CWG Issue 1514 (original) (raw)

This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-04-13

1514. Ambiguity between enumeration definition and zero-length bit-field

Section: 11.4.10 [class.bit]Status: C++14Submitter: John SpicerDate: 2012-07-03

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

Consider an example like:

struct S { typedef int T; enum E : T {}; enum E : T {}; // #1 };

The declaration at #1 is ambiguous: it could either be an invalid redefinition of enum E or a zero-length bit-field of typeenum E (since T{} is zero-valued constant expression). The current Standard does not appear to have a rule to disambiguate the two.

Proposed resolution (October, 2012) [superseded]:

Change 9.8.1 [dcl.enum] paragraph 1 as follows:

...the attributes in that attribute-specifier-seq are thereafter considered attributes of the enumeration whenever it is named. In a member-specification, an ambiguity can arise from the similarity between the declaration of an enumeration with an_enum-base_ and the declaration of a zero-length unnamed bit-field of enumeration type. The ambiguity appears as a choice between an enum-specifier and a member-declaration for a bit-field. The resolution is that a : following enum identifier is parsed as part of an enum-base. [Example:

struct S { enum E : int {}; enum E : int {}; // error: redeclaration of enumeration };

—_end example_]

Notes from the April, 2013 meeting:

The ambiguity does not occur only with an empty set of braces but also when there is a single identifier that could be taken as the name of a constant in a containing scope or the declaration of an enumerator.

The resolution above sounds as if it is to be applied only if an ambiguity occurs; it should instead be always applied.

Proposed resolution (June, 2013):

Change 9.8.1 [dcl.enum] paragraph 1 as follows:

...the attributes in that attribute-specifier-seq are thereafter considered attributes of the enumeration whenever it is named.A : following “enum identifier_” is parsed as part of an enum-base. [Note: This resolves a potential ambiguity between the declaration of an enumeration with an_enum-base and the declaration of an unnamed bit-field of enumeration type. [Example:

struct S { enum E : int {}; enum E : int {}; // error: redeclaration of enumeration };

—_end example_] —_end note_]