Proposed resolution (approved by CWG 2024-10-25)

1. Change in 7.2.1 [basic.lval] paragraph 7 as follows:

   Whenever Unless otherwise specified (7.6.1.11 [expr.const.cast]), whenever a prvalue appears as an operand of an operator that expects a glvalue for that operand, the temporary materialization conversion (7.3.5 [conv.rval]) is applied to convert the expression to an xvalue.

2. Change in 7.6.1.11 [expr.const.cast] paragraph 1 as follows:

   The result of the expression const_cast<T>(v) is of type T. If T is an lvalue reference to object type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) standard conversions are performed on the expression v. The temporary materialization conversion (7.3.5 [conv.rval]) is not performed on v, other than as specified below. Conversions that can be performed explicitly using const_cast are listed below. No other conversion shall be performed explicitly using const_cast.

3. Change in 7.6.1.11 [expr.const.cast] paragraph 3 as follows:

   For two similar object pointer or pointer to data member types T1 and T2 (7.3.6 [conv.qual]), a prvalue of type T1 may can be explicitly converted to the type T2 using a const_cast if, considering the qualification-decompositions of both types, each Pi1 is the same as Pi2 for all i. The result of a const_cast refers to the original entity. If v is a null pointer or null member pointer, the result is a null pointer or null member pointer, respectively. Otherwise, the result points to or past the end of the same object, or points to the same member, respectively, as v. [Example 1:

     typedef int *A[3];                // array of 3 pointer to int
     typedef const int *const CA[3];   // array of 3 const pointer to const int
     CA &&r = A{};        // OK, reference binds to temporary array object
                          // after qualification conversion to type CA
     A &&r1 = const_cast<A>(CA{});     // error: temporary array decayed to pointer
   
     A &&r2 = const_cast<A&&>(CA{});    // OK
   

   -- end example]

4. Change in 7.6.1.11 [expr.const.cast] paragraph 4 as follows:

   For two object types T1 and T2, if a pointer to T1 can be explicitly converted to the type “pointer to T2” using a const_cast, then the following conversions can also be made:

   • an lvalue of type T1 can be explicitly converted to an lvalue of type T2 using the cast const_cast<T2&>;
   • a glvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>; and
   • if T1 is a class or array type, a prvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>. The temporary materialization conversion is performed on v.

   The result of a reference const_cast refers to the original same object if as the (possibly converted) operand is a glvalue and to the result of applying the temporary materialization conversion (7.3.5 [conv.rval]) otherwise.

   [Example 2:

     typedef int *A[3];                // array of 3 pointer to int
     typedef const int *const CA[3];   // array of 3 const pointer to const int
   
     auto &&r2 = const_cast<A&&>(CA{}); // OK, temporary materialization conversion is performed
   

   -- end example]

5. Remove 7.6.1.11 [expr.const.cast] paragraph 5:

   A null pointer value (6.8.4 [basic.compound]) is converted to the null pointer value of the destination type. The null member pointer value (7.3.13 [conv.mem]) is converted to the null member pointer value of the destination type.

6. Change in 9.4.4 [dcl.init.ref] bullet 5.3 as follows:

   [Example 5: ...

     constexpr int f() {
       const int &x = 42;
       const_cast<int &>(x) = 1;  // undefined behavior
       return x;
     }
     constexpr int z = f();          // error: not a constant expression
   

     typedef int *AP[3];        // array of 3 pointer to int
     typedef const int *const ACPC[3]; // array of 3 const pointer to const int
     ACPC &&r = AP{};          // binds directly
   

   -- end example]

This resolution also resolve issue 1965.

CWG 2024-10-11

Subclause 7.2.1 [basic.lval] paragraph 7 should be amended with "unless otherwise specified" and cross-references to the exceptions.

Suggested resolution [SUPERSEDED]:

1. Change in 7.6.1.11 [expr.const.cast] paragraph 1 as follows:

   The result of the expression const_cast<T>(v) is of type T. If T is an lvalue reference to object type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) standard conversions are performed on the expression v. The temporary materialization conversion (7.3.5 [conv.rval]) is not performed on v, other than as specified below. Conversions that can be performed explicitly using const_cast are listed below. No other conversion shall be performed explicitly using const_cast.

