\rSec0[module]{Modules}%

\gramSec[gram.module]{Modules}

\rSec1[module.unit]{Module units and purviews}

\begin{bnf}

\nontermdef{module-declaration}\br

\opt{export-keyword} module-keyword module-name \opt{module-partition} \opt{attribute-specifier-seq} \terminal{;}

\end{bnf}

\begin{bnf}

\nontermdef{module-name}\br

\opt{module-name-qualifier} identifier

\end{bnf}

\begin{bnf}

\nontermdef{module-partition}\br

\terminal{:} \opt{module-name-qualifier} identifier

\end{bnf}

\begin{bnf}

\nontermdef{module-name-qualifier}\br

identifier \terminal{.}\br

module-name-qualifier identifier \terminal{.}

\end{bnf}

A \defn{module unit} is a translation unit that contains

a \grammarterm{module-declaration}.

A \defnadj{named}{module} is the

collection of module units with the same \grammarterm{module-name}.

The identifiers \tcode{module} and \tcode{import}

shall not appear as \grammarterm{identifier}{s}

in a \grammarterm{module-name} or \grammarterm{module-partition}.

\indextext{module!reserved name of}%

All \grammarterm{module-name}{s} either beginning with an \grammarterm{identifier}

consisting of \tcode{std} followed by zero or more \grammarterm{digit}{s} or

containing a reserved identifier\iref{lex.name}

are reserved and shall not be specified in a \grammarterm{module-declaration};

no diagnostic is required.

If any \grammarterm{identifier} in a reserved \grammarterm{module-name}

is a reserved identifier,

the module name is reserved for use by \Cpp{} implementations;

otherwise it is reserved for future standardization.

The optional \grammarterm{attribute-specifier-seq}

appertains to the \grammarterm{module-declaration}.

A \defn{module interface unit} is a module unit whose

\grammarterm{module-declaration} starts with \grammarterm{export-keyword};

any other module unit is a \defn{module implementation unit}.

A named module shall contain exactly one module interface unit

with no \grammarterm{module-partition}, known as the

\defn{primary module interface unit} of the module;

no diagnostic is required.

A \defn{module partition} is

a module unit whose \grammarterm{module-declaration} contains

a \grammarterm{module-partition}.

A named module shall not contain multiple module partitions with

the same \grammarterm{module-partition}.

All module partitions of a module

that are module interface units

shall be directly or indirectly exported

by the primary module interface unit\iref{module.import}.

No diagnostic is required for a violation of these rules.

\begin{note}

Module partitions can be imported only by

other module units in the same module.

The division of a module into module units

is not visible outside the module.

\end{note}

\begin{example}

\begin{codeblocktu}{Translation unit \#1}

export module A;

export import :Foo;

export int baz();

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#2}

export module A:Foo;

import :Internals;

export int foo() { return 2 * (bar() + 1); }

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#3}

module A:Internals;

int bar();

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#4}

module A;

import :Internals;

int bar() { return baz() - 10; }

int baz() { return 30; }

\end{codeblocktu}

Module \tcode{A} contains four translation units:

\begin{itemize}

\item a primary module interface unit,

\item a module partition \tcode{A:Foo}, which is a module interface unit

forming part of the interface of module \tcode{A},

\item a module partition \tcode{A:Internals}, which does not contribute

to the external interface of module \tcode{A}, and

\item a module implementation unit providing

a definition of \tcode{bar} and \tcode{baz},

which cannot be imported because

it does not have a partition name.

\end{itemize}

\end{example}

A \defnadj{module unit}{purview} is

the sequence of \grammarterm{token}{s}

starting at the \grammarterm{module-declaration}

and extending to the end of the translation unit.

The \defnx{purview}{purview!named module}

of a named module \tcode{M} is the set of module unit purviews

of \tcode{M}'s module units.

The \defnadj{global}{module} is the collection of all

\grammarterm{global-module-fragment}{s}

and all translation units that are not module units.

Declarations appearing in such a context

are said to be in the \defnx{purview}{purview!global module} of the global module.

\begin{note}

The global module has no name, no module interface unit, and is not

introduced by any \grammarterm{module-declaration}.

\end{note}

A \defn{module} is either a named module or the global module.

