The following wording implements guidance by EWG in Belfast to address the following NB comments:
Change in 13.2 [temp.param] paragraph 4:
A type C has strong structural equality if, given a glvalue x of type const C, either:
C is a non-class type and x <=> x is a valid expression of type std::strong_ordering or std::strong_- equality, or
C is a class type where all of the following hold:
All of C’s base class subobjects and non-static data members have strong structural equality.
C has no mutable or volatile non-static data members.
At the end of the definition of C, overload resolution performed for the expression x == x succeeds and finds either a friend or public member == operator that is defined as defaulted in the definition of C.
A non-type template-parameter shall have one of the following (optionally cv-qualified) types:Change in 13.4.2 [temp.arg.nontype] paragraph 2:
a literal type that has strong structural equality (11.11.1),
an lvalue reference type,
- a type that contains a placeholder type (184.108.40.206), or
- a placeholder for a deduced class type (220.127.116.11).
[Note: Other types are disallowed either explicitly below or implicitly by the rules governing the form of template-arguments (13.4 [temp.arg]). — end note]The top-level cv-qualifiers on the template-parameter are ignored when determining its type.
A template-argument for a non-type template-parameter shall be a converted constant expression (7.7) of the type of the template-parameter . For a non-type template-parameter of reference or pointer type, or for each non-static data member of reference or pointer type in a non-type template-parameter of class type or subobject thereof, the reference or pointer value shall not refer to or be the address of (respectively):
a subobject (6.7.2),
- a temporary object (6.7.7),
- a string literal (5.13.5),
- the result of a typeid expression (18.104.22.168),
- a predefined __func__ variable (9.5.1)
A structural type is one of the following:Change in 13.2 [temp.param] paragraph 6:
- a scalar type, or
- an lvalue reference type, or
- a literal class type with the following properties:
- all base classes and non-static data members are public and non-mutable and
- the types of all bases classes and non-static data members are structural types or (possibly multi-dimensional) array thereof.
An id-expression naming a non-type template-parameter of class type T denotes a static storage duration object of type const T, known as a template parameter object, whose value is that of the corresponding template argument after it has been converted to the type of the template-parameter. All such template parameters in the program of the same type with the same value denote the same template parameter object. [Note: If an id-expression names a non-type non-reference template-parameter, then it is a prvalue if it has non-class type. Otherwise, if it is of class type T, it is an lvalue and has type const T (22.214.171.124). — end note] [Example:Change in 13.6 [temp.type] paragraph 1:
Two template-ids refer to the same class, function, or variable if
- their template-names, operator-function-ids, or literal-operator-ids refer to the same template and
- their corresponding type template-arguments are the same type and
their corresponding non-type template-arguments of pointer-to-member type refer to the same class member or are both the null member pointer value and
their corresponding non-type template-arguments of reference type refer to the same object or function and
remainingcorresponding non-type template-arguments have the same type and valueafter conversion to the type of the template-parameter , where they are considered to have the same value if they compare equal with the == operator (7.6.10), and
- their corresponding template template-arguments refer to the same template.
Two values are template-argument-equivalent if they are of the same type and
- they are of integral type and their values are the same, or
- they are of floating-point type and their values are identical, or
- they are of type
- they are of enumeration type
Tand their values are the same [ Footnote: The identity of enumerators is not preserved. ], or
- they are of pointer type and they have the same pointer value, or
- they are of pointer-to-member type and they refer to the same class member or are both the null member pointer value, or
- they are of reference type and they refer to the same object or function, or
- they are of array type and their corresponding elements are template-argument-equivalent[ Footnote: An array as a template-parameter decays to a pointer. ], or
- 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
- they are of class type and their corresponding direct subobjects and reference members are template-argument-equivalent.