Audience: EWG, CWG
S. Davis Herring <>
November 11, 2022

History

Since r0:

• Removed outdated change to [basic.def]
• Allowed bare export of certain declarations requested by EWG (and deduction-guides, whose status is unclear, for the sake of discussion)

Introduction

This paper addresses CWG2443 by forbidding applying export directly to certain meaningless declarations. Conversely, it also allows all kinds to appear with export {}. For consistency extern "C" is given the same restrictions when used directly.

Approach

The expansion of permissions here was originally proposed in P1766, which SG2 considered in Cologne but which has never been seen by EWG (perhaps because the rest of it was considered “accepted”). EWG did see the corresponding NB comment, but the vote was too close to progress it. No issues have surfaced with the general direction, but the wording does not specify a coherent set of restrictions; as such, this wording makes most of the change (largely by separating the relevant candidates in the declaration grammar). EWG has seen this proposal several times, with varied outcomes; the most recent (in Kona 2022) requested to allow certain declarations unlikely to mislead programmers into thinking they have some effect.

Wording

Relative to N4917.

#[basic]

#[basic.pre]

Change paragraph 5:

Every name is introduced by a declaration, which is a

1. name-declaration, […]

#[basic.lookup.unqual]

Change paragraph 1:

[…]

[Note: A using-directive is exported if and only if it appears in a header unit. — end note]

#[dcl.dcl]

#[dcl.pre]

Change the grammar in paragraph 1:

[…]

declaration:

name-declaration
special-declaration

name-declaration:

block-declaration
nodeclspec-function-declaration
function-definition
template-declaration
deduction-guide
explicit-instantiation
explicit-specialization
export-declaration
linkage-specification
namespace-definition
empty-declaration
attribute-declaration
module-import-declaration

special-declaration:

explicit-instantiation
explicit-specialization
export-declaration

block-declaration:

simple-declaration
asm-declaration
namespace-alias-definition
using-declaration
using-enum-declaration
using-directive
static_assert-declaration
alias-declaration
opaque-enum-declaration

[…]

#[dcl.link]

Change the grammar in paragraph 2:

linkage-specification:

extern string-literal { declaration-seq_opt }
extern string-literal name-declaration

Change paragraph 4:

A module-import-declaration shall not be directly contained in a linkage-specification. A module-import-declaration appearing in a linkage specification with other than C++ language linkage is conditionally-supported with implementation-defined semantics.

#[module.interface]

Change the grammar before paragraph 1:

export-declaration:

export name-declaration
export { declaration-seq_opt }
export-keyword module-import-declaration

Change paragraph 1:

An export-declaration shall inhabit a namespace scope and appear in the purview of a module interface unit. An export-declaration shall not appear directly or indirectly within an unnamed namespace or a private-module-fragment. An export-declaration has the declarative effects of its name-declaration, declaration-seq (if any), or module-import-declaration. The name-declaration of an export-declaration shall not declare a partial specialization ([temp.decls.general]). The declaration or declaration-seq of an export-declaration shall not contain an export-declaration or module-import-declaration.

[Note: An export-declaration does not establish a scope. — end note]

Change paragraph 3:

An exported declaration that is not a module-import-declaration shall declare at least one name. If thean exported declaration is not within a header unit, it shall not declare a name with internal linkage.

Change paragraph 4:

[…]

export module M;
export namespace {}             // error: does not introduce any namesnamespace has internal linkage
export namespace {
  int a1;                       // error: export of name with internal linkage
}
namespace {
  export int a2;                // error: export of name with internal linkage
}
export static int b;            // error: b explicitly declared static
export int f();                 // OK
export namespace N { }          // OK
export using namespace N;       // error: does not declare a nameOK

[…]

#[temp]

#[temp.expl.spec]

Change paragraph 2:

The declaration in an explicit-specialization shall not be an export-declaration. An explicit specialization shall not use a storage-class-specifier ([dcl.stc]) other than thread_local.