A declaration is \defnx{attached}{attached!declaration} to a module as follows:

\begin{itemize}

If the declaration is a non-dependent friend declaration

that nominates a function with a \grammarterm{declarator-id}

that is a \grammarterm{qualified-id} or \grammarterm{template-id} or

that nominates a class

other than with an \grammarterm{elaborated-type-specifier} with neither

a \grammarterm{nested-name-specifier} nor a \grammarterm{simple-template-id},

it is attached to the module to which the friend is attached\iref{basic.link}.

\item Otherwise, if the declaration

\begin{itemize}

\item declares a namespace whose name has external linkage,

\item declares a type alias,

\item declares a namespace alias, or

\item appears within a \grammarterm{linkage-specification}\iref{dcl.link}

\end{itemize}

it is attached to the global module.

\item Otherwise, the declaration is

attached to the module in whose purview it appears.

\end{itemize}

A \grammarterm{module-declaration}

that contains neither an \grammarterm{export-keyword}

nor a \grammarterm{module-partition}

implicitly imports the primary module interface unit of the module

as if by a \grammarterm{module-import-declaration}.

\begin{example}

\begin{codeblocktu}{Translation unit \#1}

module B:Y; // does not implicitly import \tcode{B}

int y();

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#2}

export module B;

import :Y; // OK, does not create interface dependency cycle

int n = y();

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#3}

module B:X1; // does not implicitly import \tcode{B}

int &a = n; // error: \tcode{n} not visible here

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#4}

module B:X2; // does not implicitly import \tcode{B}

import B;

int &b = n; // OK

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#5}

module B; // implicitly imports \tcode{B}

int &c = n; // OK

\end{codeblocktu}

\end{example}

\rSec1[module.interface]{Export declaration}%

\begin{bnf}

\nontermdef{export-declaration}\br

\keyword{export} name-declaration\br

\keyword{export} \terminal{\{} \opt{declaration-seq} \terminal{\}}\br

export-keyword module-import-declaration

\end{bnf}

An \grammarterm{export-declaration} shall inhabit

a namespace scope and appear in the purview of a module interface unit.

An \grammarterm{export-declaration} shall not appear directly

or indirectly within an unnamed namespace

or a \grammarterm{private-module-fragment}.

An \grammarterm{export-declaration}

has the declarative effects of its

\grammarterm{name-declaration},

\grammarterm{declaration-seq} (if any), or

\grammarterm{module-import-declaration}.

The \grammarterm{name-declaration} of an \grammarterm{export-declaration}

shall not declare a partial specialization\iref{temp.decls.general}.

The \grammarterm{declaration-seq} of

an \grammarterm{export-declaration}

shall not contain an \grammarterm{export-declaration} or

\grammarterm{module-import-declaration}.

\begin{note}

An \grammarterm{export-declaration} does not establish a scope.

\end{note}

A declaration is \defnx{exported}{declaration!exported} if it is

declared within an \grammarterm{export-declaration} and

inhabits a namespace scope or it is

\begin{itemize}

\item a \grammarterm{namespace-definition} that contains an

exported declaration, or

\item a declaration within a header unit\iref{module.import}

that introduces at least one name.

\end{itemize}

If an exported declaration is not within a header unit,

it shall not declare a name with internal linkage.

\begin{example}

\begin{codeblocktu}{Source file \tcode{"a.h"}}

export int x;

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#1}

module;

#include "a.h" // error: declaration of \tcode{x} is not in the

// purview of a module interface unit

export module M;

export namespace {} // error: namespace has internal linkage

namespace {

export int a2; // error: export of name with internal linkage

}

export static int b; // error: \tcode{b} explicitly declared static

export int f(); // OK

export namespace N { } // OK

export using namespace N; // OK

\end{codeblocktu}

\end{example}

If an exported declaration is a \grammarterm{using-declaration}\iref{namespace.udecl}

and is not within a header unit,

all entities named by the

\grammarterm{using-declarator}{s} (if any)

shall either be a type alias or

have been introduced with a name having external linkage.

\begin{example}

\begin{codeblocktu}{Source file \tcode{"b.h"}}

int f();

\end{codeblocktu}

\begin{codeblocktu}{Importable header \tcode{"c.h"}}

int g();

