[Bug c++/82745] Fails to warn on narrowing conversion by std::forward (e.g. when calling make_unique)

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82745

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to helge from comment #4)
> (In reply to Jonathan Wakely from comment #2)
> > Narrowing conversions only cause a diagnostic in braced-init-list, not in
> > other contexts. This is how C++ has worked since day one.
> 
> I must admit that I'm no compiler expert, nor do I have any deep knowledge
> of the standard.  However, I think you must be mistaken here.

I assure you that "narrowing conversion" is a new term introduced by C++11, in
the context of list initialization.

Like C, C++ has always allowed implicit conversions that lose precision. They
are only disallowed inside a braced-init-list, because C++11 introduced that
that as a new form of initialization that doesn't affect historical code (well,
most historical code, except for aggregate initialization).

-Wsystem-headers will give you -Wconversion warnings for the code inside
<memory>:

/usr/include/c++/7/bits/unique_ptr.h:825:30: warning: conversion to ‘unsigned
char’ from ‘unsigned int’ may alter its value [-Wconversion]
     { return unique_ptr<_Tp>(new _Tp(std::forward<_Args>(__args)...)); }
                              ^

This is the same issue as PR 58876, namely that the problem happens inside a
system header, so the diagnostics are suppressed by default. It has nothing to
do with perfect forwarding or std::forward.