Annex G (informative) Ill-formed, no diagnostic required [ifndr]

G.3 [basic]: Basics [ifndr.basic]

G.3.3[ifndr:basic.def.odr.injected.match]
Specified in: [basic.def.odr]

1
#
Defining a definable item D with an injected declaration ([expr.const.reflect]) in one translation unit and a definition in a different translation unit when D is not attached to a named module or neither definition is reachable from the other is ill-formed, no diagnostic required.
2
#
[Example 1: 

Translation unit #1:struct S; consteval { std::meta::define_aggregate(^^S, {}); } // #1

Translation unit #2:struct S {}; // IFNDR, definition here, injected declaration at #1 — end example]