P1907R1 Inconsistencies with non-type template parameters

papers/nxxxx.md

@@ -43,6 +43,15 @@ CWG motion 2: [P1234R5 "Paper name"](http://wg21.link/p1234r5), resolving 3 NB c
 
 ...
 
+## Feature test macros
+
+The feature test macro `__cpp_nontype_template_parameter_class` has been removed
+to indicate that the feature added by [P0732R2](http://wg21.link/p0732r2)
+is no longer present in the same form.
+
+The value of the feature test macro `__cpp_nontype_template_args` has been increased
+to `201911L` to indicate support for [P1907R1](http://wg21.link/p1907r1).
+
 ## Disposition of editorial NB comments on C++ 2020 CD1
 
 Listed below are draft disposition for all comments that were

source/classes.tex

@@ -6700,30 +6700,6 @@
 the defaulted three-way comparison operator function is defined as deleted.
 \end{note}
 
-\pnum
-\indextext{structural comparison operator|see{operator, structural comparison}}%
-A type \tcode{C} has \defnadj{strong structural}{equality} if,
-given a glvalue \tcode{x} of type \tcode{const C}, either:
-\begin{itemize}
-\item
-\tcode{C} is a non-class type and
-\tcode{x <=> x} is a valid expression
-of type \tcode{std::strong_ordering} or \tcode{std::strong_equality}, or
-
-\item
-\tcode{C} is a class type
-where all of the following hold:
-\begin{itemize}
-\item All of \tcode{C}'s base class subobjects and non-static data members
-have strong structural equality.
-\item \tcode{C} has no mutable or volatile non-static data members.
-\item At the end of the definition of \tcode{C},
-overload resolution performed for the expression \tcode{x == x} succeeds and
-finds either a friend or public member \tcode{==} operator
-that is defined as defaulted in the definition of \tcode{C}.
-\end{itemize}
-\end{itemize}
-
 \pnum
 The direct base class subobjects of \tcode{C},
 in the order of their declaration in the \grammarterm{base-specifier-list} of \tcode{C},

source/preprocessor.tex

@@ -1668,9 +1668,8 @@
 \defnxname{cpp_modules}                           & \tcode{201907L} \\ \rowsep
 \defnxname{cpp_namespace_attributes}              & \tcode{201411L} \\ \rowsep
 \defnxname{cpp_noexcept_function_type}            & \tcode{201510L} \\ \rowsep
-\defnxname{cpp_nontype_template_args}             & \tcode{201411L} \\ \rowsep
+\defnxname{cpp_nontype_template_args}             & \tcode{201911L} \\ \rowsep
 \defnxname{cpp_nontype_template_parameter_auto}   & \tcode{201606L} \\ \rowsep
-\defnxname{cpp_nontype_template_parameter_class}  & \tcode{201806L} \\ \rowsep
 \defnxname{cpp_nsdmi}                             & \tcode{200809L} \\ \rowsep
 \defnxname{cpp_range_based_for}                   & \tcode{201603L} \\ \rowsep
 \defnxname{cpp_raw_strings}                       & \tcode{200710L} \\ \rowsep

source/templates.tex

@@ -367,28 +367,31 @@
 A non-type \grammarterm{template-parameter}
 shall have one of the following (optionally cv-qualified) types:
 \begin{itemize}
-\item a literal type that
-  has strong structural equality\iref{class.compare.default},
-
-\item an lvalue reference type,
-
+\item a structural type (see below),
 \item a type that contains a placeholder type\iref{dcl.spec.auto}, or
-
 \item a placeholder for a deduced class type\iref{dcl.type.class.deduct}.
 \end{itemize}
-
-\pnum
-\begin{note}
-Other types are disallowed either explicitly below or implicitly by
-the rules governing the form of
-\grammarterm{template-argument}{s}\iref{temp.arg}.
-\end{note}
 The top-level
 \grammarterm{cv-qualifier}{s}
 on the
 \grammarterm{template-parameter}
 are ignored when determining its type.
 
