13 Templates [temp]

13.8 Name resolution [temp.res]

13.8.1 General [temp.res.general]

1
#
Three kinds of names can be used within a template definition:
2
#
A name used in a template declaration or definition and that is dependent on a template-parameter is assumed not to name a type unless the applicable name lookup finds a type name or the name is qualified by the keyword typename.
[Example 1: // no B declared here class X; template<class T> class Y { class Z; // forward declaration of member class void f() { X* a1; // declare pointer to X T* a2; // declare pointer to T Y* a3; // declare pointer to Y<T> Z* a4; // declare pointer to Z typedef typename T::A TA; TA* a5; // declare pointer to T's A typename T::A* a6; // declare pointer to T's A T::A* a7; // error: no visible declaration of a7 // T​::​A is not a type name; multiplication of T​::​A by a7 B* a8; // error: no visible declarations of B and a8 // B is not a type name; multiplication of B by a8 } }; — end example]
3
#
A typename-specifier denotes the type or class template denoted by the simple-type-specifier ([dcl.type.simple]) formed by omitting the keyword typename.
The usual qualified name lookup ([basic.lookup.qual]) is used to find the qualified-id even in the presence of typename.
[Example 2: struct A { struct X { }; int X; }; struct B { struct X { }; }; template<class T> void f(T t) { typename T::X x; } void foo() { A a; B b; f(b); // OK: T​::​X refers to B​::​X f(a); // error: T​::​X refers to the data member A​::​X not the struct A​::​X } — end example]
4
#
A qualified name used as the name in a class-or-decltype ([class.derived]) or an elaborated-type-specifier is implicitly assumed to name a type, without the use of the typename keyword.
In a nested-name-specifier that immediately contains a nested-name-specifier that depends on a template parameter, the identifier or simple-template-id is implicitly assumed to name a type, without the use of the typename keyword.
[Note 1:
The typename keyword is not permitted by the syntax of these constructs.
— end note]
5
#
A qualified-id is assumed to name a type if
A qualified name is said to be in a type-id-only context if it appears in a type-id, new-type-id, or defining-type-id and the smallest enclosing type-id, new-type-id, or defining-type-id is a new-type-id, defining-type-id, trailing-return-type, default argument of a type-parameter of a template, or type-id of a static_­cast, const_­cast, reinterpret_­cast, or dynamic_­cast.
[Example 3: template<class T> T::R f(); // OK, return type of a function declaration at global scope template<class T> void f(T::R); // ill-formed, no diagnostic required: attempt to declare // a void variable template template<class T> struct S { using Ptr = PtrTraits<T>::Ptr; // OK, in a defining-type-id T::R f(T::P p) { // OK, class scope return static_cast<T::R>(p); // OK, type-id of a static_­cast } auto g() -> S<T*>::Ptr; // OK, trailing-return-type }; template<typename T> void f() { void (*pf)(T::X); // variable pf of type void* initialized with T​::​X void g(T::X); // error: T​::​X at block scope does not denote a type // (attempt to declare a void variable) } — end example]
6
#
A qualified-id that refers to a member of an unknown specialization, that is not prefixed by typename, and that is not otherwise assumed to name a type (see above) denotes a non-type.
[Example 4: template <class T> void f(int i) { T::x * i; // expression, not the declaration of a variable i } struct Foo { typedef int x; }; struct Bar { static int const x = 5; }; int main() { f<Bar>(1); // OK f<Foo>(1); // error: Foo​::​x is a type } — end example]
7
#
Within the definition of a class template or within the definition of a member of a class template following the declarator-id, the keyword typename is not required when referring to a member of the current instantiation ([temp.dep.type]).
[Example 5: template<class T> struct A { typedef int B; B b; // OK, no typename required }; — end example]
8
#
The validity of a template may be checked prior to any instantiation.
[Note 2:
Knowing which names are type names allows the syntax of every template to be checked in this way.
— end note]
The program is ill-formed, no diagnostic required, if:
  • (8.1)
    no valid specialization can be generated for a template or a substatement of a constexpr if statement within a template and the template is not instantiated, or
  • (8.2)
    no substitution of template arguments into a type-constraint or requires-clause would result in a valid expression, or
  • (8.3)
    every valid specialization of a variadic template requires an empty template parameter pack, or
  • (8.4)
    a hypothetical instantiation of a template immediately following its definition would be ill-formed due to a construct that does not depend on a template parameter, or
  • (8.5)
