[temp.inst] - C++17 → C++20

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@@ -1,15 +1,30 @@
 ### Implicit instantiation <a id="temp.inst">[[temp.inst]]</a>
-Unless a class template specialization has been explicitly
-instantiated ([[temp.explicit]]) or explicitly specialized (
-[[temp.expl.spec]]), the class template specialization is implicitly
-instantiated when the specialization is referenced in a context that
-requires a completely-defined object type or when the completeness of
-the class type affects the semantics of the program.
-[*Note 1*: In particular, if the semantics of an expression depend on
 the member or base class lists of a class template specialization, the
 class template specialization is implicitly generated. For instance,
 deleting a pointer to class type depends on whether or not the class
 declares a destructor, and a conversion between pointers to class type
 depends on the inheritance relationship between the two classes
@@ -32,45 +47,66 @@ void g(D<int>* p, D<char>* pp, D<double>* ppp) {
 ```
 — *end example*]
 If a class template has been declared, but not defined, at the point of
-instantiation ([[temp.point]]), the instantiation yields an incomplete
-class type ([[basic.types]]).
 [*Example 2*:
 ``` cpp
 template<class T> class X;
 X<char> ch;         // error: incomplete type X<char>
 ```
 — *end example*]
-[*Note 2*: Within a template declaration, a local class (
-[[class.local]]) or enumeration and the members of a local class are
-never considered to be entities that can be separately instantiated
-(this includes their default arguments, *noexcept-specifier*s, and
-non-static data member initializers, if any). As a result, the dependent
-names are looked up, the semantic constraints are checked, and any
-templates used are instantiated as part of the instantiation of the
-entity within which the local class or enumeration is
-declared. — *end note*]
-The implicit instantiation of a class template specialization causes the
-implicit instantiation of the declarations, but not of the definitions,
-default arguments, or *noexcept-specifier*s of the class member
-functions, member classes, scoped member enumerations, static data
-members, member templates, and friends; and it causes the implicit
-instantiation of the definitions of unscoped member enumerations and
-member anonymous unions. However, for the purpose of determining whether
-an instantiated redeclaration is valid according to  [[basic.def.odr]]
-and [[class.mem]], a declaration that corresponds to a definition in the
-template is considered to be a definition.
 [*Example 3*:
 ``` cpp
 template<class T, class U>
 struct Outer {
   template<class X, class Y> struct Inner;
   template<class Y> struct Inner<T, Y>;         // #1a
@@ -90,38 +126,47 @@ same partial specialization.
 ``` cpp
 template<typename T> struct Friendly {
   template<typename U> friend int f(U) { return sizeof(T); }
 };
 Friendly<char> fc;
-Friendly<float> ff;                             // ill-formed: produces second definition of f(U)
 ```
 — *end example*]
-Unless a member of a class template or a member template has been
-explicitly instantiated or explicitly specialized, the specialization of
-the member is implicitly instantiated when the specialization is
-referenced in a context that requires the member definition to exist; in
 particular, the initialization (and any associated side effects) of a
 static data member does not occur unless the static data member is
 itself used in a way that requires the definition of the static data
 member to exist.
-Unless a function template specialization has been explicitly
-instantiated or explicitly specialized, the function template
-specialization is implicitly instantiated when the specialization is
-referenced in a context that requires a function definition to exist. A
-function whose declaration was instantiated from a friend function
-definition is implicitly instantiated when it is referenced in a context
-that requires a function definition to exist. Unless a call is to a
-function template explicit specialization or to a member function of an
-explicitly specialized class template, a default argument for a function
-template or a member function of a class template is implicitly
-instantiated when the function is called in a context that requires the
-value of the default argument.
-[*Example 4*:
 ``` cpp
 template<class T> struct Z {
   void f();
   void g();
@@ -141,22 +186,46 @@ void h() {
 Nothing in this example requires `class` `Z<double>`, `Z<int>::g()`, or
 `Z<char>::f()` to be implicitly instantiated.
