This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-11-11

2901. Unclear semantics for near-match aliased access

Section: 7.2.1  [basic.lval]     Status: tentatively ready     Submitter: Jan Schultke     Date: 2024-06-14

(From submission #548.)

Subclause 7.2.1 [basic.lval] paragraph 11 specifies:

... If a program attempts to access (3.1 [defns.access]) the stored value of an object through a glvalue through which it is not type-accessible, the behavior is undefined. ...

Thus, access (read or write) to an int object using an lvalue of type unsigned int is valid. However, 7.3.2 [conv.lval] bullet 3.4 does not specify the resulting value when, for example, the object contains the value -1:

Similarly, 7.6.19 [expr.ass] paragraph 2 is silent for the assignment case:

In simple assignment (=), the object referred to by the left operand is modified (3.1 [defns.access]) by replacing its value with the result of the right operand.

Any concerns about accesses to the object representation are handled in the context of P1839.

Proposed resolution (approved by CWG 2024-09-13):

  1. Change in 7.3.2 [conv.lval] bullet 3.4 as follows:

    • Otherwise, the object indicated by the glvalue is read (3.1 [defns.access]), and the value contained in the object is the prvalue result. Let V be the value contained in the object. If T is an integer type, the prvalue result is the value of type T congruent (6.8.2 [basic.fundamental]) to V, and V otherwise. ...

  2. Change in 7.6.1.6 [expr.post.incr] paragraph 1 as follows:

    The value of a postfix ++ expression is the value of obtained by applying the lvalue-to-rvalue conversion (7.3.2 [conv.lval]) to its operand. [Note 1: The value obtained is a copy of the original value. —end note] ...

  3. Change in 7.6.19 [expr.ass] paragraph 2 as follows:

    In simple assignment (=), let V be the result of the right operand; the object referred to by the left operand is modified (3.1 [defns.access]) by replacing its value with the result of the right operand V or, if the object is of integer type, with the value congruent (6.8.2 [basic.fundamental]) to V.