Merge 2019-07 CWG Motion 8

 P1099R5 Using enum

Fixes #2987.

Author: zygoloid

source/basic.tex

@@ -176,6 +176,8 @@
 \item it is
 a \grammarterm{using-directive}\iref{namespace.udir},
 \item it is
+a \grammarterm{using-enum-declaration}\iref{enum.udecl},
+\item it is
 an explicit instantiation declaration\iref{temp.explicit}, or
 \item it is
 an explicit specialization\iref{temp.expl.spec} whose

source/classes.tex

@@ -440,6 +440,7 @@
     \opt{attribute-specifier-seq} \opt{decl-specifier-seq} \opt{member-declarator-list} \terminal{;}\br
     function-definition\br
     using-declaration\br
+    using-enum-declaration\br
     static_assert-declaration\br
     template-declaration\br
     deduction-guide\br

source/declarations.tex

@@ -38,6 +38,7 @@
     asm-declaration\br
     namespace-alias-definition\br
     using-declaration\br
+    using-enum-declaration\br
     using-directive\br
     static_assert-declaration\br
     alias-declaration\br
@@ -1418,6 +1419,11 @@
     class-key \opt{attribute-specifier-seq} \opt{nested-name-specifier} identifier\br
     class-key simple-template-id\br
     class-key nested-name-specifier \opt{\keyword{template}} simple-template-id\br
+    elaborated-enum-specifier
+\end{bnf}
+
+\begin{bnf}
+\nontermdef{elaborated-enum-specifier}\br
     \keyword{enum} \opt{nested-name-specifier} identifier
 \end{bnf}
 
@@ -6584,7 +6590,9 @@
 the type of the \grammarterm{id-expression} \tcode{y} is ``\tcode{const volatile double}''.
 \end{example}
 
-\rSec1[dcl.enum]{Enumeration declarations}%
+\rSec1[enum]{Enumerations}%
+
+\rSec2[dcl.enum]{Enumeration declarations}%
 \indextext{enumeration}%
 \indextext{\idxcode{\{\}}!enum declaration@\tcode{enum} declaration}%
 \indextext{\idxcode{enum}!type of}
@@ -6904,6 +6912,60 @@
 \end{codeblock}
 \end{example}
 
+\rSec2[enum.udecl]{The \tcode{using enum} declaration}%
+\indextext{enumeration!using declaration}%
+
+\begin{bnf}
+\nontermdef{using-enum-declaration}\br
+    \terminal{using} elaborated-enum-specifier \terminal{;}
+\end{bnf}
+
+\pnum
+The \grammarterm{elaborated-enum-specifier}
+shall not name a dependent type
+and the type shall have a reachable \grammarterm{enum-specifier}.
+
+\pnum
+A \grammarterm{using-enum-declaration}
+introduces the enumerator names of the named enumeration
+as if by a \grammarterm{using-declaration} for each enumerator.
+
+\pnum
+\begin{note}
+A \grammarterm{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.
+\begin{example}
+\begin{codeblock}
+enum class fruit { orange, apple };
+struct S {
+  using enum fruit;             // OK, introduces \tcode{orange} and \tcode{apple} into \tcode{S}
+};
+void f() {
+  S s;
+  s.orange;                     // OK, names \tcode{fruit::orange}
+  S::orange;                    // OK, names \tcode{fruit::orange}
+}
+\end{codeblock}
+\end{example}
+\end{note}
+
+\pnum
+\begin{note}
+Two \grammarterm{using-enum-declaration}s
+that introduce two enumerators of the same name conflict.
+\begin{example}
+\begin{codeblock}
+enum class fruit { orange, apple };
+enum class color { red, orange };
+void f() {
+  using enum fruit;             // OK
+  using enum color;             // ill-formed: \tcode{color::orange} and \tcode{fruit::orange} conflict
+}
+\end{codeblock}
+\end{example}
+\end{note}
+
 \rSec1[basic.namespace]{Namespaces}%
 \indextext{namespaces|(}
 
@@ -7319,7 +7381,7 @@
 \end{codeblock}
 \end{example}
 
-\rSec2[namespace.udir]{Using directive}%
+\rSec2[namespace.udir]{Using namespace directive}%
 \indextext{using-directive|(}
 
 \begin{bnf}
@@ -7599,8 +7661,20 @@
 \pnum
 In a \grammarterm{using-declaration} used as a
 \grammarterm{member-declaration},
-each \grammarterm{using-declarator}{'s} \grammarterm{nested-name-specifier}
-shall name a base class of the class being defined. If a
+each \grammarterm{using-declarator}
+shall either name an enumerator
+or have a \grammarterm{nested-name-specifier}
+naming a base class of the class being defined.
+\begin{example}
+\begin{codeblock}
+enum class button { up, down };
+struct S {
+  using button::up;
+  button b = up;                // OK
+};
+\end{codeblock}
+\end{example}
+If a
 \grammarterm{using-declarator} names a constructor, its
 \grammarterm{nested-name-specifier} shall name a direct base class of the class
 being defined.
@@ -7667,10 +7741,9 @@
 A \grammarterm{using-declaration} shall not name a namespace.
 
 \pnum
-A \grammarterm{using-declaration} shall not name a scoped enumerator.
-
-\pnum
-A \grammarterm{using-declaration} that names a class member shall be a
+A \grammarterm{using-declaration} that names a class member
+other than an enumerator
+shall be a
 \grammarterm{member-declaration}.
 \begin{example}
 \begin{codeblock}

source/preprocessor.tex

@@ -1722,6 +1722,7 @@
 \defnxname{cpp_unicode_characters}                & \tcode{200704L} \\ \rowsep
 \defnxname{cpp_unicode_literals}                  & \tcode{200710L} \\ \rowsep
 \defnxname{cpp_user_defined_literals}             & \tcode{200809L} \\ \rowsep
+\defnxname{cpp_using_enum}                        & \tcode{201907L} \\ \rowsep
 \defnxname{cpp_variable_templates}                & \tcode{201304L} \\ \rowsep
 \defnxname{cpp_variadic_templates}                & \tcode{200704L} \\ \rowsep
 \defnxname{cpp_variadic_using}                    & \tcode{201611L} \\ \rowsep