[Moved to DR at the April, 2013 meeting.]

Proposed resolution (December, 2012):

1. Change 6.8 [basic.types] paragraph 5 as follows:

A class that has been declared but not defined, an enumeration type in certain contexts (9.7.1 [dcl.enum]), or an array of unknown size or of incomplete element type, is an incompletely-defined object type.⁴³ Incompletely-defined object types...

2. Add the following as a new paragraph preceding 9.7.1 [dcl.enum] paragraph 6:

An enumeration whose underlying type is fixed is an incomplete type from its point of declaration (6.4.2 [basic.scope.pdecl]) to immediately after its enum-base (if any), at which point it becomes a complete type. An enumeration whose underlying type is not fixed is an incomplete type from its point of declaration to immediately after the closing } of its enum-specifier, at which point it becomes a complete type.

For an enumeration whose underlying type is not fixed...

This resolution also resolves issue 977.

According to 6.4.2 [basic.scope.pdecl] paragraph 2,

The point of declaration for an enumeration is immediately after the identifier (if any) in either its enum-specifier (9.7.1 [dcl.enum]) or its first opaque-enum-declaration (9.7.1 [dcl.enum]), whichever comes first.

This would make the following example well-formed:

  template<typename T> struct S { typedef char I; };
  enum E: S<E>::I { e };

Presumably that would make E an incomplete enumeration type for the instantiation of S<E> (a concept not otherwise found in the Standard). However, some implementations reject this example, presumably making the point of declaration after the enum-base. We either need to make it ill-formed or describe incomplete enumeration types.

See also issue 977.