P2893R3 Variadic friends

Author: jensmaurer

source/classes.tex

@@ -461,6 +461,7 @@
 \nontermdef{member-declaration}\br
     \opt{attribute-specifier-seq} \opt{decl-specifier-seq} \opt{member-declarator-list} \terminal{;}\br
     function-definition\br
+    friend-type-declaration\br
     using-declaration\br
     using-enum-declaration\br
     static_assert-declaration\br
@@ -503,6 +504,24 @@
     \terminal{=} \terminal{0}
 \end{bnf}
 
+\begin{bnf}
+\nontermdef{friend-type-declaration}\br
+    \keyword{friend} friend-type-specifier-list \terminal{;}
+\end{bnf}
+
+\begin{bnf}
+\nontermdef{friend-type-specifier-list}\br
+    friend-type-specifier \opt{\terminal{...}}\br
+    friend-type-specifier-list \terminal{,} friend-type-specifier \opt{\terminal{...}}
+\end{bnf}
+
+\begin{bnf}
+\nontermdef{friend-type-specifier}\br
+    simple-type-specifier\br
+    elaborated-type-specifier\br
+    typename-specifier
+\end{bnf}
+
 \pnum
 \indextext{definition!class}%
 The \grammarterm{member-specification} in a class definition declares the
@@ -610,6 +629,11 @@
 or if $P$ is in a complete-class context of \tcode{C}.
 Otherwise, \tcode{C} is incomplete at $P$.
 
+\pnum
+If a \grammarterm{member-declaration} matches
+the syntactic requirements of \grammarterm{friend-type-declaration},
+it is a \grammarterm{friend-type-declaration}.
+
 \pnum
 In a \grammarterm{member-declarator},
 an \tcode{=} immediately following the \grammarterm{declarator}
@@ -4841,7 +4865,7 @@
 
 \pnum
 A friend declaration that does not declare a function
-shall have one of the following forms:
+shall be a \grammarterm{friend-type-declaration}.
 
 \begin{ncsimplebnf}
 \keyword{friend} elaborated-type-specifier \terminal{;}\br
@@ -4854,14 +4878,15 @@
 \grammarterm{declaration} in
 a \grammarterm{template-declaration}\iref{temp.pre,temp.friend}.
 \end{note}
-If the
-type specifier in a friend declaration designates a (possibly
+If a \grammarterm{friend-type-specifier} in a friend declaration
+designates a (possibly
 cv-qualified) class type, that class is declared as a friend; otherwise, the
-friend declaration is ignored.
+\grammarterm{friend-type-specifier} is ignored.
 \begin{example}
 \begin{codeblock}
 class C;
 typedef C Ct;
+class E;
 
 class X1 {
   friend C;                     // OK, \tcode{class C} is a friend
@@ -4873,12 +4898,22 @@
   friend class D;               // OK, elaborated-type-specifier declares new class
 };
 
-template <typename T> class R {
-  friend T;
+template <typename ... Ts> class R {
+  friend Ts...;
+};
+
+template <class... Ts, class... Us>
+class R<R<Ts...>, R<Us...>> {
+  friend Ts::Nested..., Us...;
 };
 
 R<C> rc;                        // \tcode{class C} is a friend of \tcode{R<C>}
+R<C, E> rce;                    // classes \tcode{C} and \tcode{E} are friends of \tcode{R<C, E>}
 R<int> Ri;                      // OK, \tcode{"friend int;"} is ignored
+
+struct E { struct Nested; };
+
+R<R<E>, R<C, int>> rr;          // \tcode{E::Nested} and \tcode{C} are friends of \tcode{R<R<E>, R<C, int>>}
 \end{codeblock}
 \end{example}
 

source/declarations.tex

@@ -28,6 +28,7 @@
     block-declaration\br
     nodeclspec-function-declaration\br
     function-definition\br
+    friend-type-declaration\br
     template-declaration\br
     deduction-guide\br
     linkage-specification\br
@@ -117,6 +118,11 @@
 the declaration that are \emph{not} nested within scopes nested within
 the declaration.
 
+\pnum
+If a \grammarterm{name-declaration} matches
+the syntactic requirements of \grammarterm{friend-type-declaration},
+it is a \grammarterm{friend-type-declaration}.
+
 \pnum
 A
 \grammarterm{simple-declaration} or
@@ -1520,13 +1526,13 @@
 The target scope of $E$ is the nearest enclosing namespace or block scope.
 
 \pnum
-If an \grammarterm{elaborated-type-specifier} appears with
-the \keyword{friend} specifier as an entire \grammarterm{member-declaration},
-the \grammarterm{member-declaration} shall have one of the following forms:
+A \grammarterm{friend-type-specifier}
+that is an \grammarterm{elaborated-type-specifier}
+shall have one of the following forms:
 \begin{ncsimplebnf}
-\keyword{friend} class-key \opt{nested-name-specifier} identifier \terminal{;}\br
-\keyword{friend} class-key simple-template-id \terminal{;}\br
-\keyword{friend} class-key nested-name-specifier \opt{\keyword{template}} simple-template-id \terminal{;}
+class-key \opt{nested-name-specifier} identifier\br
+class-key simple-template-id\br
+class-key nested-name-specifier \opt{\keyword{template}} simple-template-id
 \end{ncsimplebnf}
 Any unqualified lookup for the \grammarterm{identifier} (in the first case)
 does not consider scopes that contain the target scope; no name is bound.

source/preprocessor.tex

@@ -1869,6 +1869,7 @@
 \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_friend}                   & \tcode{202403L} \\ \rowsep
 \defnxname{cpp_variable_templates}                & \tcode{201304L} \\ \rowsep
 \defnxname{cpp_variadic_templates}                & \tcode{200704L} \\ \rowsep
 \defnxname{cpp_variadic_using}                    & \tcode{201611L} \\

source/templates.tex

@@ -59,11 +59,7 @@
 \end{note}
 
 \pnum
-The
-\grammarterm{declaration}
-in a
-\grammarterm{template-declaration}
-(if any)
+The \grammarterm{declaration} in a \grammarterm{template-declaration} (if any)
 shall
 \begin{itemize}
 \item declare or define a function, a class, or a variable, or
@@ -73,6 +69,8 @@
 
 \item define a member template of a class or class template, or
 
+\item be a \grammarterm{friend-type-declaration}, or
+
 \item be a \grammarterm{deduction-guide}, or
 
 \item be an \grammarterm{alias-declaration}.
@@ -2739,6 +2737,9 @@
 \item In a \grammarterm{using-declaration}\iref{namespace.udecl};
 the pattern is a \grammarterm{using-declarator}.
 
+\item In a \grammarterm{friend-type-declaration}\iref{class.mem.general};
+the pattern is a \grammarterm{friend-type-specifier}.
+
 \item In a template parameter pack that is a pack expansion\iref{temp.param}:
 \begin{itemize}
 \item
@@ -4408,6 +4409,8 @@
 \grammarterm{elaborated-type-specifier},
 \grammarterm{class-or-decltype}, or
 \item
+a \grammarterm{simple-type-specifier} of a \grammarterm{friend-type-specifier}, or
+\item
 a \grammarterm{type-specifier} of a
 \begin{itemize}
   \item \grammarterm{new-type-id},