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#1}

export module X;

export int h();

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#2}

module;

#include "b.h"

export module M;

import "c.h";

import X;

export using ::f, ::g, ::h; // OK

struct S;

export using ::S; // error: \tcode{S} has module linkage

namespace N {

export int h();

static int h(int); // \#1

}

export using N::h; // error: \#1 has internal linkage

\end{codeblocktu}

\end{example}

\begin{note}

The underlying entity of an exported type alias

need not have a name with external linkage.

\begin{example}

\begin{codeblock}

export module M;

struct S;

export using T = S; // OK, exports name \tcode{T} denoting type \tcode{S}

\end{codeblock}

\end{example}

\end{note}

A redeclaration of an entity $X$

is implicitly exported

if $X$ was introduced by an exported declaration;

otherwise it shall not be exported

unless it is a type alias, a namespace, or a namespace alias.

\begin{example}

\begin{codeblock}

export module M;

struct S { int n; };

typedef S S;

export typedef S S; // OK

export struct S; // error: exported declaration follows non-exported declaration

namespace N { // external linkage, attached to global module, not exported

void f();

}

namespace N { // OK, exported namespace redeclaring non-exported namespace

export void g();

}

\end{codeblock}

\end{example}

\begin{note}

Names introduced by exported declarations never have module linkage.

They have external linkage, no linkage, or

(in the case of header units) internal linkage; see \ref{basic.link}.

Namespace-scope declarations exported by a module can be found by name lookup

in any translation unit importing that module\iref{basic.lookup}.

Class and enumeration member names can be found by name lookup in any

context in which a definition of the type is reachable.

\end{note}

\begin{example}

\begin{codeblocktu}{Interface unit of \tcode{M}}

export module M;

export struct X {

static void f();

struct Y { };

};

namespace {

struct S { };

}

export void f(S); // OK

struct T { };

export T id(T); // OK

export struct A; // \tcode{A} exported as incomplete

export auto rootFinder(double a) {

return [=](double x) { return (x + a/x)/2; };

}

export const int n = 5; // OK, \tcode{n} has external linkage

\end{codeblocktu}

\begin{codeblocktu}{Implementation unit of \tcode{M}}

module M;

struct A {

int value;

};

\end{codeblocktu}

\begin{codeblocktu}{Main program}

import M;

int main() {

X::f(); // OK, \tcode{X} is exported and definition of \tcode{X} is reachable

X::Y y; // OK, \tcode{X::Y} is exported as a complete type

auto f = rootFinder(2); // OK

return A{45}.value; // error: \tcode{A} is incomplete

}

\end{codeblocktu}

\end{example}

\begin{note}

Declarations in an exported \grammarterm{namespace-definition}

or in an exported \grammarterm{linkage-specification}\iref{dcl.link}

are exported and subject to the rules of exported declarations.

\begin{example}

\begin{codeblock}

export module M;

int g;

export namespace N {

int x; // OK

using ::g; // error: \tcode{::g} has module linkage

}

\end{codeblock}

\end{example}

\end{note}

\rSec1[module.import]{Import declaration}%

\begin{bnf}

\nontermdef{module-import-declaration}\br

import-keyword module-name \opt{attribute-specifier-seq} \terminal{;}\br

import-keyword module-partition \opt{attribute-specifier-seq} \terminal{;}\br

import-keyword header-name \opt{attribute-specifier-seq} \terminal{;}

\end{bnf}

A \grammarterm{module-import-declaration}

shall inhabit the global namespace scope.

In a module unit, all \grammarterm{module-import-declaration}{s}

and \grammarterm{export-declaration}{s} exporting

\grammarterm{module-import-declaration}{s}

shall appear before all other \grammarterm{declaration}{s} in

the \grammarterm{declaration-seq} of the

\grammarterm{translation-unit}

and of the \grammarterm{private-module-fragment} (if any).

The optional \grammarterm{attribute-specifier-seq}

appertains to the \grammarterm{module-import-declaration}.

A \grammarterm{module-import-declaration} \defnx{imports}{import} a set of

translation units determined as described below.

\begin{note}

Namespace-scope declarations exported by the imported translation units

can be found by name lookup\iref{basic.lookup}

in the importing translation unit

and declarations within the imported translation units

become reachable\iref{module.reach}

in the importing translation unit

after the import declaration.

