CWG Issue 2828 (original) (raw)

This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 118f. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2025-11-07

2828. Ambiguous interpretation of C-style cast

Section: 7.6.3 [expr.cast]Status: CD7Submitter: Jim XDate: 2022-03-21

[Accepted as a DR at the March, 2024 meeting.]

(From editorial issue5355.)

Consider:

int*** ptr = 0; auto t = (int constconstconst*)ptr;

There is more than one way how this can be interpreted as a static_cast followed by a const_cast, namely:

const_cast<int const * const * const * >(static_cast<int * * const * >(ptr)); const_cast<int const * const * const * >(static_cast<int * const * const * >(ptr));

Subclause 7.6.3 [expr.cast] paragraph 4 makes such a program ill-formed:

... If a conversion can be interpreted in more than one way as astatic_cast followed by a const_cast, the conversion is ill-formed.

Proposed resolution (approved by CWG 2024-03-20):

Change in 7.6.3 [expr.cast] paragraph 4 as follows:

... If a conversion can be interpreted in more than one of the ways listed above, the interpretation that appears first in the list is used, even if a cast resulting from that interpretation is ill-formed. If aconversion can be interpreted in more than one way as a static_cast followed by a const_cast is used and the conversion can be interpreted in more than one way as such, the conversion is ill-formed. [ Example 1 :

struct A { }; struct I1 : A { }; struct I2 : A { }; struct D : I1, I2 { }; A* foo( D* p ) { return (A*)( p ); // ill-formed static_cast interpretation }

int*** ptr = 0; auto t = (int constconstconst*)ptr; // OK, const_cast interpretation

struct S { operator const int*(); operator volatile int*(); }; int p = (int)S(); // error: two possible interpretations using static_cast followed by const_cast

-- end example ]