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cppdraft/module/reach.md

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+[module.reach]
+
+# 10 Modules [[module]](./#module)
+
+## 10.7 Reachability [module.reach]
+
+[1](#1)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L1006)
+
+A translation unit U is[*necessarily reachable*](#def:reachable,necessarily,translation_unit "10.7 Reachability [module.reach]") from a point P ifU is a module interface unit on which the translation unit containing P has an interface dependency, or
+the translation unit containing P imports U,
+in either case prior to P ([[module.import]](module.import "10.3 Import declaration"))[.](#1.sentence-1)
+
+[*Note [1](#note-1)*:
+
+While module interface units are reachable even when they are only
+transitively imported via a non-exported import declaration,
+namespace-scope names from such module interface units are not found by
+name lookup ([[basic.lookup]](basic.lookup "6.5 Name lookup"))[.](#1.sentence-2)
+
+— *end note*]
+
+[2](#2)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L1021)
+
+All translation units that are necessarily reachable are[*reachable*](#def:reachable,translation_unit "10.7 Reachability [module.reach]")[.](#2.sentence-1)
+
+Additional translation units on which the
+point within the program has an interface dependency may be considered reachable,
+but it is unspecified which are and under what circumstances[.](#2.sentence-2)[87](#footnote-87 "Implementations are therefore not required to prevent the semantic effects of additional translation units involved in the compilation from being observed.")
+
+[*Note [2](#note-2)*:
+
+It is advisable to avoid
+depending on the reachability of any additional translation units
+in programs intending to be portable[.](#2.sentence-3)
+
+— *end note*]
+
+[3](#3)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L1038)
+
+A declaration D is[*reachable from*](#def:reachable_from,declaration "10.7 Reachability [module.reach]") a point P if
+
+- [(3.1)](#3.1)
+
+P is a non-synthesized point and
+  * [(3.1.1)](#3.1.1)
+
+D appears prior to P in the same translation unit, or
+
+  * [(3.1.2)](#3.1.2)
+
+D is not discarded ([[module.global.frag]](module.global.frag "10.4 Global module fragment")),
+appears in a translation unit that is
+reachable from P,
+and
+does not appear within a [*private-module-fragment*](module.private.frag#nt:private-module-fragment "10.5 Private module fragment [module.private.frag]"); or
+
+- [(3.2)](#3.2)
+
+D is the injected declaration
+for which P is the corresponding synthesized point[.](#3.sentence-1)
+
+[*Example [1](#example-1)*: class Incomplete;
+
+consteval {int n = nonstatic_data_members_of( define_aggregate(^^Incomplete, {data_member_spec(^^int, {.name="x"})}),
+ std::meta::access_context::current()).size();
+
+ Incomplete y; // error: type of y is incomplete}/* P */
+
+The value of n is 1[.](#3.sentence-2)
+
+The member Incomplete​::​x members-of-precedes ([[meta.reflection.member.queries]](meta.reflection.member.queries "21.4.10 Reflection member queries"))
+the synthesized point P associated with the injected declaration
+produced by the call to define_aggregate[.](#3.sentence-3)
+
+— *end example*]
+
+[4](#4)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L1077)
+
+A declaration is [*reachable*](#def:reachable,declaration "10.7 Reachability [module.reach]") if it is reachable from
+any point in the instantiation context ([[module.context]](module.context "10.6 Instantiation context"))[.](#4.sentence-1)
+
+[*Note [3](#note-3)*:
+
+Whether a declaration is exported has no bearing on whether it is reachable[.](#4.sentence-2)
+
+— *end note*]
+
+[5](#5)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L1085)
+
+The accumulated properties of all reachable declarations of
+an entity within a context
+determine the behavior of the entity within that context[.](#5.sentence-1)
+
+[*Note [4](#note-4)*:
+
+These reachable semantic properties include type completeness,
+type definitions, initializers,
+default arguments of functions or template declarations, attributes,
+names bound, etc[.](#5.sentence-2)
+
+Since default arguments are evaluated in the context of the call expression,
+the reachable semantic properties of the corresponding parameter types apply in
+that context[.](#5.sentence-3)
+
+[*Example [2](#example-2)*:
+
+Translation unit #1:export module M:A;export struct B;
+
+Translation unit #2:module M:B;struct B {operator int();};
+
+Translation unit #3:module M:C;import :A;
+B b1; // error: no reachable definition of struct B
+
+Translation unit #4:export module M;export import :A;import :B;
+B b2;export void f(B b = B());
+
+Translation unit #5:import M;
+B b3; // error: no reachable definition of struct Bvoid g() { f(); } // error: no reachable definition of struct B — *end example*]
+
+— *end note*]
+
+[6](#6)
+
+[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/modules.tex#L1132)
+
+[*Note [5](#note-5)*:
+
+Declarations of an entity can be reachable
+even where they cannot be found by name lookup[.](#6.sentence-1)
+
+— *end note*]
+
+[*Example [3](#example-3)*:
+
+Translation unit #1:export module A;struct X {};export using Y = X;
+
+Translation unit #2:import A;
+Y y; // OK, definition of X is reachable X x; // error: X not visible to unqualified lookup — *end example*]
+
+[87)](#footnote-87)[87)](#footnoteref-87)
+
+Implementations are therefore not required to prevent the semantic
+effects of additional translation units involved in the compilation from being
+observed[.](#footnote-87.sentence-1)