CWG Issue 2084 (original) (raw)

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

2025-11-05

2084. NSDMIs and deleted union default constructors

Section: 11.4.5 [class.ctor]Status: CD4Submitter: Daveed VandevoordeDate: 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:

• X is a union-like class that has a variant member with a non-trivial default constructor,
• ...

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:

• X is a union that has a variant member with a non-trivial default constructor and no variant member of X has a default member initializer,
• X is a union-like non-union class that has a variant member M with a non-trivial default constructor and no variant member of the anonymous union containing M has a default member initializer,
• ...