https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126357
Bug ID: 126357
Summary: Calling member function before base subobject
initialization in constant evaluation not rejected
Product: gcc
Version: 16.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: gongke at ios dot ac.cn
Target Milestone: ---
Consider this program:
```cpp
#include <cstddef>
struct A {
constexpr A(int x) : val(x) {}
int val;
};
struct B : A {
constexpr int f() const { return 42; }
constexpr B() : A(f()) {} // undefined behavior: calls member function f()
but base A not yet initialized
};
constexpr int foo() {
constexpr B b{};
return b.val;
}
int main() {
constexpr auto x = foo();
return 0;
}
```
C++20 final draft N4861 [class.base.init]/16:
> Member functions (including virtual member functions, 11.7.2) can be called
> for an object under construction. Similarly, an object under construction can
> be the operand of the typeid operator (7.6.1.7) or of a dynamic_cast
> (7.6.1.6). However, if these operations are performed in a ctor-initializer
> (or in a function called directly or indirectly from a ctor-initializer )
> before all the mem-initializer s for base classes have completed, the program
> has undefined behavior. [Example:
> ```cpp
> class A {
> public:
> A(int);
> };
> class B : public A {
> int j;
> public:
> int f();
> B() : A(f()), // undefined behavior: calls member function but base A not
> yet initialized
> j(f()) { } // well-defined: bases are all initialized
> };
> ```
So this is UB and should be rejected in constant evaluation. Neither GCC nor
Clang rejects it. [godbolt](https://godbolt.org/z/er876Wbj6)
