https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126639
Bug ID: 126639
Summary: Undiagnosed undefined behaviour in pointer-to-member
conversion
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ivan.lazaric.gcc at gmail dot com
Target Milestone: ---
```cpp
struct A { char a; };
struct B { char b; };
struct AB : A, B {};
constexpr auto ap = &A::a;
constexpr auto abp = (char AB::*)ap;
constexpr auto bp = (char B::*)abp; // this is UB
static_assert(bp == nullptr);
```
This code compiles, because after pointer-to-member conversion
gcc only tracks the offset, the offset in bp becomes -1,
which coincides with nullptr representation in ptr-to-members.
https://eel.is/c++draft/conv.mem#2
> If class D does not contain the original member and is not a base class of
> the class containing the original member, the behavior is undefined
clang diagnoses this
Godbolt: https://godbolt.org/z/64hfcjKq6