\end{note}

A \grammarterm{module-import-declaration} that specifies

a \grammarterm{module-name} \tcode{M}

imports all module interface units of \tcode{M}.

A \grammarterm{module-import-declaration} that specifies

a \grammarterm{module-partition} shall only appear after

the \grammarterm{module-declaration} in a module unit of

some module \tcode{M}.

Such a declaration imports the so-named

module partition of \tcode{M}.

A \grammarterm{module-import-declaration} that specifies

a \grammarterm{header-name} \tcode{H} imports

a synthesized \defn{header unit},

which is a translation unit formed by applying

phases 1 to 7 of translation\iref{lex.phases}

to the source file or header nominated by \tcode{H},

which shall not contain a \grammarterm{module-declaration}.

\begin{note}

A header unit is a separate translation unit with

an independent set of defined macros.

All declarations within a header unit are implicitly

exported\iref{module.interface},

and are attached to the global module\iref{module.unit}.

\end{note}

An \defnadj{importable}{header} is a member of an

\impldef{how the set of importable headers is determined}

set of headers that

includes all importable \Cpp{} library headers\iref{headers}.

\tcode{H} shall identify an importable header.

Given two such \grammarterm{module-import-declaration}{s}:

\begin{itemize}

if their \grammarterm{header-name}{s} identify

different headers or source files\iref{cpp.include},

they import distinct header units;

otherwise, if they appear in the same translation unit,

they import the same header unit;

otherwise, it is unspecified whether they import the same header unit.

\begin{note}

It is therefore possible that multiple copies exist of entities

declared with internal linkage in an importable header.

\end{note}

\end{itemize}

\begin{note}

A \grammarterm{module-import-declaration} nominating

a \grammarterm{header-name} is also recognized by the

preprocessor, and results in macros defined at the

end of phase 4 of translation of the header unit

being made visible as described in \ref{cpp.import}.

Any other \grammarterm{module-import-declaration}

does not make macros visible.

\end{note}

A declaration of a name with internal linkage is

permitted within a header unit despite all

declarations being implicitly exported\iref{module.interface}.

\begin{note}

A definition that appears in multiple translation units

cannot in general refer to such names\iref{basic.def.odr}.

\end{note}

A header unit shall not contain

a definition of a non-inline function or variable

whose name has external linkage.

When a \grammarterm{module-import-declaration} imports

a translation unit $T$, it also imports

all translation units imported by

exported \grammarterm{module-import-declaration}{s}

in $T$; such translation units are

said to be \defnx{exported}{module!exported} by $T$.

Additionally, when a \grammarterm{module-import-declaration}

in a module unit of some module $M$ imports another module unit $U$ of $M$,

it also imports all translation units imported by

non-exported \grammarterm{module-import-declaration}{s}

in the module unit purview of $U$.

\begin{footnote}

This is consistent

with the lookup rules for imported names\iref{basic.lookup}.

\end{footnote}

These rules can in turn lead to the importation of yet more

translation units.

\begin{note}

Such indirect importation does not make macros available,

because a translation unit is

a sequence of tokens in translation phase 7\iref{lex.phases}.

Macros can be made available by directly importing header units

as described in \ref{cpp.import}.

\end{note}

A module implementation unit shall not be exported.

\begin{example}

\begin{codeblocktu}{Translation unit \#1}

module M:Part;

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#2}

export module M;

export import :Part; // error: exported partition \tcode{:Part} is an implementation unit

\end{codeblocktu}

\end{example}

A module implementation unit of a module \tcode{M}

that is not a module partition

shall not contain a \grammarterm{module-import-declaration}

nominating \tcode{M}.

\begin{example}

\begin{codeblock}

module M;

import M; // error: cannot import \tcode{M} in its own unit

\end{codeblock}

\end{example}

A translation unit has an \defn{interface dependency} on a translation unit \tcode{U}

if it contains a declaration (possibly a \grammarterm{module-declaration})

that imports \tcode{U} or if it has

an interface dependency on a translation unit that has an interface dependency on \tcode{U}.