    the interpretation of such a construct in the hypothetical instantiation is different from the interpretation of the corresponding construct in any actual instantiation of the template.
    [Note 3:
    This can happen in situations including the following:
    • (8.5.1)
      a type used in a non-dependent name is incomplete at the point at which a template is defined but is complete at the point at which an instantiation is performed, or
    • (8.5.2)
      lookup for a name in the template definition found a using-declaration, but the lookup in the corresponding scope in the instantiation does not find any declarations because the using-declaration was a pack expansion and the corresponding pack is empty, or
    • (8.5.3)
      an instantiation uses a default argument or default template argument that had not been defined at the point at which the template was defined, or
    • (8.5.4)
      constant expression evaluation within the template instantiation uses
      • (8.5.4.1)
        the value of a const object of integral or unscoped enumeration type or
      • (8.5.4.2)
        the value of a constexpr object or
      • (8.5.4.3)
        the value of a reference or
      • (8.5.4.4)
        the definition of a constexpr function,
      and that entity was not defined when the template was defined, or
    • (8.5.5)
      a class template specialization or variable template specialization that is specified by a non-dependent simple-template-id is used by the template, and either it is instantiated from a partial specialization that was not defined when the template was defined or it names an explicit specialization that was not declared when the template was defined.
    — end note]
Otherwise, no diagnostic shall be issued for a template for which a valid specialization can be generated.
[Note 4:
If a template is instantiated, errors will be diagnosed according to the other rules in this document.
Exactly when these errors are diagnosed is a quality of implementation issue.
— end note]
[Example 6: int j; template<class T> class X { void f(T t, int i, char* p) { t = i; // diagnosed if X​::​f is instantiated, and the assignment to t is an error p = i; // may be diagnosed even if X​::​f is not instantiated p = j; // may be diagnosed even if X​::​f is not instantiated } void g(T t) { +; // may be diagnosed even if X​::​g is not instantiated } }; template<class... T> struct A { void operator++(int, T... t); // error: too many parameters }; template<class... T> union X : T... { }; // error: union with base class template<class... T> struct A : T..., T... { }; // error: duplicate base class — end example]
9
#
When looking for the declaration of a name used in a template definition, the usual lookup rules ([basic.lookup.unqual], [basic.lookup.argdep]) are used for non-dependent names.
The lookup of names dependent on the template parameters is postponed until the actual template argument is known ([temp.dep]).
[Example 7: #include <iostream> using namespace std; template<class T> class Set { T* p; int cnt; public: Set(); Set<T>(const Set<T>&); void printall() { for (int i = 0; i<cnt; i++) cout << p[i] << '\n'; } };
In the example, i is the local variable i declared in printall, cnt is the member cnt declared in Set, and cout is the standard output stream declared in iostream.
However, not every declaration can be found this way; the resolution of some names is postponed until the actual template-arguments are known.
For example, even though the name operator<< is known within the definition of printall() and a declaration of it can be found in <iostream>, the actual declaration of operator<< needed to print p[i] cannot be known until it is known what type T is ([temp.dep]).
— end example]
10
#
If a name does not depend on a template-parameter (as defined in [temp.dep]), a declaration (or set of declarations) for that name shall be in scope at the point where the name appears in the template definition; the name is bound to the declaration (or declarations) found at that point and this binding is not affected by declarations that are visible at the point of instantiation.
[Example 8: void f(char); template<class T> void g(T t) { f(1); // f(char) f(T(1)); // dependent f(t); // dependent dd++; // not dependent; error: declaration for dd not found } enum E { e }; void f(E); double dd; void h() { g(e); // will cause one call of f(char) followed by two calls of f(E) g('a'); // will cause three calls of f(char) } — end example]
11
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[Note 5:
For purposes of name lookup, default arguments and noexcept-specifiers of function templates and default arguments and noexcept-specifiers of member functions of class templates are considered definitions ([temp.decls]).
— end note]
140)
This includes friend function declarations.
 ⮥

