Merge 2017-07 CWG Motion 11

Author: zygoloid

source/basic.tex

@@ -427,6 +427,7 @@
 with external linkage~(\ref{dcl.inline}), inline variable with external
 linkage~(\ref{dcl.inline}), class template
 (Clause~\ref{temp}), non-static function template~(\ref{temp.fct}),
+concept (\ref{temp.concept}),
 static data member of a class template~(\ref{temp.static}), member
 function of a class template~(\ref{temp.mem.func}), or template
 specialization for which some template parameters are not

source/classes.tex

@@ -466,6 +466,7 @@
 \begin{bnf}
 \nontermdef{member-declarator}\br
     declarator virt-specifier-seq\opt pure-specifier\opt\br
+    declarator requires-clause\br
     declarator brace-or-equal-initializer\opt\br
     identifier\opt{} attribute-specifier-seq\opt{} \terminal{:} constant-expression brace-or-equal-initializer\opt{}
 \end{bnf}

source/compatibility.tex

@@ -1802,6 +1802,27 @@
 \effect A valid \CppXIV program using these identifiers
 may be ill-formed in this International Standard.
 
+\rSec1[diff.cpp17]{\Cpp and ISO \CppXVII}
+
+\pnum
+\indextext{summary!compatibility with ISO \CppXVII}%
+This subclause lists the differences between \Cpp and
+ISO \CppXVII (ISO/IEC 14882:2017, \doccite{Programming Languages --- \Cpp}),
+by the chapters of this document.
+
+\rSec2[diff.cpp17.lex]{Clause~\ref{lex}: lexical conventions}
+
+\ref{lex.key}
+\change New keywords.\\
+\rationale Required for new features.
+The \tcode{requires} keyword is added
+to introduce constraints through a \grammarterm{requires-clause} or
+a \grammarterm{requires-expression}. The \tcode{concept} keyword is
+added to enable the definition of concepts (\ref{temp.concept}).
+\effect
+Valid ISO \CppXVII code using \tcode{concept} or \tcode{requires}
+as an identifier is not valid in this International Standard.
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 \rSec1[diff.library]{C standard library}
 \indextext{library!C standard}%

source/declarators.tex

@@ -27,7 +27,8 @@
 
 \begin{bnf}
 \nontermdef{init-declarator}\br
-    declarator initializer\opt
+    declarator initializer\opt\br
+    declarator requires-clause
 \end{bnf}
 
 \pnum
@@ -95,6 +96,32 @@
 \end{codeblock}
 \end{note}
 
+\pnum
+The optional \grammarterm{requires-clause} (Clause~\ref{temp}) in an
+\grammarterm{init-declarator} or \grammarterm{member-declarator}
+shall not be present when the declarator does not declare a 
+function (\ref{dcl.fct}).
+% 
+When present after a declarator, the \grammarterm{requires-clause} 
+is called the \defnx{trailing \grammarterm{requires-clause}{}}{%
+trailing requires-clause@trailing \grammarterm{requires-clause}}.
+The trailing \grammarterm{requires-clause} introduces the
+\grammarterm{constraint-expression} that results from interpreting
+its \grammarterm{constraint-logical-or-expression} as a
+\grammarterm{constraint-expression}.
+% 
+\begin{example}
+\begin{codeblock}
+void f1(int a) requires true;               // OK
+auto f2(int a) -> bool requires true;       // OK
+auto f3(int a) requires true -> bool;       // error: \grammarterm{requires-clause} precedes \grammarterm{trailing-return-type}
+void (*pf)() requires true;                 // error: constraint on a variable
+void g(int (*)() requires true);            // error: constraint on a \grammarterm{parameter-declaration}
+  
+auto* p = new void(*)(char) requires true;  // error: not a function declaration
+\end{codeblock}
+\end{example}
+
 \pnum
 Declarators have the syntax
 
@@ -1403,8 +1430,8 @@
 \indextext{type!function}%
 A single name can be used for several different functions in a single scope;
 this is function overloading (Clause~\ref{over}).
-All declarations for a function shall agree exactly
-in both the return type and the parameter-type-list.
+All declarations for a function shall have equivalent return types, 
+parameter-type-lists, and \grammarterm{requires-clause}{s} (\ref{temp.over.link}).
 The type of a function is determined using the following rules.
 The type of each parameter (including function parameter packs) is
 determined from its own
@@ -1483,7 +1510,8 @@
 The return type, the parameter-type-list, the \grammarterm{ref-qualifier},
 the \grammarterm{cv-qualifier-seq}, and
 the exception specification,
-but not the default arguments~(\ref{dcl.fct.default}),
+but not the default arguments~(\ref{dcl.fct.default})
+or \grammarterm{requires-clause}{s}~(Clause~\ref{temp}),
 are part of the function type.
 \begin{note}
 Function types are checked during the assignments and initializations of
@@ -1990,7 +2018,8 @@
 %
 \begin{bnf}
 \nontermdef{function-definition}\br
-    attribute-specifier-seq\opt decl-specifier-seq\opt declarator virt-specifier-seq\opt function-body
+    attribute-specifier-seq\opt decl-specifier-seq\opt declarator virt-specifier-seq\opt function-body\br
+    attribute-specifier-seq\opt decl-specifier-seq\opt declarator requires-clause function-body
 \end{bnf}
 
 \begin{bnf}
@@ -2104,12 +2133,8 @@
 \indextext{definition!function!explicitly-defaulted}%
 
 \pnum
-A function definition of the form:
-
-\begin{ncbnf}
-    attribute-specifier-seq\opt decl-specifier-seq\opt declarator virt-specifier-seq\opt{} \terminal{ = default ;}
-\end{ncbnf}
-
+A function definition
+with the \grammarterm{function-body} \tcode{= default ;}
 is called an \grammarterm{explicitly-defaulted} definition.
 A function that is explicitly defaulted shall
 
@@ -2202,12 +2227,8 @@
 \indextext{definition!function!deleted}%
 
 \pnum
-A function definition of the form:
-
-\begin{ncbnf}
-    attribute-specifier-seq\opt decl-specifier-seq\opt declarator virt-specifier-seq\opt{} \terminal{ = delete ;}
-\end{ncbnf}
-
+A function definition
+with the \grammarterm{function-body} \tcode{= delete ;}
 is called a \term{deleted definition}. A function with a
 deleted definition is also called a \term{deleted function}.
 

source/derived.tex

@@ -690,6 +690,16 @@
 \end{codeblock}
 \end{example}
 
+\pnum
+A virtual function shall not have a \grammarterm{requires-clause}.
+\begin{example}
+\begin{codeblock}
+struct A {
+  virtual void f() requires true; // error: constrained virtual function
+};
+\end{codeblock}
+\end{example}
+
 \pnum
 Even though destructors are not inherited, a destructor in a derived
 class overrides a base class destructor declared virtual;