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

2024-10-26

2672. Lambda body SFINAE is still required, contrary to intent and note

[Accepted as a DR at the November, 2023 meeting.]

Subclause 13.10.3.1 [temp.deduct.general] paragraph 9 specifies:

A lambda-expression appearing in a function type or a template parameter is not considered part of the immediate context for the purposes of template argument deduction. [Note 7: The intent is to avoid requiring implementations to deal with substitution failure involving arbitrary statements. ... -- end note ]

However, the intent of the note is not satisfied by the normative rule, because a lambda-expression appearing in a requires-expression has the same concerns as one in a function signature.

Suggested resolution: Change the rule to say that substitution into the body of a lambda is never in the immediate context of substitution into the lambda-expression and move the rule somewhere more general.

Possible resolution (reviewed by CWG 2023-08-25) [SUPERSEDED]:

1. Change in 13.5.2.3 [temp.constr.atomic] paragraph 3 as follows:

   To determine if an atomic constraint is satisfied, the parameter mapping and template arguments are first substituted into its expression. If substitution results in an invalid type or expression in the immediate context of the atomic constraint (13.10.3.1 [temp.deduct.general]), the constraint is not satisfied. Otherwise, the lvalue-to-rvalue conversion (7.3.2 [conv.lval]) is performed if necessary, and E shall be a constant expression of type bool. The constraint is satisfied if and only if evaluation of E results in true.

2. Change in 13.10.3.1 [temp.deduct.general] paragraph 9 as follows:

   A lambda-expression appearing in a function type or a template parameter Substituting into the body of a lambda-expression is not considered part of never in the immediate context for the purposes of template argument deduction of substitution into the lambda-expression. [Note 7: The intent is to avoid requiring implementations to deal with substitution failure involving arbitrary statements.

Proposed resolution (approved by CWG 2023-11-09):

1. Change in 7.5.8.1 [expr.prim.req.general] paragraph 5 as follows:

   The substitution of template arguments into a requires-expression may can result in the formation of invalid types or expressions in the immediate context of its requirements (13.10.3.1 [temp.deduct.general]) or the violation of the semantic constraints of those requirements. In such cases, the requires-expression evaluates to false; it does not cause the program to be ill-formed. The substitution and semantic constraint checking proceeds in lexical order and stops when a condition that determines the result of the requires-expression is encountered. If substitution (if any) and semantic constraint checking succeed, the requires-expression evaluates to true.

2. Change in 13.5.2.3 [temp.constr.atomic] paragraph 3 as follows:

   To determine if an atomic constraint is satisfied, the parameter mapping and template arguments are first substituted into its expression. If substitution results in an invalid type or expression in the immediate context of the atomic constraint (13.10.3.1 [temp.deduct.general]), the constraint is not satisfied. Otherwise, the lvalue-to-rvalue conversion (7.3.2 [conv.lval]) is performed if necessary, and E shall be a constant expression of type bool. The constraint is satisfied if and only if evaluation of E results in true.

3. Change in 13.10.3.1 [temp.deduct.general] paragraph 9 as follows:

   A lambda-expression appearing in a function type or a template parameter is not considered part of the immediate context for the purposes of template argument deduction. When substituting into a lambda-expression, substitution into its body is not in the immediate context. [Note 7: The intent is to avoid requiring implementations to deal with substitution failure involving arbitrary statements.