13.8.2 Locally declared names [temp.local]

1
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Like normal (non-template) classes, class templates have an injected-class-name ([class.pre]).
The injected-class-name can be used as a template-name or a type-name.
When it is used with a template-argument-list, as a template-argument for a template template-parameter, or as the final identifier in the elaborated-type-specifier of a friend class template declaration, it is a template-name that refers to the class template itself.
Otherwise, it is a type-name equivalent to the template-name followed by the template-parameters of the class template enclosed in <>.
2
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Within the scope of a class template specialization or partial specialization, when the injected-class-name is used as a type-name, it is equivalent to the template-name followed by the template-arguments of the class template specialization or partial specialization enclosed in <>.
[Example 1: template<template<class> class T> class A { }; template<class T> class Y; template<> class Y<int> { Y* p; // meaning Y<int> Y<char>* q; // meaning Y<char> A<Y>* a; // meaning A<​::​Y> class B { template<class> friend class Y; // meaning ​::​Y }; }; — end example]
3
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The injected-class-name of a class template or class template specialization can be used as either a template-name or a type-name wherever it is in scope.
[Example 2: template <class T> struct Base { Base* p; }; template <class T> struct Derived: public Base<T> { typename Derived::Base* p; // meaning Derived​::​Base<T> }; template<class T, template<class> class U = T::template Base> struct Third { }; Third<Derived<int> > t; // OK: default argument uses injected-class-name as a template — end example]
4
#
A lookup that finds an injected-class-name ([class.member.lookup]) can result in an ambiguity in certain cases (for example, if it is found in more than one base class).
If all of the injected-class-names that are found refer to specializations of the same class template, and if the name is used as a template-name, the reference refers to the class template itself and not a specialization thereof, and is not ambiguous.
[Example 3: template <class T> struct Base { }; template <class T> struct Derived: Base<int>, Base<char> { typename Derived::Base b; // error: ambiguous typename Derived::Base<double> d; // OK }; — end example]
5
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When the normal name of the template (i.e., the name from the enclosing scope, not the injected-class-name) is used, it always refers to the class template itself and not a specialization of the template.
[Example 4: template<class T> class X { X* p; // meaning X<T> X<T>* p2; X<int>* p3; ::X* p4; // error: missing template argument list // ​::​X does not refer to the injected-class-name }; — end example]
6
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The name of a template-parameter shall not be redeclared within its scope (including nested scopes).
A template-parameter shall not have the same name as the template name.
[Example 5: template<class T, int i> class Y { int T; // error: template-parameter redeclared void f() { char T; // error: template-parameter redeclared } }; template<class X> class X; // error: template-parameter redeclared — end example]
7
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In the definition of a member of a class template that appears outside of the class template definition, the name of a member of the class template hides the name of a template-parameter of any enclosing class templates (but not a template-parameter of the member if the member is a class or function template).
[Example 6: template<class T> struct A { struct B { /* ... */ }; typedef void C; void f(); template<class U> void g(U); }; template<class B> void A<B>::f() { B b; // A's B, not the template parameter } template<class B> template<class C> void A<B>::g(C) { B b; // A's B, not the template parameter C c; // the template parameter C, not A's C } — end example]
8
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In the definition of a member of a class template that appears outside of the namespace containing the class template definition, the name of a template-parameter hides the name of a member of this namespace.
[Example 7: namespace N { class C { }; template<class T> class B { void f(T); }; } template<class C> void N::B<C>::f(C) { C b; // C is the template parameter, not N​::​C } — end example]
9
#
In the definition of a class template or in the definition of a member of such a template that appears outside of the template definition, for each non-dependent base class ([temp.dep.type]), if the name of the base class or the name of a member of the base class is the same as the name of a template-parameter, the base class name or member name hides the template-parameter name.
[Example 8: struct A { struct B { /* ... */ }; int a; int Y; }; template<class B, class a> struct X : A { B b; // A's B a b; // error: A's a isn't a type name }; — end example]

