r243463 - Do not give a -Wredundant-move warning when removing the move will result in an

[Arthur O'Dwyer]

When the criteria for elision of a copy operation are met[,] *or would be
met save for the fact that the source object is a function parameter,* and
the object to be copied is designated by an lvalue, overload resolution to
select the constructor for the copy is first performed as if the object
were designated by an rvalue. If overload resolution fails, or if the type
of the first parameter of the selected constructor is not an rvalue
reference to the object’s type (possibly cv-qualified), overload resolution
is performed again, considering the object as an lvalue.  —N3797, 12.8
[class.copy]/32


N4296 says the same thing, but much more verbosely:

When the criteria for elision of a copy/move operation are met, but not for
an exception-declaration, and the object to be copied is designated by an
lvalue, or when the expression in a return statement is a (possibly
parenthesized) id-expression that names an object with automatic storage
duration declared in the body or parameter-declaration-clause of the
innermost enclosing function or lambda-expression, overload resolution to
select the constructor for the copy is first performed as if the object
were designated by an rvalue.
 Looks to me as if GCC is correct and Clang is (/has always been) incorrect.

–Arthur


On Wed, Jul 29, 2015 at 2:57 AM, Sebastian Redl <
sebastian.redl at getdesigned.at> wrote:

>
>
> On 28.07.2015 21:06, Richard Trieu wrote:
>
>> Author: rtrieu
>> Date: Tue Jul 28 14:06:16 2015
>> New Revision: 243463
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=243463&view=rev
>> Log:
>> Do not give a -Wredundant-move warning when removing the move will result
>> in an
>> error.
>>
>> If the object being moved has a move constructor and a deleted copy
>> constructor,
>> std::move is required, otherwise Clang will give a deleted constructor
>> error.
>>
>>  Is that actually correct behavior by Clang? GCC 5.2 compiles the
> following without problems:
>
> struct A {
>     A() {}
>     A(A&&) {}
> };
>
> struct B {
>   B(A) {}
> };
>
> B fn(A a) {
>     return a;
> }
>
> int main() {
>     fn(A());
> }
>
> Sebastian
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>
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