Annex F (informative) Core undefined behavior [ub]

F.5 [dcl]: Declarations [ub.dcl]

F.5.4[ub:dcl.ref.incompatible.type]
Specified in: [dcl.ref]

Initializing a reference to an object with a value that is not type-accessible ([basic.lval]) through the type of the reference has undefined behavior.
[Example 1: float g; int& i = reinterpret_cast<int&>(g); // undefined behavior — end example]