[dcl.type.decltype] (original) (raw)

:

As a result, storage is not allocated for the prvalue and it is not destroyed.

Thus, a class type is not instantiated as a result of being the type of a function call in this context.

In this context, the common purpose of writing the expression is merely to refer to its type.

In that sense, adecltype-specifier is analogous to a use of a typedef-name, so the usual reasons for requiring a complete type do not apply.

In particular, it is not necessary to allocate storage for a temporary object or to enforce the semantic constraints associated with invoking the type's destructor.

— end note