[committed] libstdc++: Make std::function work better with -fno-rtti

[Jonathan Wakely]

On 29/10/20 14:49 +0000, Jonathan Wakely wrote:
>This change allows std::function::target<F>() to work even without RTTI,
>using the same approach as std::any. Because we know what the manager
>function would be for a given type, we can check if the stored pointer
>has the expected address. If it does, we don't need to use RTTI. If it
>isn't equal, we still need to do the RTTI check (when RTTI is enabled)
>to handle the case where the same function has different addresses in
>different shared objects.
>
>This also changes the implementation of the manager function to return a
>null pointer result when asked for the type_info of the target object.
>This not only avoids a warning with -Wswitch -Wsystem-headers, but also
>avoids prevents std::function::target_type() from dereferencing an
>uninitialized pointer when the linker keeps an instantiation of the
>manager function that was compiled without RTTI.
>
>Finally, this fixes a bug in the non-const overload of function::target
>where calling it with a function type F was ill-formed, due to
>attempting to use const_cast<F*>(ptr). The standard only allows
>const_cast<T*> when T is an object type.  The solution is to use
>*const_cast<F**>(&ptr) instead, because F* is an object type even if F
>isn't. I've also used _GLIBCXX17_CONSTEXPR in function::target so that
>it doesn't bother instantiating anything for types that can never be a
>valid target.
>
>libstdc++-v3/ChangeLog:
>
>	* include/bits/std_function.h (_Function_handler<void, void>):
>	Define explicit specialization used for invalid target types.
>	(_Base_manager::_M_manager) [!__cpp_rtti]: Return null.
>	(function::target_type()): Check for null pointer.
>	(function::target()): Define unconditionall. Fix bug with
>	const_cast of function pointer type.
>	(function::target() const): Define unconditionally, but
>	only use RTTI if enabled.
>	* testsuite/20_util/function/target_no_rtti.cc: New test.
>
>Tested powerpc64le-linux. Committed to trunk.
>
>2
>1
>0

Oops, sorry for the duplicate mail about this. My mailer ate my
attempt to script something.