[enum.udecl] - C++17 → C++20

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+### The `using enum` declaration <a id="enum.udecl">[[enum.udecl]]</a>
+``` bnf
+using-enum-declaration:
+    'using' elaborated-enum-specifier ';'
+```
+The *elaborated-enum-specifier* shall not name a dependent type and the
+type shall have a reachable *enum-specifier*.
+A *using-enum-declaration* introduces the enumerator names of the named
+enumeration as if by a *using-declaration* for each enumerator.
+[*Note 1*:
+A *using-enum-declaration* in class scope adds the enumerators of the
+named enumeration as members to the scope. This means they are
+accessible for member lookup.
+[*Example 1*:
+``` cpp
+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*]
+[*Note 2*:
+Two *using-enum-declaration*s that introduce two enumerators of the same
+name conflict.
+[*Example 2*:
+``` cpp
+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*]

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