 — *end example*]
-Unless a variable template specialization has been explicitly
-instantiated or explicitly specialized, the variable template
-specialization is implicitly instantiated when the specialization is
-used. A default template argument for a variable template is implicitly
-instantiated when the variable template is referenced in a context that
-requires the value of the default argument.
-If the function selected by overload resolution ([[over.match]]) can be
 determined without instantiating a class template definition, it is
 unspecified whether that instantiation actually takes place.
-[*Example 5*:
 ``` cpp
 template <class T> struct S {
   operator int();
 };
@@ -173,34 +242,41 @@ void g(S<int>& sr) {
 — *end example*]
 If a function template or a member function template specialization is
 used in a way that involves overload resolution, a declaration of the
-specialization is implicitly instantiated ([[temp.over]]).
 An implementation shall not implicitly instantiate a function template,
 a variable template, a member template, a non-virtual member function, a
 member class, a static data member of a class template, or a
-substatement of a constexpr if statement ([[stmt.if]]), unless such
-instantiation is required. It is unspecified whether or not an
-implementation implicitly instantiates a virtual member function of a
-class template if the virtual member function would not otherwise be
-instantiated. The use of a template specialization in a default argument
-shall not cause the template to be implicitly instantiated except that a
-class template may be instantiated where its complete type is needed to
-determine the correctness of the default argument. The use of a default
-argument in a function call causes specializations in the default
-argument to be implicitly instantiated.
 Implicitly instantiated class, function, and variable template
 specializations are placed in the namespace where the template is
 defined. Implicitly instantiated specializations for members of a class
 template are placed in the namespace where the enclosing class template
 is defined. Implicitly instantiated member templates are placed in the
 namespace where the enclosing class or class template is defined.
-[*Example 6*:
 ``` cpp
 namespace N {
   template<class T> class List {
   public:
@@ -217,11 +293,11 @@ public:
 void g(Map<const char*,int>& m) {
   int i = m.get("Nicholas");
 }
 ```
-a call of `lt.get()` from `Map<const char*,int>::get()` would place
 `List<int>::get()` in the namespace `N` rather than in the global
 namespace.
 — *end example*]
@@ -230,19 +306,19 @@ argument to be used, the dependent names are looked up, the semantics
 constraints are checked, and the instantiation of any template used in
 the default argument is done as if the default argument had been an
 initializer used in a function template specialization with the same
 scope, the same template parameters and the same access as that of the
 function template `f` used at that point, except that the scope in which
-a closure type is declared ([[expr.prim.lambda.closure]]) – and
-therefore its associated namespaces – remain as determined from the
-context of the definition for the default argument. This analysis is
-called *default argument instantiation*. The instantiated default
-argument is then used as the argument of `f`.
 Each default argument is instantiated independently.
-[*Example 7*:
 ``` cpp
 template<class T> void f(T x, T y = ydef(T()), T z = zdef(T()));
 class  A { };
@@ -250,34 +326,34 @@ class  A { };
 A zdef(A);
 void g(A a, A b, A c) {
   f(a, b, c);       // no default argument instantiation
   f(a, b);          // default argument z = zdef(T()) instantiated
-  f(a);             // ill-formed; ydef is not declared
 }
 ```
 — *end example*]
 The *noexcept-specifier* of a function template specialization is not
 instantiated along with the function declaration; it is instantiated
-when needed ([[except.spec]]). If such an *noexcept-specifier* is
-needed but has not yet been instantiated, the dependent names are looked
-up, the semantics constraints are checked, and the instantiation of any
 template used in the *noexcept-specifier* is done as if it were being
 done as part of instantiating the declaration of the specialization at
 that point.
-[*Note 3*:  [[temp.point]] defines the point of instantiation of a
 template specialization. — *end note*]
 There is an *implementation-defined* quantity that specifies the limit
-on the total depth of recursive instantiations ([[implimits]]), which
 could involve more than one template. The result of an infinite
 recursion in instantiation is undefined.
