Annex F (informative) Core undefined behavior [ub]

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

F.5.3[ub:dcl.ref.incompatible.function]
Specified in: [dcl.ref]

Initializing a reference to a function with a value that is a function that is not call-compatible ([expr.call]) with the type of the reference has undefined behavior.
[Example 1: void f(float x); void (&g)(int) = reinterpret_cast<void (&)(int)>(f); // undefined behavior — end example]