[expr.prim.lambda] - C++14 → C++17

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@@ -1,119 +1,99 @@
 ### Lambda expressions <a id="expr.prim.lambda">[[expr.prim.lambda]]</a>
-Lambda expressions provide a concise way to create simple function
-objects.
-``` cpp
-#include <algorithm>
-#include <cmath>
-void abssort(float* x, unsigned N) {
-  std::sort(x, x + N,
-    [](float a, float b) {
-      return std::abs(a) < std::abs(b);
-    });
-}
-```
 ``` bnf
 lambda-expression:
     lambda-introducer lambda-declaratorₒₚₜ compound-statement
 ```
 ``` bnf
 lambda-introducer:
     '[' lambda-captureₒₚₜ ']'
 ```
-``` bnf
-lambda-capture:
-    capture-default
-    capture-list
-    capture-default ',' capture-list
-```
-``` bnf
-capture-default:
-    '&'
-    '='
-```
-``` bnf
-capture-list:
-    capture '...'ₒₚₜ
-    capture-list ',' capture '...'ₒₚₜ
-```
-``` bnf
-capture:
-    simple-capture
-    init-capture
-```
-``` bnf
-simple-capture:
-    identifier
-    '&' identifier
-    'this'
-```
-``` bnf
-init-capture:
-    identifier initializer
-    '&' identifier initializer
-```
 ``` bnf
 lambda-declarator:
-    '(' parameter-declaration-clause ')' 'mutable'ₒₚₜ
- exception-specificationₒₚₜ attribute-specifier-seqₒₚₜ trailing-return-typeₒₚₜ
 ```
-The evaluation of a *lambda-expression* results in a prvalue temporary (
-[[class.temporary]]). This temporary is called the *closure object*. A
-*lambda-expression* shall not appear in an unevaluated operand (Clause
-[[expr]]), in a *template-argument*, in an *alias-declaration*, in a
-typedef declaration, or in the declaration of a function or function
-template outside its function body and default arguments. The intention
-is to prevent lambdas from appearing in a signature. A closure object
-behaves like a function object ([[function.objects]]).
-The type of the *lambda-expression* (which is also the type of the
-closure object) is a unique, unnamed non-union class type — called the
-*closure type* — whose properties are described below. This class type
-is neither an aggregate ([[dcl.init.aggr]]) nor a literal type (
-[[basic.types]]). The closure type is declared in the smallest block
-scope, class scope, or namespace scope that contains the corresponding
-*lambda-expression*. This determines the set of namespaces and classes
-associated with the closure type ([[basic.lookup.argdep]]). The
-parameter types of a *lambda-declarator* do not affect these associated
-namespaces and classes. An implementation may define the closure type
-differently from what is described below provided this does not alter
-the observable behavior of the program other than by changing:
 - the size and/or alignment of the closure type,
 - whether the closure type is trivially copyable (Clause  [[class]]),
 - whether the closure type is a standard-layout class (Clause
   [[class]]), or
 - whether the closure type is a POD class (Clause  [[class]]).
 An implementation shall not add members of rvalue reference type to the
 closure type.
-If a *lambda-expression* does not include a *lambda-declarator*, it is
-as if the *lambda-declarator* were `()`. The lambda return type is
-`auto`, which is replaced by the *trailing-return-type* if provided
-and/or deduced from `return` statements as described in
-[[dcl.spec.auto]].
-``` cpp
-auto x1 = [](int i){ return i; };     // OK: return type is int
-auto x2 = []{ return { 1, 2 }; };     // error: deducing return type from braced-init-list
-int j;
-auto x3 = []()->auto&& { return j; }; // OK: return type is int&
-```
 The closure type for a non-generic *lambda-expression* has a public
 inline function call operator ([[over.call]]) whose parameters and
 return type are described by the *lambda-expression*’s
 *parameter-declaration-clause* and *trailing-return-type* respectively.
 For a generic lambda, the closure type has a public inline function call
@@ -127,13 +107,16 @@ of the function call operator template are derived from the
 *lambda-expression*'s *trailing-return-type* and
 *parameter-declaration-clause* by replacing each occurrence of `auto` in
 the *decl-specifier*s of the *parameter-declaration-clause* with the
 name of the corresponding invented *template-parameter*.
