[Accepted as a DR at the June, 2023 meeting.]

Paper P2242 (Non-literal variables (and labels and gotos) in constexpr functions) added 7.7 [expr.const] bullet 5.2:

• a control flow that passes through a declaration of a variable with static (6.7.6.2 [basic.stc.static]) or thread (6.7.6.3 [basic.stc.thread]) storage duration;

It seems that block-scope extern (i.e. non-defining) declarations are covered by the above bullet, but only definitions should be in view here.