+\pnum
+A \defnadj{structural}{type} is one of the following:
+\begin{itemize}
+\item a scalar type, or
+\item an lvalue reference type, or
+\item a literal class type with the following properties:
+\begin{itemize}
+\item
+all base classes and non-static data members are public and non-mutable and
+\item
+the types of all bases classes and non-static data members are
+structural types or (possibly multi-dimensional) array thereof.
+\end{itemize}
+\end{itemize}
+
 \pnum
 An \grammarterm{id-expression} naming
 a non-type \grammarterm{template-parameter} of class type \tcode{T}
@@ -399,6 +402,7 @@
 to the type of the \grammarterm{template-parameter}.
 All such template parameters in the program of the same type
 with the same value denote the same template parameter object.
+A template parameter object shall have constant destruction\iref{expr.const}.
 \begin{note}
 If an \grammarterm{id-expression} names
 a non-type non-reference \grammarterm{template-parameter},
@@ -409,9 +413,9 @@
 \begin{example}
 \begin{codeblock}
 using X = int;
-struct A { friend bool operator==(const A&, const A&) = default; };
+struct A {};
 template<const X& x, int i, A a> void f() {
-  i++;                          // error: change of template-parameter value
+  i++;                          // error: change of \grammarterm{template-parameter} value
 
   &x;                           // OK
   &i;                           // error: address of non-reference template-parameter
@@ -424,16 +428,17 @@
 \end{example}
 
 \pnum
+\begin{note}
 A non-type
 \grammarterm{template-parameter}
-shall not be declared to have floating-point or void type.
+cannot be declared to have type \cv{} \tcode{void}.
 \begin{example}
 \begin{codeblock}
-template<double d> class X;     // error
-template<double* pd> class Y;   // OK
-template<double& rd> class Z;   // OK
+template<void v> class X;       // error
+template<void* pv> class Y;     // OK
 \end{codeblock}
 \end{example}
+\end{note}
 
 \pnum
 A non-type
@@ -1184,26 +1189,27 @@
 for a non-type \grammarterm{template-parameter}
 shall be a converted constant expression\iref{expr.const}
 of the type of the \grammarterm{template-parameter}.
+\begin{note}
+If the \grammarterm{template-argument}
+represents a set of overloaded functions
+(or a pointer or member pointer to such),
+the matching function is selected from the set\iref{over.over}.
+\end{note}
+
+\pnum
 For a non-type \grammarterm{template-parameter} of reference or pointer type,
 or for each non-static data member of reference or pointer type
 in a non-type \grammarterm{template-parameter} of class type or subobject thereof,
 the reference or pointer value shall not refer to
 or be the address of (respectively):
 \begin{itemize}
-\item a subobject\iref{intro.object},
 \item a temporary object\iref{class.temporary},
 \item a string literal\iref{lex.string},
-\item the result of a \tcode{typeid} expression\iref{expr.typeid}, or
-\item a predefined \mname{func} variable\iref{dcl.fct.def.general}.
+\item the result of a \tcode{typeid} expression\iref{expr.typeid},
+\item a predefined \mname{func} variable\iref{dcl.fct.def.general}, or
+\item a subobject\iref{intro.object} of one of the above.
 \end{itemize}
 
-\begin{note}
-If the \grammarterm{template-argument}
-represents a set of overloaded functions
-(or a pointer or member pointer to such),
-the matching function is selected from the set\iref{over.over}.
-\end{note}
-
 \pnum
 \begin{example}
 \begin{codeblock}
@@ -1228,9 +1234,10 @@
 A<&f> a;                        // selects \tcode{f(int)}
 
 template<auto n> struct B { @\commentellip@ };
-B<5> b1;                        // OK: template parameter type is \tcode{int}
-B<'a'> b2;                      // OK: template parameter type is \tcode{char}
-B<2.5> b3;                      // error: template parameter type cannot be \tcode{double}
+B<5> b1;                        // OK, template parameter type is \tcode{int}
+B<'a'> b2;                      // OK, template parameter type is \tcode{char}
+B<2.5> b3;                      // OK, template parameter type is \tcode{double}
+B<void(0)> b4;                  // error: template parameter type cannot be \tcode{void}
 \end{codeblock}
 \end{example}
 
@@ -1245,40 +1252,21 @@
   @\commentellip@
 };
 
-X<const char*, "Studebaker"> x; // error: string literal as template-argument
+X<const char*, "Studebaker"> x; // error: string literal as \grammarterm{template-argument}
+X<const char*, "Knope" + 1> x2; // error: subobject of string literal as \grammarterm{template-argument}
 
 const char p[] = "Vivisectionist";
 X<const char*, p> y;            // OK
 
 struct A {
   constexpr A(const char*) {}
-  friend bool operator==(const A&, const A&) = default;
 };
 
