13 Templates [temp]

13.8 Name resolution [temp.res]

13.8.1 General [temp.res.general]

1
#
A name that appears in a declaration D of a template T is looked up from where it appears in an unspecified declaration of T that either is D itself or is reachable from D and from which no other declaration of T that contains the usage of the name is reachable.
If the name is dependent (as specified in [temp.dep]), it is looked up for each specialization (after substitution) because the lookup depends on a template parameter.
[Note 1: 
Some dependent names are also looked up during parsing to determine that they are dependent or to interpret following < tokens.
Uses of other names might be type-dependent or value-dependent ([temp.dep.expr], [temp.dep.constexpr]).
A using-declarator is never dependent in a specialization and is therefore replaced during lookup for that specialization ([basic.lookup]).
— end note]
[Example 1: struct A { operator int(); }; template<class B, class T> struct D : B { T get() { return operator T(); } // conversion-function-id is dependent }; int f(D<A, int> d) { return d.get(); } // OK, lookup finds A​::​operator int — end example]
[Example 2: 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]
[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]
2
#
If the validity or meaning of the program would be changed by considering a default argument or default template argument introduced in a declaration that is reachable from the point of instantiation of a specialization ([temp.point]) but is not found by lookup for the specialization, the program is ill-formed, no diagnostic required.
3
#
The component names of a typename-specifier are its identifier (if any) and those of its nested-name-specifier and simple-template-id (if any).
A typename-specifier denotes the type or class template denoted by the simple-type-specifier ([dcl.type.simple]) formed by omitting the keyword typename.
[Note 2: 
The usual qualified name lookup ([basic.lookup.qual]) applies even in the presence of typename.
— end note]
[Example 4: 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 or unqualified name is said to be in a type-only context if it is the terminal name of
[Example 5: 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]
5
#
A qualified-id whose terminal name is dependent and that is in a type-only context is considered to denote a type.
A name that refers to a using-declarator whose terminal name is dependent is interpreted as a typedef-name if the using-declarator uses the keyword typename.
[Example 6: 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]
6
#
The validity of a templated entity may be checked prior to any instantiation.
[Note 3: 
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
  • (6.1)
    no valid specialization, ignoring static_assert-declarations that fail ([dcl.pre]), can be generated for a templated entity or a substatement of a constexpr if statement ([stmt.if]) within a templated entity and the innermost enclosing template is not instantiated, or
  • (6.2)
    no valid specialization, ignoring static_assert-declarations that fail, can be generated for a default template-argument and the default template-argument is not used in any instantiation, or
  • (6.3)
    no specialization of an alias template ([temp.alias]) is valid and no specialization of the alias template is named in the program, or
  • (6.4)
    any constraint-expression in the program, introduced or otherwise, has (in its normal form) an atomic constraint A where no satisfaction check of A could be well-formed and no satisfaction check of A is performed, or
  • (6.5)
    every valid specialization of a variadic template requires an empty template parameter pack, or
  • (6.6)
    a hypothetical instantiation of a templated entity immediately following its definition would be ill-formed due to a construct (other than a static_assert-declaration that fails) that does not depend on a template parameter, or
  • (6.7)
    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 templated entity.
[Note 4: 
This can happen in situations including the following:
  • (6.8)
    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
  • (6.9)
    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
  • (6.10)
    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
  • (6.11)
    constant expression evaluation within the template instantiation uses
    • (6.11.1)
      the value of a const object of integral or unscoped enumeration type or
    • (6.11.2)
      the value of a constexpr object or
    • (6.11.3)
      the value of a reference or
    • (6.11.4)
      the definition of a constexpr function,
    and that entity was not defined when the template was defined, or
  • (6.12)
    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]
[Note 5: 
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 7: 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 X<T>::g(t); // OK X<T>::h(); // 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]
7
#
[Note 6: 
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]
119)119)
This includes friend function declarations.

13.8.2 Locally declared names [temp.local]

1
#
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 argument list ([temp.decls.general], [temp.arg.general]) of the class template enclosed in <>.
2
#
When the injected-class-name of a class template specialization or partial specialization 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
#
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 named.
[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::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
#
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
#
The name of a template-parameter shall not be bound to any following declaration whose locus is contained by the scope to which the template-parameter belongs.
[Example 5: template<class T, int i> class Y { int T; // error: template-parameter hidden void f() { char T; // error: template-parameter hidden } friend void T(); // OK, no name bound }; template<class X> class X; // error: hidden by template-parameter — end example]
7
#
Unqualified name lookup considers the template parameter scope of a template-declaration immediately after the outermost scope associated with the template declared (even if its parent scope does not contain the template-parameter-list).
[Note 1: 
The scope of a class template, including its non-dependent base classes ([temp.dep.type], [class.member.lookup]), is searched before its template parameter scope.
— end note]
[Example 6: struct B { }; namespace N { typedef void V; template<class T> struct A : B { typedef void C; void f(); template<class U> void g(U); }; } template<class V> void N::A<V>::f() { // N​::​V not considered here V v; // V is still the template parameter, not N​::​V } template<class B> template<class C> void N::A<B>::g(C) { B b; // B is the base class, not the template parameter C c; // C is the template parameter, not A's C } — 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
#
A dependent call is an expression, possibly formed as a non-member candidate for an operator ([over.match.oper]), of the form:
postfix-expression ( expression-list )
where the postfix-expression is an unqualified-id and
The component name of an unqualified-id ([expr.prim.id.unqual]) is dependent if
[Note 1: 
Such names are looked up only 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]