A translation unit shall not have an interface dependency on itself.

\begin{example}

\begin{codeblocktu}{Interface unit of \tcode{M1}}

export module M1;

import M2;

\end{codeblocktu}

\begin{codeblocktu}{Interface unit of \tcode{M2}}

export module M2;

import M3;

\end{codeblocktu}

\begin{codeblocktu}{Interface unit of \tcode{M3}}

export module M3;

import M1; // error: cyclic interface dependency $\mathtt{M3} \rightarrow \mathtt{M1} \rightarrow \mathtt{M2} \rightarrow \mathtt{M3}$

\end{codeblocktu}

\end{example}

\rSec1[module.global.frag]{Global module fragment}

\begin{bnf}

\nontermdef{global-module-fragment}\br

module-keyword \terminal{;} \opt{declaration-seq}

\end{bnf}

A \grammarterm{global-module-fragment} specifies the contents of the

\defn{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 \defn{decl-reachable} from a declaration $S$

in the same translation unit if

\begin{itemize}

$D$ does not declare a function or function template and

$S$ contains an

\grammarterm{id-expression},

\grammarterm{namespace-name},

\grammarterm{type-name},

\grammarterm{template-name}, or

\grammarterm{concept-name}

naming $D$, or

$D$ declares a function or function template that

is named by an expression\iref{basic.def.odr}

appearing in $S$, or

$S$ contains a dependent call \tcode{E}\iref{temp.dep}

and $D$ is found by any name lookup performed

for an expression synthesized from \tcode{E}

by replacing each type-dependent argument or operand

with a value of a placeholder type

with no associated namespaces or entities, or

\begin{note}

This includes the lookup for \tcode{\keyword{operator}==} performed

when considering rewriting an \tcode{!=} expression,

the lookup for \tcode{\keyword{operator}<=>}

performed when considering rewriting a relational comparison, and

the lookup for \tcode{\keyword{operator}!=}

when considering whether an \tcode{\keyword{operator}==} is a rewrite target.

\end{note}

$S$ contains an expression that

takes the address of an overload set\iref{over.over}

that contains $D$ and

for which the target type is dependent, or

there exists a declaration $M$ that is not a \grammarterm{namespace-definition}

for which $M$ is decl-reachable from $S$ and either

\begin{itemize}

$D$ is decl-reachable from $M$, or

$D$ and $M$ declare the same entity,

and $D$ neither is a friend declaration

nor inhabits a block scope, or

$D$ declares a namespace $N$ and $M$ is a member of $N$, or

one of $D$ and $M$ declares a class or class template $C$

and the other declares a member or friend of $C$, or

one of $D$ and $M$ declares an enumeration $E$

and the other declares an enumerator of $E$, or

$D$ declares a function or variable and $M$ is declared in $D$,

\begin{footnote}

A declaration can appear within a \grammarterm{lambda-expression}

in the initializer of a variable.

\end{footnote}

or

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

$M$ declares a class template

and $D$ is a deduction guide for that template, or

one of $D$ and $M$ declares a class or enumeration type

and the other introduces a typedef name for linkage purposes for that type.

\end{itemize}

\end{itemize}

In this determination, it is unspecified

\begin{itemize}

whether a reference to an

\grammarterm{alias-declaration},

\tcode{typedef} declaration,

\grammarterm{using-declaration}, or

\grammarterm{namespace-alias-definition}

is replaced by the declarations they name

prior to this determination,

whether a \grammarterm{simple-template-id}

that does not denote a dependent type

and whose \grammarterm{template-name} names an alias template

is replaced by its denoted type

prior to this determination,

whether a \grammarterm{decltype-specifier}

that does not denote a dependent type

is replaced by its denoted type

prior to this determination,

whether a non-value-dependent constant expression

is replaced by the result of constant evaluation

prior to this determination,

and

whether

a \grammarterm{splice-expression},

a \grammarterm{splice-type-specifier},

a \grammarterm{splice-scope-specifier}, or

any \grammarterm{splice-specifier} or

\grammarterm{splice-specialization-specifier}

outside of the preceding is replaced in any non-dependent context

by the construct that it designates prior to this determination.

