diagnostic divergence between -std=c++20 and -std=c++17 when referencing member of std::vector of incomplete type

[ldalessa]

In the following code example, I perform a value initialization of a std::vector of incomplete type.

#include <vector>

struct B;

struct A {
    std::vector<B> _;
};

A a{};  // <--here

struct B {};

https://godbolt.org/z/ab5Erqc1n

This runs afoul of the second half of https://eel.is/c++draft/vector#overview-4. "T shall be complete before any member of the resulting specialization of vector is referenced." I do not know if this is ill-formed or undefined behavior.

Staring in clang-15, this is diagnosed as invalid, but only with -std=c++20. Thisimplies to me that this diagnostic is probably incidental. For consistency it would be helpful if this diagnostic could be emitted for all standard versions, as a Quality of Implementation improvement.

As a reference I have added https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108669 to normalize this diagnostic across clang and gcc.

[ldalessa]

Just as an update, in the gcc issue Johnathan pointed out that this is not ill-formed, in the library "shall" requires something from the user (me) and failure means library UB. So this is really just a QoI diagnostic request to normalize the error across language standards.