13.8.3 Dependent names [temp.dep]

13.8.3.1 General [temp.dep.general]

1
#
Inside a template, some constructs have semantics which may differ from one instantiation to another.
Such a construct depends on the template parameters.
In particular, types and expressions may depend on the type and/or value of template parameters (as determined by the template arguments) and this determines the context for name lookup for certain names.
An expression may be type-dependent (that is, its type may depend on a template parameter) or value-dependent (that is, its value when evaluated as a constant expression ([expr.const]) may depend on a template parameter) as described below.
2
#
In an expression of the form:
postfix-expression ( expression-list )
where the postfix-expression is an unqualified-id, the unqualified-id denotes a dependent name if
If an operand of an operator is a type-dependent expression, the operator also denotes a dependent name.
[Note 1:
Such names are unbound and are looked up at the point of the template instantiation ([temp.point]) in both the context of the template definition and the context of the point of instantiation ([temp.dep.candidate]).
— end note]
3
#
[Example 1: template<class T> struct X : B<T> { typename T::A* pa; void f(B<T>* pb) { static int i = B<T>::i; pb->j++; } };
The base class name B<T>, the type name T​::​A, the names B<T>​::​i and pb->j explicitly depend on the template-parameter.
— end example]
4
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In the definition of a class or class template, the scope of a dependent base class is not examined during unqualified name lookup either at the point of definition of the class template or member or during an instantiation of the class template or member.
[Example 2: typedef double A; template<class T> class B { typedef int A; }; template<class T> struct X : B<T> { A a; // a has type double };
The type name A in the definition of X<T> binds to the typedef name defined in the global namespace scope, not to the typedef name defined in the base class B<T>.
— end example]
[Example 3: struct A { struct B { /* ... */ }; int a; int Y; }; int a; template<class T> struct Y : T { struct B { /* ... */ }; B b; // The B defined in Y void f(int i) { a = i; } // ​::​a Y* p; // Y<T> }; Y<A> ya;
The members A​::​B, A​::​a, and A​::​Y of the template argument A do not affect the binding of names in Y<A>.
— end example]

