C issue 0251: are struct fred and union fred the same type ?

Issue 0251: are `struct fred` and `union fred` the same type ?

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Clive D.W. Feather <clive@demon.net>, UK C Panel
Date: 2001-09-07
Submitted against: C99
Status: Fixed
Fixed in: C99 TC3
Converted from: summary-c99.htm, dr_251.htm

Problem

Consider the code:

 union fred { int a; }
    int main (void)
    {
        struct fred *ptr;  /* Line X */
        // ...

I can see nothing that forbids this code. In particular, 6.7.2.3#8 reads:

[#8] If a type specifier of the form
        struct-or-union identifier
or
        enum identifier
occurs other than as part of one of the above forms, and a declaration of the identifier as a tag is visible, then it specifies the same type as that other declaration, and does not redeclare the tag.

At line X a declaration of fred as a tag is visible, so this line specifies the same type as that other declaration, even though this uses struct and that uses union !

It has been further pointed out to me that nothing in the Standard actually says that union x is a union type as opposed to a structure type, and vice versa.

Suggested Technical Corrigendum

Append to 6.7.2.1#6:

The keywords struct and union indicate that the type being specified is, respectively, a structure type or a union type.

Add a new paragraph following 6.7.2.3#1:

[#1a] Where two declarations that use the same tag declare the same type, they shall both use the same choice of struct, union, or enum.

Comment from WG14 on 2006-03-29:

Committee Discussion

The Committee is inclined to accept the suggested TC, but the issue is still being debated.

Technical Corrigendum