https://bugs.llvm.org/show_bug.cgi?id=32072
Casey Carter <ca...@carter.net> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
CC| |ca...@carter.net
Resolution|--- |INVALID
--- Comment #3 from Casey Carter <ca...@carter.net> ---
This is not a bug in clang: it's a defect in the specification of
std::function's conversion constructor template that I worked around in the
MSVC STL in August of 2017. It is fixed in the current release of VS2017.
In detail: std::function<T(Args...)> has a conversion constructor template that
accepts an argument of any type that is callable with Args... and returns a
type that is convertible to T.
At the closing brace in the definition of:
struct B {
std::function<C()> f;
};
the compiler is required to generate the implicitly-defined copy constructor
for B. To determine if that copy constructor should be deleted, it performs
overload resolution to determine if each of the members of B is copy
constructible. For f, that means it must determine if an object of type
std::function<C()> can be initialized from an lvalue expression of type const
std::function<C()>.
There are two candidate constructors for that initialization: std::function's
implicitly-defined copy constructor, and its conversion constructor template.
To determine if the conversion constructor template is viable it has to check
the constraints:
* "Can I call a std::function<C(Args...)> with Args...?" Yes.
* "Is the return type C convertible to C?" If implemented with
is_convertible_v<C, C>, which we do, results in undefined behavior when C is an
incomplete type.
We now guard the conversion constructor template against being used for copy
and move construction. I suspect the other Standard Libraries do as well, but
that behavior isn't guaranteed by the Standard.
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