Issue 0094: Is there an inconsistency between the constraints on passing values versus returning values?

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

Authors: Ron Guilmette, WG14
Date: 1993-12-03
Submitted against: C90
Status: Closed
Converted from: dr.htm, dr_094.html

ANSI/ISO C Defect report #rfg1:

There appears to be an inconsistency between the constraints on “passing” values versus “returning” values. The constraints for function calls clearly indicate that a diagnostic is required if any given actual argument is passed (to a prototyped function) into a corresponding formal parameter whose type is not assignment compatible with respect to the type of the passed value. In the case of values returned by a return statement however, there seems to be no such compatibility constraint imposed upon the expression given in the return statement and the corresponding (declared) function return type.

A new constraint should be added to the C Standard like:

If present, the expression given in a return statement shall have a type such that its value may be assigned to an object with the unqualified version of the return type of the containing function.

(This exactly mirrors the existing constraint on parameter matching imposed upon calls to prototyped functions.)

Comment from WG14 on 1997-09-23:

Response

The constraint in the description of the return statement is unneeded. Early on, the Committee decided that if a behavior was described as being equivalent to another construct, all of the constraints of that construct would apply. This “chaining” process means that any violation of a constraint in any section referred to explicitly or by the phrases “equivalent behavior” or “as if” will generate a diagnostic.

The Semantics section of the return statement (subclause 6.6.6.4) states: “If the expression has a type different from that of the function in which it appears, it is converted as if it were assigned to an object of that type.” The constraints in the section on simple assignment (subclause 6.3.16.1) are sufficient to assure assignment compatibility of the two types.