-[*Example 8*:
 ``` cpp
 template<class T> class X {
   X<T>* p;          // OK
   X<T*> a;          // implicit generation of X<T> requires
@@ -286,5 +362,63 @@ template<class T> class X {
 };
 ```
 — *end example*]

 ### Implicit instantiation <a id="temp.inst">[[temp.inst]]</a>
+A template specialization E is a *declared specialization* if there is a
+reachable explicit instantiation definition [[temp.explicit]] or
+explicit specialization declaration [[temp.expl.spec]] for E, or if
+there is a reachable explicit instantiation declaration for E and E is
+not
+- an inline function,
+- declared with a type deduced from its initializer or return value
+  [[dcl.spec.auto]],
+- a potentially-constant variable [[expr.const]], or
+- a specialization of a templated class.
+[*Note 1*: An implicit instantiation in an importing translation unit
+cannot use names with internal linkage from an imported translation unit
+[[basic.link]]. — *end note*]
+Unless a class template specialization is a declared specialization, the
+class template specialization is implicitly instantiated when the
+specialization is referenced in a context that requires a
+completely-defined object type or when the completeness of the class
+type affects the semantics of the program.
+[*Note 2*: In particular, if the semantics of an expression depend on
 the member or base class lists of a class template specialization, the
 class template specialization is implicitly generated. For instance,
 deleting a pointer to class type depends on whether or not the class
 declares a destructor, and a conversion between pointers to class type
 depends on the inheritance relationship between the two classes
 ```
 — *end example*]
 If a class template has been declared, but not defined, at the point of
+instantiation [[temp.point]], the instantiation yields an incomplete
+class type [[basic.types]].
 [*Example 2*:
 ``` cpp
 template<class T> class X;
 X<char> ch;         // error: incomplete type X<char>
 ```
 — *end example*]
+[*Note 3*: Within a template declaration, a local class [[class.local]]
+or enumeration and the members of a local class are never considered to
+be entities that can be separately instantiated (this includes their
+default arguments, *noexcept-specifier*s, and non-static data member
+initializers, if any, but not their *type-constraint*s or
+*requires-clause*s). As a result, the dependent names are looked up, the
+semantic constraints are checked, and any templates used are
+instantiated as part of the instantiation of the entity within which the
+local class or enumeration is declared. — *end note*]
+The implicit instantiation of a class template specialization causes
+- the implicit instantiation of the declarations, but not of the
+  definitions, of the non-deleted class member functions, member
+  classes, scoped member enumerations, static data members, member
+  templates, and friends; and
+- the implicit instantiation of the definitions of deleted member
+  functions, unscoped member enumerations, and member anonymous unions.
+The implicit instantiation of a class template specialization does not
+cause the implicit instantiation of default arguments or
+*noexcept-specifier*s of the class member functions.
 [*Example 3*:
+``` cpp
+template<class T>
+struct C {
+  void f() { T x; }
+  void g() = delete;
+};
+C<void> c;                      // OK, definition of C<void>::f is not instantiated at this point
+template<> void C<int>::g() { } // error: redefinition of C<int>::g
+```
+— *end example*]
+However, for the purpose of determining whether an instantiated
+redeclaration is valid according to  [[basic.def.odr]] and
+[[class.mem]], a declaration that corresponds to a definition in the
+template is considered to be a definition.
+[*Example 4*:
 ``` cpp
 template<class T, class U>
 struct Outer {
   template<class X, class Y> struct Inner;
   template<class Y> struct Inner<T, Y>;         // #1a
 ``` cpp
 template<typename T> struct Friendly {
   template<typename U> friend int f(U) { return sizeof(T); }
 };
 Friendly<char> fc;
+Friendly<float> ff;                             // error: produces second definition of f(U)
 ```
 — *end example*]
+Unless a member of a class template or a member template is a declared
+specialization, the specialization of the member is implicitly
+instantiated when the specialization is referenced in a context that
+requires the member definition to exist or if the existence of the
+definition of the member affects the semantics of the program; in
 particular, the initialization (and any associated side effects) of a
 static data member does not occur unless the static data member is
 itself used in a way that requires the definition of the static data
 member to exist.