 X<A, "Pyrophoricity"> z;        // OK, string literal is a constructor argument to \tcode{A}
 \end{codeblock}
 \end{example}
 \end{note}
 
-\pnum
-\begin{note}
-The address of an array element or non-static data member is not an acceptable
-\grammarterm{template-argument}.
-\begin{example}
-\begin{codeblock}
-template<int* p> class X { };
-
-int a[10];
-struct S { int m; static int s; } s;
-
-X<&a[2]> x3;                    // error: address of array element
-X<&s.m> x4;                     // error: address of non-static member
-X<&s.s> x5;                     // OK: address of static member
-X<&S::s> x6;                    // OK: address of static member
-\end{codeblock}
-\end{example}
-\end{note}
-
 \pnum
 \begin{note}
 A temporary object
@@ -1291,10 +1279,16 @@
 \begin{codeblock}
 template<const int& CRI> struct B { @\commentellip@ };
 
-B<1> b2;                        // error: temporary would be required for template argument
+B<1> b1;                        // error: temporary would be required for template argument
 
 int c = 1;
-B<c> b1;                        // OK
+B<c> b2;                        // OK
+
+struct X { int n; };
+struct Y { const int &r; };
+template<Y y> struct C { @\commentellip@ };
+C<Y{X{1}.n}> c;                 // error: subobject of temporary object used to initialize
+                                // reference member of template parameter
 \end{codeblock}
 \end{example}
 \end{note}
@@ -1939,25 +1933,68 @@
 Two \grammarterm{template-id}{s} refer to the same
 class, function, or variable if
 \begin{itemize}
-\item {their \grammarterm{template-name}{s},
-\grammarterm{operator-function-id}{s}, or \grammarterm{literal-operator-id}{s}
-refer to the same template and}
-\item {their corresponding type \grammarterm{template-argument}{s} are the
-same type and}
-\item {their corresponding non-type \grammarterm{template-argument}{s} of
-pointer-to-member type refer to the same class member or are both the null member
-pointer value and}
-\item {their corresponding non-type \grammarterm{template-argument}{s} of
-reference type refer to the same object or function and}
-\item {their remaining corresponding
-non-type \grammarterm{template-argument}{s}
-have the same type and value
-after conversion to the type of the \grammarterm{template-parameter},
-where they are considered to have the same value if they compare equal
-with the \tcode{==} operator\iref{expr.eq}, and}
-\item {their corresponding template \grammarterm{template-argument}{s} refer
-to the same template.}
+\item
+their \grammarterm{template-name}{s},
+\grammarterm{operator-function-id}{s}, or
+\grammarterm{literal-operator-id}{s}
+refer to the same template, and
+
+\item
+their corresponding type \grammarterm{template-argument}{s}
+are the same type, and
+
+\item
+their corresponding non-type \grammarterm{template-argument}{s}
+are template-argument-equivalent (see below)
+after conversion to the type of the \grammarterm{template-parameter}, and
+
+\item
+their corresponding template \grammarterm{template-argument}{s}
+refer to the same template.
+\end{itemize}
+
+\pnum
+Two values are \defn{template-argument-equivalent} if
+they are of the same type and
+\begin{itemize}
+\item
+they are of integral type and their values are the same, or
+
+\item
+they are of floating-point type and their values are identical, or
+
+\item
+they are of type \tcode{std::nullptr_t}, or
+
+\item
+they are of enumeration type and their values are the same,%
+\footnote{The identity of enumerators is not preserved.} or
+
+\item
+they are of pointer type and they have the same pointer value, or
+
+\item
+they are of pointer-to-member type and they refer to the same class member
+or are both the null member pointer value, or
+
+\item
+they are of reference type and they refer to the same object or function, or
+
+\item
+they are of array type and their corresponding elements are template-argument-equivalent,%
+\footnote{An array as a \grammarterm{template-parameter} decays to a pointer.} or
+
+\item
+they are of union type and either
+they both have no active member or
+they have the same active member and their active members are template-argument-equivalent, or
+
+\item
+they are of class type and
+their corresponding direct subobjects and reference members are template-argument-equivalent.
 \end{itemize}
+
+\pnum
 \begin{example}
 \begin{codeblock}
 template<class E, int size> class buffer { @\commentellip@ };