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

2491. Export of typedef after its first declaration

[Accepted as a DR at the October, 2021 meeting.]

According to 10.2 [module.interface] paragraph 6,

A redeclaration of an entity or typedef-name X is implicitly exported if X was introduced by an exported declaration; otherwise it shall not be exported. [Example 4:

  export module M;
  struct S { int n; };
  typedef S S;
  export typedef S S; // OK, does not redeclare an entity
  export struct S;    // error: exported declaration follows non-exported declaration

—end example]

The normative text says that exporting a typedef that was not exported on its first declaration is ill-formed, but the example does so and states that it is “OK”. This is a contradiction that was introduced by the changes in paper P1787R6; the previous normative text supported the usage in the example.

(See also editorial issue 4540.)

Proposed resolution, August, 2021:

Change 10.2 [module.interface] paragraph 6 as follows:

A redeclaration of an entity or typedef-name X is implicitly exported if X was introduced by an exported declaration; otherwise it shall not be exported.