 ``` cpp
 auto glambda = [](auto a, auto&& b) { return a < b; };
 bool b = glambda(3, 3.14);                             // OK
 auto vglambda = [](auto printer) {
   return [=](auto&& ... ts) {                          // OK: ts is a function parameter pack
     printer(std::forward<decltype(ts)>(ts)...);
     return [=]() {
@@ -145,42 +128,99 @@ auto glambda = [](auto a, auto&& b) { return a < b; };
                    { std::cout << v1 << v2 << v3; } );
 auto q = p(1, 'a', 3.14);                              // OK: outputs 1a3.14
 q();                                                   // OK: outputs 1a3.14
 ```
-This function call operator or operator template is declared `const` (
 [[class.mfct.non-static]]) if and only if the *lambda-expression*’s
 *parameter-declaration-clause* is not followed by `mutable`. It is
-neither virtual nor declared `volatile`. Any *exception-specification*
 specified on a *lambda-expression* applies to the corresponding function
 call operator or operator template. An *attribute-specifier-seq* in a
 *lambda-declarator* appertains to the type of the corresponding function
-call operator or operator template. Names referenced in the
-*lambda-declarator* are looked up in the context in which the
-*lambda-expression* appears.
 The closure type for a non-generic *lambda-expression* with no
-*lambda-capture* has a public non-virtual non-explicit const conversion
-function to pointer to function with C++language linkage ([[dcl.link]])
-having the same parameter and return types as the closure type’s
-function call operator. The value returned by this conversion function
-shall be the address of a function that, when invoked, has the same
-effect as invoking the closure type’s function call operator. For a
-generic lambda with no *lambda-capture*, the closure type has a public
-non-virtual non-explicit const conversion function template to pointer
-to function. The conversion function template has the same invented
 *template-parameter-list*, and the pointer to function has the same
 parameter types, as the function call operator template. The return type
 of the pointer to function shall behave as if it were a
 *decltype-specifier* denoting the return type of the corresponding
-function call operator template specialization. If the generic lambda
-has no *trailing-return-type* or the *trailing-return-type* contains a
-placeholder type, return type deduction of the corresponding function
-call operator template specialization has to be done. The corresponding
-specialization is that instantiation of the function call operator
-template with the same template arguments as those deduced for the
-conversion function template. Consider the following:
 ``` cpp
 auto glambda = [](auto a) { return a; };
 int (*fp)(int) = glambda;
 ```
@@ -202,10 +242,14 @@ struct Closure {
   template<class T> operator fptr_t<T>() const
     { return &lambda_call_operator_invoker; }
 };
 ```
 ``` cpp
 void f1(int (*)(int))   { }
 void f2(char (*)(int))  { }
 void g(int (*)(int))    { }  // #1
@@ -220,33 +264,63 @@ f2(glambda);  // error: ID is not convertible
 g(glambda);   // error: ambiguous
 h(glambda);   // OK: calls #3 since it is convertible from ID
 int& (*fpi)(int*) = [](auto* a) -> auto& { return *a; }; // OK
 ```
 The value returned by any given specialization of this conversion
-function template shall be the address of a function that, when invoked,
 has the same effect as invoking the generic lambda’s corresponding
-function call operator template specialization. This will result in the
-implicit instantiation of the generic lambda’s body. The instantiated
-generic lambda’s return type and parameter types shall match the return
-type and parameter types of the pointer to function.
 ``` cpp
 auto GL = [](auto a) { std::cout << a; return a; };
 int (*GL_int)(int) = GL;  // OK: through conversion function template
 GL_int(3);                // OK: same as GL(3)
 ```
 The *lambda-expression*’s *compound-statement* yields the
 *function-body* ([[dcl.fct.def]]) of the function call operator, but
 for purposes of name lookup ([[basic.lookup]]), determining the type
 and value of `this` ([[class.this]]) and transforming *id-expression*s
 referring to non-static class members into class member access
 expressions using `(*this)` ([[class.mfct.non-static]]), the
 *compound-statement* is considered in the context of the
 *lambda-expression*.
 ``` cpp
 struct S1 {
   int x, y;
   int operator()(int);
   void f() {
@@ -256,46 +330,135 @@ struct S1 {
     };
   }
 };
 ```
 Further, a variable `__func__` is implicitly defined at the beginning of
 the *compound-statement* of the *lambda-expression*, with semantics as
 described in  [[dcl.fct.def.general]].
