Defect Report #427

Submitter: Shao Miller <[email protected]>
Submission Date: 2013-01-24
Source: WG 14
Reference Document: N1671
Version: 1.2
Date: April 2014
Subject: Function Parameter and Return Value Assignments

Summary
The wording for the the assignments of function arguments to function parameters and for the assignment of a return statement's expression to the value of the function call can potentially be confused.

6.5.2.2p2:

If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be assigned to an object with the unqualified version of the type of its corresponding parameter.

The appearance of "may be assigned" can lead to the question (#1) of whether or not the constraints and semantics under both 6.5.16 and 6.5.16.1 might apply. The Forward references indicate 6.5.16.1, so this question might be unwarranted.

The appearance of "unqualified version of the type of its corresponding parameter" does not match 6.9.1p10, which doesn't use "unqualified" (see below).

6.5.16p2:

An assignment operator shall have a modifiable lvalue as its left operand.

If 6.5.2.2p2's mention of "assigned" implies this constraint as a secondary constraint, it is not clear which "modifiable lvalue" or even "lvalue" would ever satisfy the constraint. The "modifiable lvalue" does not appear to be the parameter, because:

• the mention of "unqualified version" suggests a theoretical object, while the parameter might be const-qualified, or
• the parameter might have an unqualified structure or union type having at least one const-qualified member (possibly via recursion)

6.7.3p4:

The properties associated with qualified types are meaningful only for expressions that are lvalues.132)

132) The implementation may place a const object that is not volatile in a read-only region of storage. Moreover, the implementation need not allocate storage for such an object if its address is never used.

This can suggest that 6.5.2.2p2's "an object with the unqualified version of the type" implies an lvalue, but (question #2) is it a modifiable lvalue? Question #3: If the type is a structure or union type with a const-qualified member (possibly via recursion), are the members considered to be unqualified, too? If so, this is an important difference from pointer types where the referenced type (or its referenced type, recursively) would not be considered unqualified. Also worth consideration would be an array object (which is not qualified) having elements matching such a structure or union type (possibly via recursion).

The return type of a function might be const-qualified, or might be a structure or union type having such a member (possibly via recursion). Question #4: Should the return type of a function be adjusted to be an unqualified version of the type? Such an adjustment might have implications for type compatibility and composite type and might be better off left alone. (const is being used for illustrative purposes, but all type qualifiers can equally be considerations.)

6.8.6.4p3:

If a return statement with an expression is executed, the value of the expression is returned to the caller as the value of the function call expression. If the expression has a type different from the return type of the function in which it appears, the value is converted as if by assignment to an object having the return type of the function.160)

160) The return statement is not an assignment. The overlap restriction of subclause 6.5.16.1 does not apply to the case of function return. The representation of floating-point values may have wider range or precision than implied by the type; a cast may be used to remove this extra range and precision.

If the return type of a function is const-qualified, or is a structure or union type having such a member (possibly via recursion), then "as if by assignment" works for 6.5.16.1, but the constraint of 6.5.16p2 requires a "modifiable lvalue".

The footnote reminds us that a return statement with an expression is not an assignment, but it is not clear that only 6.5.16.1 applies for the "as if by assignment" case.

6.9.1p10:

On entry to the function, the size expressions of each variably modified parameter are evaluated and the value of each argument expression is converted to the type of the corresponding parameter as if by assignment. (Array expressions and function designators as arguments were converted to pointers before the call.)

6.9.1p11:

After all parameters have been assigned, the compound statement that constitutes the body of the function definition is executed.

A const-qualified lvalue cannot normally be assigned-to. An lvalue for an object having a structure or union type containing a const-qualified member (possible via recursion) cannot normally be assigned-to.

6.9.1p10 doesn't match the use of "unqualified" in 6.5.2.2p2 (see above).

Suggested Technical Corrigendum

Sun c99 and GCC disagree on the return statement's semantics.

Change 6.5.2.2p2 to:

If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall be such that it satifies the constraints of simple assignment when considering the argument to be the right operand and considering the left operand to have the unqualified version of the type of the corresponding parameter.

(Loosely establishes an example for "as if by simple assignment".)

Change 6.8.6.4p3 to:

If a return statement with an expression is executed, the value of the expression is returned to the caller as the value of the function call expression. If the expression has a type different from the return type of the function in which it appears, the value is converted as if by simple assignment to an object having the unqualified version of the return type of the function.160)

Change 6.9.1p10 to:

On entry to the function, the size expressions of each variably modified parameter are evaluated in an unspecified order, the value of each argument expression is converted to the unqualified version of the type of the corresponding parameter as if by simple assignment, then each converted value becomes the initial value for the corresponding parameter. (Array expressions and function designators as arguments were converted to pointers before the call.)

Change 6.9.1p11 to:

After all parameters have initial values, the compound statement that constitutes the body of the function definition is executed.

Add bullet to J.1

- The order in which the size expressions of variably modified parameters are evaluated upon function entry (6.9.1).

Apr 2013 meeting

Committee Discussion

• No one on the committee understood the problem that this paper was trying to discuss.
• In particular, the actual difference in behavior referenced between Sun c99 and GCC is not elaborated upon.
• The committee will solicit further input from the author.

• As a result of further correspondence with the author, a proposed resolution was made on the WG14 reflector in message 13024 and refined in 13035.
• The defect is that the initial values of parameters to function calls are specified in the standard in terms of assignment instead of initialization, such that const and aggregates containing const would be excluded since they cannot be the subject of assignments!
• A Suggested Technical Corrigendum was composed, and follows. It is not yet a Proposed Technical Corrigendum due to the following unresolved discussion points.

• In 6.5.2.2p2 change:

  If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be assigned to an object with the unqualified version of the type of its corresponding parameter.

  to

  If the expression that denotes the called function has a type that includes a prototype, the number of arguments shall agree with the number of parameters. Each argument shall have a type such that its value may be used to initialize an object having the type of its corresponding parameter.

  In 6.5.2.2p4 change

  An argument may be an expression of any complete object type. In preparing for the call to a function, the arguments are evaluated, and each parameter is assigned to the value of the corresponding argument.
  

  to
  

  An argument may be an expression of any complete object type. In preparing for the call to a function, the arguments are evaluated, and each parameter is initialized to the value of the corresponding argument.

• It is not clear yet whether there are issues around conversions with respect to this change since this is actual practice that we are intending to simply reflect and to not introduce new constraints.
• This change might be augmented by a new example along the lines of, say, 6.5.16.1 example 3.

Committee Discussion

The issue of conversion has to do with whether there are differing promotions and type conversions that would apply when constructing an argument list that would not occur if these expressions were used as initializers in a declaration.