This proposal removes the restriction that prevented constexpr member functions in classes with virtual base classes. Previously, constructors, destructors, and other member functions in virtually-inheriting classes could not be declared constexpr. Now all such functions are eligible for constant evaluation.

For example, virtual inheritance is used in the standard library's iostream hierarchy. The restriction blocked constexpr std::stringstream and compile-time stream-based parsing. Removing this limitation enables constant evaluation for any class hierarchy, regardless of whether it uses virtual inheritance.

#include <print>

struct Base {
    int value;
    constexpr Base(int v) : value(v) {}
    constexpr virtual int get() const { return value; }
};

struct Left : virtual Base {
    constexpr Left(int v) : Base(v) {}
};

struct Right : virtual Base {
    constexpr Right(int v) : Base(v) {}
};

struct Diamond : Left, Right {
    constexpr Diamond(int v) : Base(v), Left(v), Right(v) {}
    constexpr int get() const override { return value * 2; }
};

int main() {
    constexpr auto d = Diamond(21);
    static_assert(d.get() == 42);
    std::println("Diamond value: {}", d.get());
}