13.8.3.2 Dependent types [temp.dep.type]

1
#
A name or template-id 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 its template-head ([temp.arg]) 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 class template partial specialization or a member of a class template partial specialization, the name of the class template followed by a template argument list equivalent to that of the partial specialization ([temp.spec.partial]) enclosed in <> (or an equivalent template alias specialization).
2
#
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
  • (2.1)
    it has the same type as the template parameter (ignoring cv-qualification) and
  • (2.2)
    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]
3
#
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]
4
#
A qualified ([basic.lookup.qual]) or unqualified name is a member of the current instantiation if
  • (4.1)
    its lookup context, if it is a qualified name, is the current instantiation, and
  • (4.2)
    lookup for it finds any member of a class that is the current instantiation
[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 qualified or unqualified name names 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 declaration (including a using-declarator whose terminal name is dependent) of a class that is the current instantiation.
5
#
A qualified name ([basic.lookup.qual]) is dependent if
[Example 4: struct A { using B = int; A f(); }; struct C : A {}; template<class T> void g(T t) { decltype(t.A::f())::B i; // error: typename needed to interpret B as a type } template void g(C); // …even though A is ​::​A here — end example]
6
#
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 name, the name 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 [expr.prim.id.general] — end example]
7
#
An initializer is dependent if any constituent expression ([intro.execution]) of the initializer is type-dependent.
A placeholder type ([dcl.spec.auto.general]) is dependent if it designates a type deduced from a dependent initializer.
8
#
A placeholder for a deduced class type ([dcl.type.class.deduct]) is dependent if
9
#
[Example 6: template<class T, class V> struct S { S(T); }; template<class U> struct A { template<class T> using X = S<T, U>; template<class T> using Y = S<T, int>; void f() { new X(1); // dependent new Y(1); // not dependent } }; — end example]
10
#
A type is dependent if it is
  • (10.1)
    a template parameter,
  • (10.2)
    denoted by a dependent (qualified) name,
  • (10.3)
    a nested class or enumeration that is a direct member of a class that is the current instantiation,
  • (10.4)
    a cv-qualified type where the cv-unqualified type is dependent,
  • (10.5)
    a compound type constructed from any dependent type,
  • (10.6)
    an array type whose element type is dependent or whose bound (if any) is value-dependent,
  • (10.7)
    a function type whose parameters include one or more function parameter packs,
  • (10.8)
    a function type whose exception specification is value-dependent,
  • (10.9)
    denoted by a dependent placeholder type,
  • (10.10)
    denoted by a dependent placeholder for a deduced class type,
  • (10.11)
    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,121
  • (10.12)
    a pack-index-specifier, or
  • (10.13)
    denoted by decltype(expression), where expression is type-dependent.
11
#
[Note 3: 
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]
120)120)
Every instantiation of a class template declares a different set of assignment operators.
121)121)
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 current class ([expr.prim.this]) is dependent ([temp.dep.type]).
3
#
An id-expression is type-dependent if it is a template-id that is not a concept-id and is dependent; or if its terminal name is 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, typename-specifier, or new-type-id is dependent, even if any subexpression is type-dependent:
simple-type-specifier ( expression-list )
simple-type-specifier braced-init-list
typename-specifier ( expression-list )
typename-specifier braced-init-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 )
requires-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 terminal name of its id-expression, if any, is dependent or the expression refers to a member of the current instantiation and the type of the referenced member is dependent.
[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.
— 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.
8
#
A pack-index-expression is type-dependent if its id-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-parameter is dependent if it names a template-parameter or its terminal name is dependent.

13.8.4 Dependent name resolution [temp.dep.res]

13.8.4.1 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.

13.8.4.2 Candidate functions [temp.dep.candidate]

1
#
If a dependent call ([temp.dep]) would be ill-formed or would find a better match had the lookup for its dependent name considered all the function declarations with external linkage introduced in the associated namespaces in all translation units, not just considering those declarations found in the template definition and template instantiation contexts ([basic.lookup.argdep]), then the program is ill-formed, no diagnostic required.
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]