+Unless a function template specialization is a declared specialization,
+the function template specialization is implicitly instantiated when the
+specialization is referenced in a context that requires a function
+definition to exist or if the existence of the definition affects the
+semantics of the program. A function whose declaration was instantiated
+from a friend function definition is implicitly instantiated when it is
+referenced in a context that requires a function definition to exist or
+if the existence of the definition affects the semantics of the program.
+Unless a call is to a function template explicit specialization or to a
+member function of an explicitly specialized class template, a default
+argument for a function template or a member function of a class
+template is implicitly instantiated when the function is called in a
+context that requires the value of the default argument.
+[*Note 4*: An inline function that is the subject of an explicit
+instantiation declaration is not a declared specialization; the intent
+is that it still be implicitly instantiated when odr-used
+[[basic.def.odr]] so that the body can be considered for inlining, but
+that no out-of-line copy of it be generated in the translation
+unit. — *end note*]
+[*Example 5*:
 ``` cpp
 template<class T> struct Z {
   void f();
   void g();
 Nothing in this example requires `class` `Z<double>`, `Z<int>::g()`, or
 `Z<char>::f()` to be implicitly instantiated.
 — *end example*]
+Unless a variable template specialization is a declared specialization,
+the variable template specialization is implicitly instantiated when it
+is referenced in a context that requires a variable definition to exist
+or if the existence of the definition affects the semantics of the
+program. A default template argument for a variable template is
+implicitly instantiated when the variable template is referenced in a
+context that requires the value of the default argument.
+The existence of a definition of a variable or function is considered to
+affect the semantics of the program if the variable or function is
+needed for constant evaluation by an expression [[expr.const]], even if
+constant evaluation of the expression is not required or if constant
+expression evaluation does not use the definition.
+[*Example 6*:
+``` cpp
+template<typename T> constexpr int f() { return T::value; }
+template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
+template<bool B, typename T> void g(...);
+template<bool B, typename T> void h(decltype(int{B ? f<T>() : 0}));
+template<bool B, typename T> void h(...);
+void x() {
+  g<false, int>(0); // OK, B ? f<T>() :\ 0 is not potentially constant evaluated
+  h<false, int>(0); // error, instantiates f<int> even though B evaluates to false and
+                    // list-initialization of int from int cannot be narrowing
+}
+```
+— *end example*]
+If the function selected by overload resolution [[over.match]] can be
 determined without instantiating a class template definition, it is
 unspecified whether that instantiation actually takes place.
+[*Example 7*:
 ``` cpp
 template <class T> struct S {
   operator int();
 };
 — *end example*]
 If a function template or a member function template specialization is
 used in a way that involves overload resolution, a declaration of the
+specialization is implicitly instantiated [[temp.over]].
 An implementation shall not implicitly instantiate a function template,
 a variable template, a member template, a non-virtual member function, a
 member class, a static data member of a class template, or a
+substatement of a constexpr if statement [[stmt.if]], unless such
+instantiation is required.
+[*Note 5*: The instantiation of a generic lambda does not require
+instantiation of substatements of a constexpr if statement within its
+*compound-statement* unless the call operator template is
+instantiated. — *end note*]
+It is unspecified whether or not an implementation implicitly
+instantiates a virtual member function of a class template if the
+virtual member function would not otherwise be instantiated. The use of
+a template specialization in a default argument shall not cause the
+template to be implicitly instantiated except that a class template may
+be instantiated where its complete type is needed to determine the
+correctness of the default argument. The use of a default argument in a
+function call causes specializations in the default argument to be
+implicitly instantiated.
 Implicitly instantiated class, function, and variable template
 specializations are placed in the namespace where the template is
 defined. Implicitly instantiated specializations for members of a class
 template are placed in the namespace where the enclosing class template
 is defined. Implicitly instantiated member templates are placed in the
 namespace where the enclosing class or class template is defined.
