13 Templates [temp]

13.3 Names of template specializations [temp.names]

1
#
A template specialization can be referred to by a template-id:
[Note
:
The name lookup rules are used to associate the use of a name with a template declaration; that is, to identify a name as a template-name.
— end note
]
2
#
For a template-name to be explicitly qualified by the template arguments, the name must be considered to refer to a template.
[Note
:
Whether a name actually refers to a template cannot be known in some cases until after argument dependent lookup is done ([basic.lookup.argdep]).
— end note
]
A name is considered to refer to a template if name lookup finds a template-name or an overload set that contains a function template.
A name is also considered to refer to a template if it is an unqualified-id followed by a < and name lookup either finds one or more functions or finds nothing.
3
#
When a name is considered to be a template-name, and it is followed by a <, the < is always taken as the delimiter of a template-argument-list and never as the less-than operator.
When parsing a template-argument-list, the first non-nested >128 is taken as the ending delimiter rather than a greater-than operator.
Similarly, the first non-nested >> is treated as two consecutive but distinct > tokens, the first of which is taken as the end of the template-argument-list and completes the template-id.
[Note
:
The second > token produced by this replacement rule may terminate an enclosing template-id construct or it may be part of a different construct (e.g., a cast).
— end note
]
[Example
: 
template<int i> class X { /* ... */ };

X< 1>2 > x1;                            // syntax error
X<(1>2)> x2;                            // OK

template<class T> class Y { /* ... */ };
Y<X<1>> x3;                             // OK, same as Y<X<1> > x3;
Y<X<6>>1>> x4;                          // syntax error
Y<X<(6>>1)>> x5;                        // OK
— end example
]
4
#
The keyword template is said to appear at the top level in a qualified-id if it appears outside of a template-argument-list or decltype-specifier.
In a qualified-id of a declarator-id or in a qualified-id formed by a class-head-name or enum-head-name, the keyword template shall not appear at the top level.
In a qualified-id used as the name in a typename-specifier, elaborated-type-specifier, using-declaration, or class-or-decltype, an optional keyword template appearing at the top level is ignored.
In these contexts, a < token is always assumed to introduce a template-argument-list.
In all other contexts, when naming a template specialization of a member of an unknown specialization ([temp.dep.type]), the member template name shall be prefixed by the keyword template.
[Example
: 
struct X {
  template<std::size_t> X* alloc();
  template<std::size_t> static X* adjust();
};
template<class T> void f(T* p) {
  T* p1 = p->alloc<200>();              // error: < means less than
  T* p2 = p->template alloc<200>();     // OK: < starts template argument list
  T::adjust<100>();                     // error: < means less than
  T::template adjust<100>();            // OK: < starts template argument list
}
— end example
]
5
#
A name prefixed by the keyword template shall be a template-id or the name shall refer to a class template or an alias template.
[Note
:
The keyword template may not be applied to non-template members of class templates.
— end note
]
[Note
:
As is the case with the typename prefix, the template prefix is allowed in cases where it is not strictly necessary; i.e., when the nested-name-specifier or the expression on the left of the -> or . is not dependent on a template-parameter, or the use does not appear in the scope of a template.
— end note
]
[Example
: 
template <class T> struct A {
  void f(int);
  template <class U> void f(U);
};

template <class T> void f(T t) {
  A<T> a;
  a.template f<>(t);                    // OK: calls template
  a.template f(t);                      // error: not a template-id
}

template <class T> struct B {
  template <class T2> struct C { };
};

// OK: T​::​template C names a class template:
template <class T, template <class X> class TT = T::template C> struct D { };
D<B<int> > db;
— end example
]
6
#
A template-id is valid if
A simple-template-id shall be valid unless it names a function template specialization ([temp.deduct]).
[Example
: 
template<class T, T::type n = 0> class X;
struct S {
  using type = int;
};
using T1 = X<S, int, int>;      // error: too many arguments
using T2 = X<>;                 // error: no default argument for first template parameter
using T3 = X<1>;                // error: value 1 does not match type-parameter
using T4 = X<int>;              // error: substitution failure for second template parameter
using T5 = X<S>;                // OK
— end example
]
7
#
When the template-name of a simple-template-id names a constrained non-function template or a constrained template template-parameter, but not a member template that is a member of an unknown specialization ([temp.res]), and all template-arguments in the simple-template-id are non-dependent ([temp.dep.temp]), the associated constraints ([temp.constr.decl]) of the constrained template shall be satisfied ([temp.constr.constr]).
[Example
: 
template<typename T> concept C1 = sizeof(T) != sizeof(int);

template<C1 T> struct S1 { };
template<C1 T> using Ptr = T*;

S1<int>* p;                         // error: constraints not satisfied
Ptr<int> p;                         // error: constraints not satisfied

template<typename T>
struct S2 { Ptr<int> x; };          // ill-formed, no diagnostic required

template<typename T>
struct S3 { Ptr<T> x; };            // OK, satisfaction is not required

S3<int> x;                          // error: constraints not satisfied

template<template<C1 T> class X>
struct S4 {
  X<int> x;                         // ill-formed, no diagnostic required
};

template<typename T> concept C2 = sizeof(T) == 1;

template<C2 T> struct S { };

template struct S<char[2]>;         // error: constraints not satisfied
template<> struct S<char[2]> { };   // error: constraints not satisfied
— end example
]
8
#
A concept-id is a prvalue of type bool, and does not name a template specialization.
A concept-id evaluates to true if the concept's normalized constraint-expression is satisfied ([temp.constr.constr]) by the specified template arguments and false otherwise.
[Note
:
Since a constraint-expression is an unevaluated operand, a concept-id appearing in a constraint-expression is not evaluated except as necessary to determine whether the normalized constraints are satisfied.
— end note
]
[Example
: 
template<typename T> concept C = true;
static_assert(C<int>);      // OK
— end example
]
128)
A > that encloses the type-id of a dynamic_­cast, static_­cast, reinterpret_­cast or const_­cast, or which encloses the template-arguments of a subsequent template-id, is considered nested for the purpose of this description.
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