13.8.3.2 Dependent types [temp.dep.type]

1
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A name refers to the current instantiation if it is
  • (1.1)
    in the definition of a class template, a nested class of a class template, a member of a class template, or a member of a nested class of a class template, the injected-class-name of the class template or nested class,
  • (1.2)
    in the definition of a primary class template or a member of a primary class template, the name of the class template followed by the template argument list of the primary template (as described below) enclosed in <> (or an equivalent template alias specialization),
  • (1.3)
    in the definition of a nested class of a class template, the name of the nested class referenced as a member of the current instantiation, or
  • (1.4)
    in the definition of a partial specialization or a member of a partial specialization, the name of the class template followed by the template argument list of the partial specialization enclosed in <> (or an equivalent template alias specialization).
    If the template parameter is a template parameter pack, the template argument is a pack expansion ([temp.variadic]) whose pattern is the name of the template parameter pack.
2
#
The template argument list of a primary template is a template argument list in which the template argument has the value of the template parameter of the class template.
If the template parameter is a template parameter pack, the template argument is a pack expansion whose pattern is the name of the template parameter pack.
3
#
A template argument that is equivalent to a template parameter can be used in place of that template parameter in a reference to the current instantiation.
For a template type-parameter, a template argument is equivalent to a template parameter if it denotes the same type.
For a non-type template parameter, a template argument is equivalent to a template parameter if it is an identifier that names a variable that is equivalent to the template parameter.
A variable is equivalent to a template parameter if
  • (3.1)
    it has the same type as the template parameter (ignoring cv-qualification) and
  • (3.2)
    its initializer consists of a single identifier that names the template parameter or, recursively, such a variable.
[Note 1:
Using a parenthesized variable name breaks the equivalence.
— end note]
[Example 1: template <class T> class A { A* p1; // A is the current instantiation A<T>* p2; // A<T> is the current instantiation A<T*> p3; // A<T*> is not the current instantiation ::A<T>* p4; // ​::​A<T> is the current instantiation class B { B* p1; // B is the current instantiation A<T>::B* p2; // A<T>​::​B is the current instantiation typename A<T*>::B* p3; // A<T*>​::​B is not the current instantiation }; }; template <class T> class A<T*> { A<T*>* p1; // A<T*> is the current instantiation A<T>* p2; // A<T> is not the current instantiation }; template <class T1, class T2, int I> struct B { B<T1, T2, I>* b1; // refers to the current instantiation B<T2, T1, I>* b2; // not the current instantiation typedef T1 my_T1; static const int my_I = I; static const int my_I2 = I+0; static const int my_I3 = my_I; static const long my_I4 = I; static const int my_I5 = (I); B<my_T1, T2, my_I>* b3; // refers to the current instantiation B<my_T1, T2, my_I2>* b4; // not the current instantiation B<my_T1, T2, my_I3>* b5; // refers to the current instantiation B<my_T1, T2, my_I4>* b6; // not the current instantiation B<my_T1, T2, my_I5>* b7; // not the current instantiation }; — end example]
4
#
A dependent base class is a base class that is a dependent type and is not the current instantiation.
[Note 2:
A base class can be the current instantiation in the case of a nested class naming an enclosing class as a base.
[Example 2: template<class T> struct A { typedef int M; struct B { typedef void M; struct C; }; }; template<class T> struct A<T>::B::C : A<T> { M m; // OK, A<T>​::​M }; — end example]
— end note]
5
#
A name is a member of the current instantiation if it is
  • (5.1)
    An unqualified name that, when looked up, refers to at least one member of a class that is the current instantiation or a non-dependent base class thereof.
    [Note 3:
    This can only occur when looking up a name in a scope enclosed by the definition of a class template.
    — end note]
  • (5.2)
    A qualified-id in which the nested-name-specifier refers to the current instantiation and that, when looked up, refers to at least one member of a class that is the current instantiation or a non-dependent base class thereof.
    [Note 4:
    If no such member is found, and the current instantiation has any dependent base classes, then the qualified-id is a member of an unknown specialization; see below.
    — end note]
  • (5.3)
    An id-expression denoting the member in a class member access expression for which the type of the object expression is the current instantiation, and the id-expression, when looked up, refers to at least one member of a class that is the current instantiation or a non-dependent base class thereof.
    [Note 5:
    If no such member is found, and the current instantiation has any dependent base classes, then the id-expression is a member of an unknown specialization; see below.
    — end note]
[Example 3: template <class T> class A { static const int i = 5; int n1[i]; // i refers to a member of the current instantiation int n2[A::i]; // A​::​i refers to a member of the current instantiation int n3[A<T>::i]; // A<T>​::​i refers to a member of the current instantiation int f(); }; template <class T> int A<T>::f() { return i; // i refers to a member of the current instantiation } — end example]
A name is a dependent member of the current instantiation if it is a member of the current instantiation that, when looked up, refers to at least one member of a class that is the current instantiation.
6
#
A name is a member of an unknown specialization if it is
  • (6.1)
    A qualified-id in which the nested-name-specifier names a dependent type that is not the current instantiation.
  • (6.2)
    A qualified-id in which the nested-name-specifier refers to the current instantiation, the current instantiation has at least one dependent base class, and name lookup of the qualified-id does not find any member of a class that is the current instantiation or a non-dependent base class thereof.
  • (6.3)
    An id-expression denoting the member in a class member access expression in which either
    • (6.3.1)
      the type of the object expression is the current instantiation, the current instantiation has at least one dependent base class, and name lookup of the id-expression does not find a member of a class that is the current instantiation or a non-dependent base class thereof; or
    • (6.3.2)
      the type of the object expression is not the current instantiation and the object expression is type-dependent.
7
#
If a qualified-id in which the nested-name-specifier refers to the current instantiation is not a member of the current instantiation or a member of an unknown specialization, the program is ill-formed even if the template containing the qualified-id is not instantiated; no diagnostic required.
Similarly, if the id-expression in a class member access expression for which the type of the object expression is the current instantiation does not refer to a member of the current instantiation or a member of an unknown specialization, the program is ill-formed even if the template containing the member access expression is not instantiated; no diagnostic required.
[Example 4: template<class T> class A { typedef int type; void f() { A<T>::type i; // OK: refers to a member of the current instantiation typename A<T>::other j; // error: neither a member of the current instantiation nor // a member of an unknown specialization } }; — end example]
8
#
If, for a given set of template arguments, a specialization of a template is instantiated that refers to a member of the current instantiation with a qualified-id or class member access expression, the name in the qualified-id or class member access expression is looked up in the template instantiation context.
If the result of this lookup differs from the result of name lookup in the template definition context, name lookup is ambiguous.
[Example 5: struct A { int m; }; struct B { int m; }; template<typename T> struct C : A, T { int f() { return this->m; } // finds A​::​m in the template definition context int g() { return m; } // finds A​::​m in the template definition context }; template int C<B>::f(); // error: finds both A​::​m and B​::​m template int C<B>::g(); // OK: transformation to class member access syntax // does not occur in the template definition context; see [class.mfct.non-static] — end example]
9
#
A type is dependent if it is
  • (9.1)
    a template parameter,
  • (9.2)
    a member of an unknown specialization,
  • (9.3)
    a nested class or enumeration that is a dependent member of the current instantiation,
  • (9.4)
    a cv-qualified type where the cv-unqualified type is dependent,
  • (9.5)
    a compound type constructed from any dependent type,
  • (9.6)
    an array type whose element type is dependent or whose bound (if any) is value-dependent,
  • (9.7)
    a function type whose exception specification is value-dependent,
  • (9.8)
    denoted by a simple-template-id in which either the template name is a template parameter or any of the template arguments is a dependent type or an expression that is type-dependent or value-dependent or is a pack expansion,141 or
  • (9.9)
    denoted by decltype(expression), where expression is type-dependent.
10
#
[Note 6:
Because typedefs do not introduce new types, but instead simply refer to other types, a name that refers to a typedef that is a member of the current instantiation is dependent only if the type referred to is dependent.
— end note]
141)
This includes an injected-class-name ([class.pre]) of a class template used without a template-argument-list.
 ⮥