+[*Example 8*:
 ``` cpp
 namespace N {
   template<class T> class List {
   public:
 void g(Map<const char*,int>& m) {
   int i = m.get("Nicholas");
 }
 ```
+A call of `lt.get()` from `Map<const char*,int>::get()` would place
 `List<int>::get()` in the namespace `N` rather than in the global
 namespace.
 — *end example*]
 constraints are checked, and the instantiation of any template used in
 the default argument is done as if the default argument had been an
 initializer used in a function template specialization with the same
 scope, the same template parameters and the same access as that of the
 function template `f` used at that point, except that the scope in which
+a closure type is declared [[expr.prim.lambda.closure]] – and therefore
+its associated namespaces – remain as determined from the context of the
+definition for the default argument. This analysis is called *default
+argument instantiation*. The instantiated default argument is then used
+as the argument of `f`.
 Each default argument is instantiated independently.
+[*Example 9*:
 ``` cpp
 template<class T> void f(T x, T y = ydef(T()), T z = zdef(T()));
 class  A { };
 A zdef(A);
 void g(A a, A b, A c) {
   f(a, b, c);       // no default argument instantiation
   f(a, b);          // default argument z = zdef(T()) instantiated
+  f(a);             // error: ydef is not declared
 }
 ```
 — *end example*]
 The *noexcept-specifier* of a function template specialization is not
 instantiated along with the function declaration; it is instantiated
+when needed [[except.spec]]. If such an *noexcept-specifier* is needed
+but has not yet been instantiated, the dependent names are looked up,
+the semantics constraints are checked, and the instantiation of any
 template used in the *noexcept-specifier* is done as if it were being
 done as part of instantiating the declaration of the specialization at
 that point.
+[*Note 6*:  [[temp.point]] defines the point of instantiation of a
 template specialization. — *end note*]
 There is an *implementation-defined* quantity that specifies the limit
+on the total depth of recursive instantiations [[implimits]], which
 could involve more than one template. The result of an infinite
 recursion in instantiation is undefined.
+[*Example 10*:
 ``` cpp
 template<class T> class X {
   X<T>* p;          // OK
   X<T*> a;          // implicit generation of X<T> requires
 };
 ```
 — *end example*]
+The *type-constraint*s and *requires-clause* of a template
+specialization or member function are not instantiated along with the
+specialization or function itself, even for a member function of a local
+class; substitution into the atomic constraints formed from them is
+instead performed as specified in [[temp.constr.decl]] and
+[[temp.constr.atomic]] when determining whether the constraints are
+satisfied or as specified in [[temp.constr.decl]] when comparing
+declarations.
+[*Note 7*: The satisfaction of constraints is determined during
+template argument deduction [[temp.deduct]] and overload resolution
+[[over.match]]. — *end note*]
+[*Example 11*:
+``` cpp
+template<typename T> concept C = sizeof(T) > 2;
+template<typename T> concept D = C<T> && sizeof(T) > 4;
+template<typename T> struct S {
+  S() requires C<T> { }         // #1
+  S() requires D<T> { }         // #2
+};
+S<char> s1;                     // error: no matching constructor
+S<char[8]> s2;                  // OK, calls #2
+```
+When `S<char>` is instantiated, both constructors are part of the
+specialization. Their constraints are not satisfied, and they suppress
+the implicit declaration of a default constructor for `S<char>`
+[[class.default.ctor]], so there is no viable constructor for `s1`.
+— *end example*]
+[*Example 12*:
+``` cpp
+template<typename T> struct S1 {
+  template<typename U>
+    requires false
+  struct Inner1;                // ill-formed, no diagnostic required
+};
+template<typename T> struct S2 {
+  template<typename U>
+    requires (sizeof(T[-(int)sizeof(T)]) > 1)
+  struct Inner2;                // ill-formed, no diagnostic required
+};
+```
+The class `S1<T>::Inner1` is ill-formed, no diagnostic required, because
+it has no valid specializations. `S2` is ill-formed, no diagnostic
+required, since no substitution into the constraints of its `Inner2`
+template would result in a valid expression.
+— *end example*]

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