[Accepted as a DR at the July, 2019 meeting.]

The term “usable in constant expressions” (7.7 [expr.const] paragraph 3) is only defined for variables:

A variable is usable in constant expressions after its initializing declaration is encountered if it is a constexpr variable, or it is of reference type or of const-qualified integral or enumeration type, and its initializer is a constant initializer.

However, uses of the term assume that it applies more widely. For example, 7.7 [expr.const] bullet 4.7.1 mentions “a non-volatile glvalue that refers to an object that is usable in constant expressions” (not all objects are variables), and bullet 4.10.1 speaks of a “data member of reference type” (also not a variable) that is usable in constant expressions.