2. Change in 7.6.1.11 [expr.const.cast] paragraph 3 as follows:

   For two similar object pointer or pointer to data member types T1 and T2 (7.3.6 [conv.qual]), a prvalue of type T1 may can be explicitly converted to the type T2 using a const_cast if, considering the qualification-decompositions of both types, each Pi1 is the same as Pi2 for all i. The result of a const_cast refers to the original entity. If v is a null pointer or null member pointer, the result is a null pointer or null member pointer, respectively. Otherwise, the result points to or past the end of the same object, or points to the same member, respectively, as v. [Example 1:

     typedef int *A[3];                // array of 3 pointer to int
     typedef const int *const CA[3];   // array of 3 const pointer to const int
     CA &&r = A{};        // OK, reference binds to temporary array object
                          // after qualification conversion to type CA
     A &&r1 = const_cast<A>(CA{});     // error: temporary array decayed to pointer
   
     A &&r2 = const_cast<A&&>(CA{});    // OK
   

   -- end example]

3. Change in 7.6.1.11 [expr.const.cast] paragraph 4 as follows:

   For two object types T1 and T2, if a pointer to T1 can be explicitly converted to the type “pointer to T2” using a const_cast, then the following conversions can also be made:

   • an lvalue of type T1 can be explicitly converted to an lvalue of type T2 using the cast const_cast<T2&>;
   • a glvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>; and
   • if T1 is a class or array type, a prvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>. The temporary materialization conversion is performed on v.

   The result of a reference const_cast refers to the original same object if as the (possibly converted) operand is a glvalue and to the result of applying the temporary materialization conversion (7.3.5 [conv.rval]) otherwise.

   [Example 2:

     typedef int *A[3];                // array of 3 pointer to int
     typedef const int *const CA[3];   // array of 3 const pointer to const int
   
     auto &&r2 = const_cast<A&&>(CA{}); // OK, temporary materialization conversion is performed
   

   -- end example]

4. Remove 7.6.1.11 [expr.const.cast] paragraph 5:

   A null pointer value (6.8.4 [basic.compound]) is converted to the null pointer value of the destination type. The null member pointer value (7.3.13 [conv.mem]) is converted to the null member pointer value of the destination type.

5. Change in 9.4.4 [dcl.init.ref] bullet 5.3 as follows:

   [Example 5: ...

     constexpr int f() {
       const int &x = 42;
       const_cast<int &>(x) = 1;  // undefined behavior
       return x;
     }
     constexpr int z = f();          // error: not a constant expression
   

     typedef int *AP[3];        // array of 3 pointer to int
     typedef const int *const ACPC[3]; // array of 3 const pointer to const int
     ACPC &&r = AP{};          // binds directly
   

   -- end example]

This resolution also resolve issue 1965.

CWG 2024-05-31

The existing example in 7.6.1.11 [expr.const.cast] paragraph 3 shows the temporary materialization conversion applied to an array type. The example would be made ill-formed by the suggested resolution above. More investigation is advised.

Suggested resolution [SUPERSEDED]:

1. Change in 7.6.1.11 [expr.const.cast] paragraph 1 as follows:

   The result of the expression const_cast<T>(v) is of type T. If T is an lvalue reference to object type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue (7.3.2 [conv.lval]), array-to-pointer (7.3.3 [conv.array]), and function-to-pointer (7.3.4 [conv.func]) standard conversions are performed on the expression v. The temporary materialization conversion (7.3.5 [conv.rval]) is not performed on v, other than as specified below. Conversions that can be performed explicitly using const_cast are listed below. No other conversion shall be performed explicitly using const_cast.

