CWG Issue 2845 (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

2845. Make the closure type of a captureless lambda a structural type

Section: 7.5.6.2 [expr.prim.lambda.closure]Status: CD7Submitter: Barry RevzinDate: 2023-12-29

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

Issue 2542 (approved in June, 2023) made all closure types not be structural types, i.e. unsuitable for use as non-type template parameters. This causes an inconsistency with the treatment of the pointer-to-function conversion for closure types with no captures:

template void foo() {}

void bar() { foo<[i = 3] { return i; }>(); // #1: error foo<[]{}>(); // #2: error foo<+[]{}>(); // #3: OK, a function pointer is a structural type }

Proposed resolution (approved by CWG 2024-02-02):

  1. Change in 7.5.6.2 [expr.prim.lambda.closure] paragraph 3 as follows:

    The closure type is not an aggregate type (9.5.2 [dcl.init.aggr])and not ; it is a structural type (13.2 [temp.param])if and only if the lambda has no lambda-capture. An implementation may define the closure type differently from ...

  2. Change in 13.6 [temp.type] paragraph 2 as follows:

    Two values are template-argument-equivalent if they are of the same type and

    • ...
    • they are of a closure type (7.5.6.2 [expr.prim.lambda.closure]), or
    • they are of class type and their corresponding direct subobjects and reference members are template-argument-equivalent.