\end{itemize}

A declaration \tcode{D} in a global module fragment of a module unit

is \defnx{discarded}{discarded!declaration} if \tcode{D}

is not decl-reachable from any \grammarterm{declaration}

in the \grammarterm{declaration-seq}

of the \grammarterm{translation-unit}.

\begin{note}

A discarded declaration is neither reachable

nor visible to name lookup outside the module unit,

nor in template instantiations whose points of instantiation\iref{temp.point}

are outside the module unit,

even when the instantiation context\iref{module.context}

includes the module unit.

\end{note}

\begin{example}

\begin{codeblock}

const int size = 2;

int ary1[size]; // unspecified whether \tcode{size} is decl-reachable from \tcode{ary1}

constexpr int identity(int x) { return x; }

int ary2[identity(2)]; // unspecified whether \tcode{identity} is decl-reachable from \tcode{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(); // \tcode{S}, \tcode{S2}, \tcode{g}, and \tcode{::} are decl-reachable from \tcode{f}

template<int N>

void h() noexcept(g(N) == N); // \tcode{g} and \tcode{::} are decl-reachable from \tcode{h}

\end{codeblock}

\end{example}

\begin{example}

\begin{codeblocktu}{Source file \tcode{"foo.h"}}

namespace N {

struct X {};

int d();

int e();

inline int f(X, int = d()) { return e(); }

int g(X);

int h(X);

}

\end{codeblocktu}

\begin{codeblocktu}{Module \tcode{M} interface}

module;

#include "foo.h"

export module M;

template<typename T> int use_f() {

N::X x; // \tcode{N::X}, \tcode{N}, and \tcode{::} are decl-reachable from \tcode{use_f}

return f(x, 123); // \tcode{N::f} is decl-reachable from \tcode{use_f},

// \tcode{N::e} is indirectly decl-reachable from \tcode{use_f}

// because it is decl-reachable from \tcode{N::f}, and

// \tcode{N::d} is decl-reachable from \tcode{use_f}

// because it is decl-reachable from \tcode{N::f}

// even though it is not used in this call

}

template<typename T> int use_g() {

N::X x; // \tcode{N::X}, \tcode{N}, and \tcode{::} are decl-reachable from \tcode{use_g}

return g((T(), x)); // \tcode{N::g} is not decl-reachable from \tcode{use_g}

}

template<typename T> int use_h() {

N::X x; // \tcode{N::X}, \tcode{N}, and \tcode{::} are decl-reachable from \tcode{use_h}

return h((T(), x)); // \tcode{N::h} is not decl-reachable from \tcode{use_h}, but

// \tcode{N::h} is decl-reachable from \tcode{use_h<int>}

}

int k = use_h<int>();

// \tcode{use_h<int>} is decl-reachable from \tcode{k}, so

// \tcode{N::h} is decl-reachable from \tcode{k}

\end{codeblocktu}

\begin{codeblocktu}{Module \tcode{M} implementation}

module M;

int a = use_f<int>(); // OK

int b = use_g<int>(); // error: no viable function for call to \tcode{g};

// \tcode{g} is not decl-reachable from purview of

// module \tcode{M}{'s} interface, so is discarded

int c = use_h<int>(); // OK

\end{codeblocktu}

\end{example}

\rSec1[module.private.frag]{Private module fragment}

\begin{bnf}

\nontermdef{private-module-fragment}\br

module-keyword \terminal{:} \keyword{private} \terminal{;} \opt{declaration-seq}

\end{bnf}

A \grammarterm{private-module-fragment} shall appear only

in a primary module interface unit\iref{module.unit}.

A module unit with a \grammarterm{private-module-fragment}

shall be the only module unit of its module;

no diagnostic is required.

\begin{note}

A \grammarterm{private-module-fragment} ends

the portion of the module interface unit

that can affect the behavior of other translation units.

A \grammarterm{private-module-fragment} allows a module

to be represented as a single translation unit

without making all of the contents of the module reachable to importers.

The presence of a \grammarterm{private-module-fragment} affects:

\begin{itemize}

the point by which the definition of

an inline function or variable

is required\iref{dcl.inline},

the point by which the definition of

an exported function with a placeholder return type

is required\iref{dcl.spec.auto},

whether a declaration is required not to be an exposure\iref{basic.link},

where definitions for inline functions and templates

must appear\iref{basic.def.odr,dcl.inline,temp.pre},

the instantiation contexts of templates

instantiated before it\iref{module.context}, and

the reachability of declarations within it\iref{module.reach}.

