This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 108h. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2022-07-01


2084. NSDMIs and deleted union default constructors

Section: 11.4.5  [class.ctor]     Status: CD4     Submitter: Daveed Vandevoorde     Date: 2015-02-12

[Adopted at the February, 2016 meeting.]

According to 11.4.5 [class.ctor] paragraph 4 says,

A defaulted default constructor for class X is defined as deleted if:

This should make the following example ill-formed:

  struct S {
    S();
  };
  union U {
    S s{};
  } u;

because the default constructor of U is deleted. However, both clang and g++ accept this without error. Should the rule be relaxed for a union with an NSDMI?

Notes from the May, 2015 meeting:

An NSDMI is basically syntactic sugar for a mem-initializer, so the presence of one should be treated as if a user-declared default constructor were present.

Proposed resolution (October, 2015):

Change 11.4.5 [class.ctor] paragraph 4 as follows:

...A defaulted default constructor for class X is defined as deleted if: