[Bug c++/58093] Semi-bogus warning about narrowing conversions and variadic templates

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58093

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Nick Maclaren from comment #0)
> 4.13 [conv.rank], 4.7 [conv.integral] and 5 [expr] make it pretty clear
> that this is not a narrowing conversion, which makes the diagnostic very
> confusing.  I agree that it potentially changes value, but the wording
> could be better (e.g. 'loss of sign').  That's the trivial part of this
> report.

8.5.4 [dcl.init.list] p7 makes it very clear it *is* a narrowing conversion,
because the elements in the pack expansion Dims... are not constant
expressions.

> The more serious (but still minor) one is that I don't think that this
> is a soluble problem for the variadic template writer.  It is unreasonable
> to have to convert arguments explicitly, when it's automatic under almost
> all circumstances.

Why is it not soluble? The solution is trivial:

     size_t r[] = {static_cast<size_t>(Dims)...};

Or replace the static_cast with your preferred choice of cast.

G++ is following the standard, this is not a bug.

>     size_t r[] = {Dims...};
>     for (int i = 0; i < sizeof...(Dims); ++i)
>         std::cout << " " << r[i];

This would be simpler as:

    size_t r[] = {Dims...};
    for (auto i : r)
        std::cout << " " << i;