13.8.3.3 Type-dependent expressions [temp.dep.expr]

1
#
Except as described below, an expression is type-dependent if any subexpression is type-dependent.
2
#
this is type-dependent if the class type of the enclosing member function is dependent ([temp.dep.type]).
3
#
An id-expression is type-dependent if it is not a concept-id and it contains or if it names a dependent member of the current instantiation that is a static data member of type “array of unknown bound of T” for some T ([temp.static]).
Expressions of the following forms are type-dependent only if the type specified by the type-id, simple-type-specifier or new-type-id is dependent, even if any subexpression is type-dependent:
simple-type-specifier ( expression-list )
:: new new-placement new-type-id new-initializer
:: new new-placement ( type-id ) new-initializer
dynamic_­cast < type-id > ( expression )
static_­cast < type-id > ( expression )
const_­cast < type-id > ( expression )
reinterpret_­cast < type-id > ( expression )
( type-id ) cast-expression
4
#
Expressions of the following forms are never type-dependent (because the type of the expression cannot be dependent):
literal
sizeof unary-expression
sizeof ( type-id )
sizeof ... ( identifier )
alignof ( type-id )
typeid ( expression )
typeid ( type-id )
:: delete cast-expression
:: delete [ ] cast-expression
throw assignment-expression
noexcept ( expression )
[Note 1:
For the standard library macro offsetof, see [support.types].
— end note]
5
#
A class member access expression is type-dependent if the expression refers to a member of the current instantiation and the type of the referenced member is dependent, or the class member access expression refers to a member of an unknown specialization.
[Note 2:
In an expression of the form x.y or xp->y the type of the expression is usually the type of the member y of the class of x (or the class pointed to by xp).
However, if x or xp refers to a dependent type that is not the current instantiation, the type of y is always dependent.
If x or xp refers to a non-dependent type or refers to the current instantiation, the type of y is the type of the class member access expression.
— end note]
6
#
A braced-init-list is type-dependent if any element is type-dependent or is a pack expansion.
7
#
A fold-expression is type-dependent.

