also help with this horror:
> https://twitter.com/zygoloid/status/1367301323838812160

(In reply to Jonathan Wakely from comment #4)
> And this one https://twitter.com/wakomeup/status/1274778577087627267

Archive links for these: 

http://web.archive.org/web/20220327011407/https://twitter.com/zygoloid/status/1367301323838812160
http://web.archive.org/web/20200621190630/https://twitter.com/wakomeup/status/1274778577087627267