9 Declarations [dcl.dcl]

9.7 Enumerations [enum]

9.7.2 The using enum declaration [enum.udecl]

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A using-enum-declarator names the set of declarations found by type-only lookup ([basic.lookup.general]) for the using-enum-declarator ([basic.lookup.unqual], [basic.lookup.qual]).
The using-enum-declarator shall designate a non-dependent type with a reachable enum-specifier.
[Example 1: enum E { x }; void f() { int E; using enum E; // OK } using F = E; using enum F; // OK template<class T> using EE = T; void g() { using enum EE<E>; // OK } — end example]
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A using-enum-declaration is equivalent to a using-declaration for each enumerator.
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[Note 1: 
A using-enum-declaration in class scope makes the enumerators of the named enumeration available via member lookup.
[Example 2: enum class fruit { orange, apple }; struct S { using enum fruit; // OK, introduces orange and apple into S }; void f() { S s; s.orange; // OK, names fruit​::​orange S::orange; // OK, names fruit​::​orange } — end example]
— end note]
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[Note 2: 
Two using-enum-declarations that introduce two enumerators of the same name conflict.
[Example 3: enum class fruit { orange, apple }; enum class color { red, orange }; void f() { using enum fruit; // OK using enum color; // error: color​::​orange and fruit​::​orange conflict } — end example]
— end note]