2. Change in 7.6.1.11 [expr.const.cast] paragraph 3 as follows:

   For two similar object pointer or pointer to data member types T1 and T2 (7.3.6 [conv.qual]), a prvalue of type T1 may can be explicitly converted to the type T2 using a const_cast if, considering the qualification-decompositions of both types, each Pi1 is the same as Pi2 for all i. The result of a const_cast refers to the original entity. If v is a null pointer or null member pointer, the result is a null pointer or null member pointer, respectively. Otherwise, the result points to or past the end of the same object, or points to the same member, respectively, as v.

3. Change in 7.6.1.11 [expr.const.cast] paragraph 4 as follows:

   For two object types T1 and T2, if a pointer to T1 can be explicitly converted to the type “pointer to T2” using a const_cast, then the following conversions can also be made:

   • an lvalue of type T1 can be explicitly converted to an lvalue of type T2 using the cast const_cast<T2&>;
   • a glvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>; and
   • if T1 is a class type, a prvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>. The temporary materialization conversion is performed on v.

   The result of a reference const_cast refers to the original same object if as the (possibly converted) operand is a glvalue and to the result of applying the temporary materialization conversion (7.3.5 [conv.rval]) otherwise.

4. Remove 7.6.1.11 [expr.const.cast] paragraph 5:

   A null pointer value (6.8.4 [basic.compound]) is converted to the null pointer value of the destination type. The null member pointer value (7.3.13 [conv.mem]) is converted to the null member pointer value of the destination type.

Suggested resolution [SUPERSEDED]:

1. Change in 7.6.1.11 [expr.const.cast] paragraph 3 as follows:

   For two similar types T1 and T2 (7.3.6 [conv.qual]), a prvalue of type T1 may be explicitly converted to the type T2 using a const_cast if, considering the qualification-decompositions of both types, each Pi1 is the same as Pi2 for all i. The result of a const_cast refers to the original entity.

   If T is an object pointer type or pointer to data member type, the type of v shall be similar to T and the corresponding P_i components of the qualification decompositions of T and the type of v shall be the same (7.3.6 [conv.qual]). If v is a null pointer or null member pointer, the result is a null pointer or null member pointer, respectively. Otherwise, the result points to or past the end of the same object or member, respectively, as v.

   [ Example: ... ]

2. Change in 7.6.1.11 [expr.const.cast] paragraph 4 as follows:

   For two object types T1 and T2, if a pointer to T1 can be explicitly converted to the type “pointer to T2” using a const_cast, then the following conversions can also be made:

   • an lvalue of type T1 can be explicitly converted to an lvalue of type T2 using the cast const_cast<T2&>;
   • a glvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>; and
   • if T1 is a class type, a prvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_cast<T2&&>.

   The result of a reference const_cast refers to the original object if the operand is a glvalue and to the result of applying the temporary materialization conversion (7.3.5 [conv.rval]) otherwise.

   Otherwise, T shall be a reference type. Let T1 be the type of v and T2 be the type referred to by T. A const_cast from "pointer to T1" to "pointer to T2" shall be valid. If T is an lvalue reference type, v shall be an lvalue. Otherwise, if T2 is a class type and v is a prvalue, the temporary materialization conversion (7.3.5 [conv.rval]) is applied. Otherwise, the temporary materialization conversion is not applied and v shall be a glvalue. The result refers to the same object as the (possibly converted) operand.

3. Remove 7.6.1.11 [expr.const.cast] paragraph 5:

   A null pointer value (6.8.4 [basic.compound]) is converted to the null pointer value of the destination type. The null member pointer value (7.3.13 [conv.mem]) is converted to the null member pointer value of the destination type.

(From submissions #526, #232, and #342.)

The resolution of issue 891 intended to make const_cast<int&&>(2) ill-formed. However, combined with the temporary materialization conversion turning prvalues into glvalues (7.1 [expr.pre] paragraph 7, this is now well-formed.

Also, the current rules regrettably allow const_cast<int>(0) and const_casts involving function pointers and pointers to member functions. The latter is non-normatively considered disallowed by the note in 7.6.1.11 [expr.const.cast] paragraph 9.

Major implementations except MSVC agree with the proposed direction of this issue.