13.8.3.4 Value-dependent expressions [temp.dep.constexpr]

1
#
Except as described below, an expression used in a context where a constant expression is required is value-dependent if any subexpression is value-dependent.
2
#
An id-expression is value-dependent if:
  • (2.1)
    it is a concept-id and any of its arguments are dependent,
  • (2.2)
    it is type-dependent,
  • (2.3)
    it is the name of a non-type template parameter,
  • (2.4)
    it names a static data member that is a dependent member of the current instantiation and is not initialized in a member-declarator,
  • (2.5)
    it names a static member function that is a dependent member of the current instantiation, or
  • (2.6)
    it names a potentially-constant variable ([expr.const]) that is initialized with an expression that is value-dependent.
Expressions of the following form are value-dependent if the unary-expression or expression is type-dependent or the type-id is dependent:
sizeof unary-expression
sizeof ( type-id )
typeid ( expression )
typeid ( type-id )
alignof ( type-id )
noexcept ( expression )
[Note 1:
For the standard library macro offsetof, see [support.types].
— end note]
3
#
Expressions of the following form are value-dependent if either the type-id or simple-type-specifier is dependent or the expression or cast-expression is value-dependent:
simple-type-specifier ( expression-list )
static_­cast < type-id > ( expression )
const_­cast < type-id > ( expression )
reinterpret_­cast < type-id > ( expression )
( type-id ) cast-expression
4
#
Expressions of the following form are value-dependent:
sizeof ... ( identifier )
fold-expression
5
#
An expression of the form &qualified-id where the qualified-id names a dependent member of the current instantiation is value-dependent.
An expression of the form &cast-expression is also value-dependent if evaluating cast-expression as a core constant expression succeeds and the result of the evaluation refers to a templated entity that is an object with static or thread storage duration or a member function.

13.8.3.5 Dependent template arguments [temp.dep.temp]

1
#
A type template-argument is dependent if the type it specifies is dependent.
2
#
A non-type template-argument is dependent if its type is dependent or the constant expression it specifies is value-dependent.
3
#
Furthermore, a non-type template-argument is dependent if the corresponding non-type template-parameter is of reference or pointer type and the template-argument designates or points to a member of the current instantiation or a member of a dependent type.
4
#
A template template-argument is dependent if it names a template-parameter or is a qualified-id that refers to a member of an unknown specialization.

13.8.4 Non-dependent names [temp.nondep]

1
#
Non-dependent names used in a template definition are found using the usual name lookup and bound at the point they are used.
[Example 1: void g(double); void h(); template<class T> class Z { public: void f() { g(1); // calls g(double) h++; // ill-formed: cannot increment function; this could be diagnosed // either here or at the point of instantiation } }; void g(int); // not in scope at the point of the template definition, not considered for the call g(1) — end example]

13.8.5 Dependent name resolution [temp.dep.res]

13.8.5.1 Point of instantiation [temp.point]

1
#
For a function template specialization, a member function template specialization, or a specialization for a member function or static data member of a class template, if the specialization is implicitly instantiated because it is referenced from within another template specialization and the context from which it is referenced depends on a template parameter, the point of instantiation of the specialization is the point of instantiation of the enclosing specialization.
Otherwise, the point of instantiation for such a specialization immediately follows the namespace scope declaration or definition that refers to the specialization.
2
#
If a function template or member function of a class template is called in a way which uses the definition of a default argument of that function template or member function, the point of instantiation of the default argument is the point of instantiation of the function template or member function specialization.
3
#
For a noexcept-specifier of a function template specialization or specialization of a member function of a class template, if the noexcept-specifier is implicitly instantiated because it is needed by another template specialization and the context that requires it depends on a template parameter, the point of instantiation of the noexcept-specifier is the point of instantiation of the specialization that requires it.
Otherwise, the point of instantiation for such a noexcept-specifier immediately follows the namespace scope declaration or definition that requires the noexcept-specifier.
4
#
For a class template specialization, a class member template specialization, or a specialization for a class member of a class template, if the specialization is implicitly instantiated because it is referenced from within another template specialization, if the context from which the specialization is referenced depends on a template parameter, and if the specialization is not instantiated previous to the instantiation of the enclosing template, the point of instantiation is immediately before the point of instantiation of the enclosing template.
Otherwise, the point of instantiation for such a specialization immediately precedes the namespace scope declaration or definition that refers to the specialization.
5
#
If a virtual function is implicitly instantiated, its point of instantiation is immediately following the point of instantiation of its enclosing class template specialization.
6
#
An explicit instantiation definition is an instantiation point for the specialization or specializations specified by the explicit instantiation.
7
#
A specialization for a function template, a member function template, or of a member function or static data member of a class template may have multiple points of instantiations within a translation unit, and in addition to the points of instantiation described above,
A specialization for a class template has at most one point of instantiation within a translation unit.
A specialization for any template may have points of instantiation in multiple translation units.
If two different points of instantiation give a template specialization different meanings according to the one-definition rule, the program is ill-formed, no diagnostic required.