\end{itemize}

\end{note}

\begin{example}

\begin{codeblock}

export module A;

export inline void fn_e(); // error: exported inline function \tcode{fn_e} not defined

// before private module fragment

inline void fn_m(); // error: non-exported inline function \tcode{fn_m} not defined

static void fn_s();

export struct X;

export void g(X *x) {

fn_s(); // OK, call to static function in same translation unit

}

export X *factory(); // OK

module :private;

struct X {}; // definition not reachable from importers of \tcode{A}

X *factory() {

return new X ();

}

void fn_e() {}

void fn_m() {}

void fn_s() {}

\end{codeblock}

\end{example}

\rSec1[module.context]{Instantiation context}

The \defn{instantiation context} is a set of points within the program

that determines which declarations are found by

argument-dependent name lookup\iref{basic.lookup.argdep}

and which are reachable\iref{module.reach}

in the context of a particular declaration or template instantiation.

During the implicit definition of

a defaulted function\iref{special,class.compare.default},

the instantiation context contains each point in

the instantiation context from the definition of the class and

each point in

the instantiation context of the program construct that

resulted in the implicit definition of the defaulted function.

During the implicit instantiation of a template

whose point of instantiation is specified as

that of an enclosing specialization\iref{temp.point},

the instantiation context contains each point in

the instantiation context of the enclosing specialization and,

if the template is defined in a module interface unit of a module $M$

and the point of instantiation is not in a module interface unit of $M$,

the point at the end of the

\grammarterm{declaration-seq} of the

primary module interface unit of $M$

(prior to the \grammarterm{private-module-fragment}, if any).

During the implicit instantiation of a template

that is implicitly instantiated because it is referenced

from within the implicit definition of a defaulted function,

the instantiation context contains each point in the instantiation context of

the defaulted function.

During the instantiation of any other template specialization,

the instantiation context contains the point of instantiation

of the template.

During the implicit instantiation of any construct

that resulted from the evaluation of an expression

as a core constant expression,

the instantiation context contains

each point in the evaluation context\iref{expr.const.reflect}.

\begin{note}

Implicit instantiations can result

from invocations of library functions\iref{meta.reflection}.

The evaluation context can include synthesized points

associated with injected declarations

produced by \tcode{std::meta::define_aggregate}\iref{meta.reflection.define.aggregate}.

\end{note}

In any other case, the instantiation context

at a point within the program

contains that point.

The instantiation context contains only the points specified above.

\begin{example}

\begin{codeblocktu}{Translation unit \#1}

export module stuff;

export template<typename T, typename U> void foo(T, U u) { auto v = u; }

export template<typename T, typename U> void bar(T, U u) { auto v = *u; }

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#2}

export module M1;

import "defn.h"; // provides \tcode{struct X \{\};}

import stuff;

export template<typename T> void f(T t) {

X x;

foo(t, x);

}

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#3}

export module M2;

import "decl.h"; // provides \tcode{struct X;} (not a definition)

import stuff;

export template<typename T> void g(T t) {

X *x;

bar(t, x);

}

\end{codeblocktu}

\begin{codeblocktu}{Translation unit \#4}

import M1;

import M2;

void test() {

f(0);

g(0);

}

\end{codeblocktu}

The call to \tcode{f(0)} is valid;

the instantiation context of \tcode{foo<int, X>} comprises

\begin{itemize}

\item the point at the end of translation unit \#1,

\item the point at the end of translation unit \#2, and

\item the point of the call to \tcode{f(0)},

\end{itemize}

so the definition of \tcode{X} is reachable\iref{module.reach}.

It is unspecified whether the call to \tcode{g(0)} is valid:

the instantiation context of \tcode{bar<int, X>} comprises

\begin{itemize}

\item the point at the end of translation unit \#1,

\item the point at the end of translation unit \#3, and

\item the point of the call to \tcode{g(0)},

\end{itemize}

so the definition of \tcode{X} need not be reachable,

as described in \ref{module.reach}.

\end{example}

\rSec1[module.reach]{Reachability}

\indextext{necessarily reachable|see{reachable, necessarily}}