https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109243
Bug ID: 109243
Summary: Side cast of pointer-to-member with UB is incorrectly
accepted in constant evaluation
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Keywords: accepts-invalid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: de34 at live dot cn
Target Milestone: ---
The following code snippet is incorrectly accepted by gcc (perhaps in all
versions supporting constexpr) (Godbolt link: https://godbolt.org/z/GqodcGWPe):
```
struct B1 {
char i;
};
struct B2 {
char j;
};
struct D : B1, B2 {};
constexpr auto mp = static_cast<char B2::*>(static_cast<char D::*>(&B1::i));
```
According to [expr.static.cast]/13, such static_cast (performing side cast) has
undefined behavior, and should not be accepted as a constant (sub)expression.
(Unfortunately, there're well-defined manners to create a non-null
pointer-to-member with offset -1, see
https://github.com/itanium-cxx-abi/cxx-abi/pull/163).
