Rationale (February, 2008):

In the other cases where a class type is considered complete within the definition of the class, it is possible to defer handling the construct until the end of the definition. That is not possible for types, as the type may be needed immediately in subsequent declarations.

It was also noted that the primary utility of decltype is in generic contexts; within a single class definition, other mechanisms are possible (e.g., use of a member typedef in both the declaration of the operand of the decltype and to replace the decltype itself).