https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100710
Bug ID: 100710
Summary: static_cast to derived* of base* pointing to
non-static data member of base type not rejected in
constant expression
Product: gcc
Version: 11.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: jason.e.cobb at gmail dot com
Target Milestone: ---
In the following code:
[code]
class B {};
class D : public B {
public:
B member;
};
constexpr bool test(bool do_ub = true) {
if (do_ub) {
D d;
return static_cast<D*>(&d.member) != nullptr;
}
return false;
}
static_assert(test());
[/code]
On Compiler Explorer: https://godbolt.org/z/eKEcT7bhb
GCC 11.1 accepts when it should reject due to the static_cast being UB during
the evaluation of a constant expression, as the pointer to B does not actually
point to a base class subobject of an object of type D.
>From [expr.static.cast]/11:
> If the prvalue of type “pointer to cv1 B” points to a B that is actually a
> subobject of an object of type D, the resulting pointer points to the
> enclosing object of type D. Otherwise, the behavior is undefined.
This wording is slightly broken because it says "actually a subobject" instead
of "actually a base class subobject", but there is an editorial PR to fix this:
https://github.com/cplusplus/draft/pull/4605