13.8.5.2 Candidate functions [temp.dep.candidate]

1
#
For a function call where the postfix-expression is a dependent name, the candidate functions are found using the usual lookup rules from the template definition context ([basic.lookup.unqual], [basic.lookup.argdep]).
[Note 1:
For the part of the lookup using associated namespaces ([basic.lookup.argdep]), function declarations found in the template instantiation context are found by this lookup, as described in [basic.lookup.argdep].
— end note]
If the call would be ill-formed or would find a better match had the lookup within the associated namespaces considered all the function declarations with external linkage introduced in those namespaces in all translation units, not just considering those declarations found in the template definition and template instantiation contexts, then the program has undefined behavior.
2
#
[Example 1:

Source file "X.h":namespace Q { struct X { }; }

Source file "G.h":namespace Q { void g_impl(X, X); }

Module interface unit of M1:module; #include "X.h" #include "G.h" export module M1; export template<typename T> void g(T t) { g_impl(t, Q::X{ }); // ADL in definition context finds Q​::​g_­impl, g_­impl not discarded }

Module interface unit of M2:module; #include "X.h" export module M2; import M1; void h(Q::X x) { g(x); // OK } — end example]

3
#
[Example 2:

Module interface unit of Std:export module Std; export template<typename Iter> void indirect_swap(Iter lhs, Iter rhs) { swap(*lhs, *rhs); // swap not found by unqualified lookup, can be found only via ADL }

Module interface unit of M:export module M; import Std; struct S { /* ...*/ }; void swap(S&, S&); // #1 void f(S* p, S* q) { indirect_swap(p, q); // finds #1 via ADL in instantiation context } — end example]

4
#
[Example 3:

Source file "X.h":struct X { /* ... */ }; X operator+(X, X);

Module interface unit of F:export module F; export template<typename T> void f(T t) { t + t; }

Module interface unit of M:module; #include "X.h" export module M; import F; void g(X x) { f(x); // OK: instantiates f from F, // operator+ is visible in instantiation context } — end example]

5
#
[Example 4:

Module interface unit of A:export module A; export template<typename T> void f(T t) { cat(t, t); // #1 dog(t, t); // #2 }

Module interface unit of B:export module B; import A; export template<typename T, typename U> void g(T t, U u) { f(t); }

Source file "foo.h", not an importable header:struct foo { friend int cat(foo, foo); }; int dog(foo, foo);

Module interface unit of C1:module; #include "foo.h" // dog not referenced, discarded export module C1; import B; export template<typename T> void h(T t) { g(foo{ }, t); }

Translation unit:import C1; void i() { h(0); // error: dog not found at #2 }

Importable header "bar.h":struct bar { friend int cat(bar, bar); }; int dog(bar, bar);

Module interface unit of C2:module; #include "bar.h" // imports header unit "bar.h" export module C2; import B; export template<typename T> void j(T t) { g(bar{ }, t); }

Translation unit:import C2; void k() { j(0); // OK, dog found in instantiation context: // visible at end of module interface unit of C2 } — end example]

13.8.6 Friend names declared within a class template [temp.inject]

1
#
Friend classes or functions can be declared within a class template.
When a template is instantiated, the names of its friends are treated as if the specialization had been explicitly declared at its point of instantiation.
2
#
As with non-template classes, the names of namespace-scope friend functions of a class template specialization are not visible during an ordinary lookup unless explicitly declared at namespace scope ([class.friend]).
Such names may be found under the rules for associated classes ([basic.lookup.argdep]).142
[Example 1: template<typename T> struct number { number(int); friend number gcd(number x, number y) { return 0; }; }; void g() { number<double> a(3), b(4); a = gcd(a,b); // finds gcd because number<double> is an associated class, // making gcd visible in its namespace (global scope) b = gcd(3,4); // error: gcd is not visible } — end example]
142)
Friend declarations do not introduce new names into any scope, either when the template is declared or when it is instantiated.
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