Issue 0492: Named Child struct-union with no Member

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

Authors: WG14, Clive H. Pygott
Date: 2016-03-01
Reference document: N2007
Submitted against: C11 / C17
Status: Closed
Converted from: n2396.htm

Summary

Introduction

Some weeks ago I posted the following code to the reflector after an argument at work as to whether it was legal, and if so, what was it meant to do?

       struct  S1
            {  union U11
                    {int    m11;
                     float  m12;
                    };

               int m13;
            } s1;

The issue is that U11 isn't an anonymous union (because it has a name U11) but doesn't declare a member of S1. When tried with a number of compilers the result was either that it was rejected as a constraint error or it was treated as an anonymous union, with m11 and m12 accessible as though they were members of S1 (e.g. s1.m11 = 42;). The declaration of union U11 was also added at file scope, so could be used in other structs.

After some discussion (thanks to Doug and Roberto), it was concluded that the code was intended to be a constraint error because it can be argued that it violates the first constraint in 6.7.2.1 para 2 "A struct-declaration that does not declare an anonymous structure or anonymous union shall contain a struct-declarator-list". The syntax fragment its referring to is:

struct-declaration:
specifier-qualifier-list struct-declarator-listopt ;
static_assert-declaration

In parsing the above code “union U11 {int m11; float m12;} ;” is a struct-declaration. It is believed that the intended reading is:

Hence, as U11 isn't an anonymous union and doesn't have a struct-declarator-list, it violates the quoted constraint.

However, there would appear to be a different reading that says that this struct-declaration does “contain a struct-declarator-list”. So it shouldn't be a constraint error hence the potential need for a DR to clarify the intent (discussed in the next section).

The alternative reading of the constraint requirement comes about because the specifier-qualifier-list union U11 {int m11; float m12;} is interpreted by recursively entering the same part of the syntax tree. As its interpreted, int m11; and float m12; are struct-declarations, where “m11” and “m12” are struct-declarator-lists,. So the struct-declaration for U11 does contain a struct-declarator-lists, so shouldn't be a constraint error.

The response to that argument on the reflector was that ‘whenever a constraint refers to elements of the syntax tree, it means those elements in the term currently being processed, and not any terms that may be found by recursively traversing the tree’. However, I cannot see this principle stated anywhere in the standard.

Hence, I'd argue that whether this code is legal or not is ambiguous and a DR is required, either to:

Comment from WG14 on 2017-04-07:

Apr 2016 meeting

Committee Discussion

The committee formed a strong consensus that this was not a defect.

Proposed Committee Response

There are implementations that allow this construct and other variations, but the committee is clear that since union U11 isn‘t anonymous, it is a constraint violation.