10 Modules [module]

10.4 Global module fragment [module.global.frag]

global-module-fragment:
module-keyword ; declaration-seq

_{o p t}

[Note 1:

Prior to phase 4 of translation, only preprocessing directives can appear in the declaration-seq ([cpp.pre]).

— end note]

A global-module-fragment specifies the contents of the global module fragment for a module unit.

The global module fragment can be used to provide declarations that are attached to the global module and usable within the module unit.

A declaration D is decl-reachable from a declaration S in the same translation unit if

(3.1)
D does not declare a function or function template and S contains an id-expression, namespace-name, type-name, template-name, or concept-name naming D, or
(3.2)
D declares a function or function template that is named by an expression ([basic.def.odr]) appearing in S, or
(3.3)
S contains a dependent call E ([temp.dep]) and D is found by any name lookup performed for an expression synthesized from E by replacing each type-dependent argument or operand with a value of a placeholder type with no associated namespaces or entities, or
[Note 2:
This includes the lookup for operator== performed when considering rewriting an != expression, the lookup for operator<=> performed when considering rewriting a relational comparison, and the lookup for operator!= when considering whether an operator== is a rewrite target.
— end note]
(3.4)
S contains an expression that takes the address of an overload set ([over.over]) that contains D and for which the target type is dependent, or
(3.5)
there exists a declaration M that is not a namespace-definition for which M is decl-reachable from S and either
- (3.5.1)
  D is decl-reachable from M, or
- (3.5.2)
  D and M declare the same entity, and D neither is a friend declaration nor inhabits a block scope, or
- (3.5.3)
  D declares a namespace N and M is a member of N, or
- (3.5.4)
  one of D and M declares a class or class template C and the other declares a member or friend of C, or
- (3.5.5)
  one of D and M declares an enumeration E and the other declares an enumerator of E, or
- (3.5.6)
  D declares a function or variable and M is declared in D,87 or
- (3.5.7)
  one of D and M declares a template and the other declares a partial or explicit specialization or an implicit or explicit instantiation of that template, or
- (3.5.8)
  M declares a class template and D is a deduction guide for that template, or
- (3.5.9)
  one of D and M declares a class or enumeration type and the other introduces a typedef name for linkage purposes for that type.

In this determination, it is unspecified

(3.6)
whether a reference to an alias-declaration, typedef declaration, using-declaration, or namespace-alias-definition is replaced by the declarations they name prior to this determination,
(3.7)
whether a simple-template-id that does not denote a dependent type and whose template-name names an alias template is replaced by its denoted type prior to this determination,
(3.8)
whether a decltype-specifier that does not denote a dependent type is replaced by its denoted type prior to this determination, and
(3.9)
whether a non-value-dependent constant expression is replaced by the result of constant evaluation prior to this determination.

A declaration D in a global module fragment of a module unit is discarded if D is not decl-reachable from any declaration in the declaration-seq of the translation-unit.

[Note 3:

A discarded declaration is neither reachable nor visible to name lookup outside the module unit, nor in template instantiations whose points of instantiation ([temp.point]) are outside the module unit, even when the instantiation context ([module.context]) includes the module unit.

— end note]

[Example 1: const int size = 2; int ary1[size]; // unspecified whether size is decl-reachable from ary1 constexpr int identity(int x) { return x; } int ary2[identity(2)]; // unspecified whether identity is decl-reachable from ary2 template<typename> struct S; template<typename, int> struct S2; constexpr int g(int); template<typename T, int N> S<S2<T, g(N)>> f(); // S, S2, g, and :: are decl-reachable from f template<int N> void h() noexcept(g(N) == N); // g and :: are decl-reachable from h — end example]

[Example 2:

Source file "foo.h":namespace N { struct X {}; int d(); int e(); inline int f(X, int = d()) { return e(); } int g(X); int h(X); }

Module M interface:module; #include "foo.h" export module M; template<typename T> int use_f() { N::X x; // N::X, N, and :: are decl-reachable from use_f return f(x, 123); // N::f is decl-reachable from use_f, // N::e is indirectly decl-reachable from use_f // because it is decl-reachable from N::f, and // N::d is decl-reachable from use_f // because it is decl-reachable from N::f // even though it is not used in this call } template<typename T> int use_g() { N::X x; // N::X, N, and :: are decl-reachable from use_g return g((T(), x)); // N::g is not decl-reachable from use_g } template<typename T> int use_h() { N::X x; // N::X, N, and :: are decl-reachable from use_h return h((T(), x)); // N::h is not decl-reachable from use_h, but // N::h is decl-reachable from use_h<int> } int k = use_h<int>(); // use_h<int> is decl-reachable from k, so // N::h is decl-reachable from k

Module M implementation:module M; int a = use_f<int>(); // OK int b = use_g<int>(); // error: no viable function for call to g; // g is not decl-reachable from purview of // module M's interface, so is discarded int c = use_h<int>(); // OK — end example]

87)87)

A declaration can appear within a lambda-expression in the initializer of a variable.