 If a *lambda-capture* includes a *capture-default* that is `&`, no
 identifier in a *simple-capture* of that *lambda-capture* shall be
 preceded by `&`. If a *lambda-capture* includes a *capture-default* that
 is `=`, each *simple-capture* of that *lambda-capture* shall be of the
-form “`&` *identifier*”. Ignoring appearances in *initializer*s of
-*init-capture*s, an identifier or `this` shall not appear more than once
-in a *lambda-capture*.
 ``` cpp
 struct S2 { void f(int i); };
 void S2::f(int i) {
   [&, i]{ };        // OK
   [&, &i]{ };       // error: i preceded by & when & is the default
   [=, this]{ };     // error: this when = is the default
   [i, i]{ };        // error: i repeated
 }
 ```
 A *lambda-expression* whose smallest enclosing scope is a block scope (
 [[basic.scope.block]]) is a *local lambda expression*; any other
 *lambda-expression* shall not have a *capture-default* or
 *simple-capture* in its *lambda-introducer*. The *reaching scope* of a
 local lambda expression is the set of enclosing scopes up to and
-including the innermost enclosing function and its parameters. This
-reaching scope includes any intervening *lambda-expression*s.
 The *identifier* in a *simple-capture* is looked up using the usual
 rules for unqualified name lookup ([[basic.lookup.unqual]]); each such
 lookup shall find an entity. An entity that is designated by a
 *simple-capture* is said to be *explicitly captured*, and shall be
-`this` or a variable with automatic storage duration declared in the
-reaching scope of the local lambda expression.
 An *init-capture* behaves as if it declares and explicitly captures a
 variable of the form “`auto` *init-capture* `;`” whose declarative
 region is the *lambda-expression*’s *compound-statement*, except that:
@@ -305,34 +468,45 @@ region is the *lambda-expression*’s *compound-statement*, except that:
   non-static data member, and no additional copy and destruction is
   performed, and
 - if the capture is by reference, the variable’s lifetime ends when the
   closure object’s lifetime ends.
-This enables an *init-capture* like “`x = std::move(x)`”; the second
-“`x`” must bind to a declaration in the surrounding context.
 ``` cpp
 int x = 4;
 auto y = [&r = x, x = x+1]()->int {
             r += 2;
             return x+2;
          }();  // Updates ::x to 6, and initializes y to 7.
 ```
 A *lambda-expression* with an associated *capture-default* that does not
-explicitly capture `this` or a variable with automatic storage duration
 (this excludes any *id-expression* that has been found to refer to an
 *init-capture*'s associated non-static data member), is said to
-*implicitly capture* the entity (i.e., `this` or a variable) if the
 *compound-statement*:
-- odr-uses ([[basic.def.odr]]) the entity, or
 - names the entity in a potentially-evaluated expression (
   [[basic.def.odr]]) where the enclosing full-expression depends on a
   generic lambda parameter declared within the reaching scope of the
   *lambda-expression*.
 ``` cpp
 void f(int, const int (&)[2] = {})    { }   // #1
 void f(const int&, const int (&)[1])  { }   // #2
 void test() {
   const int x = 17;
@@ -345,63 +519,85 @@ void test() {
     f(x, selector);             // OK: is a dependent expression, so captures x
   };
 }
 ```
 All such implicitly captured entities shall be declared within the
-reaching scope of the lambda expression. The implicit capture of an
-entity by a nested *lambda-expression* can cause its implicit capture by
-the containing *lambda-expression* (see below). Implicit odr-uses of
-`this` can result in implicit capture.
 An entity is *captured* if it is captured explicitly or implicitly. An
 entity captured by a *lambda-expression* is odr-used (
 [[basic.def.odr]]) in the scope containing the *lambda-expression*. If
-`this` is captured by a local lambda expression, its nearest enclosing
 function shall be a non-static member function. If a *lambda-expression*
 or an instantiation of the function call operator template of a generic
 lambda odr-uses ([[basic.def.odr]]) `this` or a variable with automatic
 storage duration from its reaching scope, that entity shall be captured
 by the *lambda-expression*. If a *lambda-expression* captures an entity
 and that entity is not defined or captured in the immediately enclosing
 lambda expression or function, the program is ill-formed.
 ``` cpp
 void f1(int i) {
   int const N = 20;
   auto m1 = [=]{
     int const M = 30;
     auto m2 = [i]{
       int x[N][M];              // OK: N and M are not odr-used
-      x[0][0] = i;              // OK: i is explicitly captured by m2
-                                // and implicitly captured by m1
     };
   };
   struct s1 {
     int f;
     void work(int n) {
       int m = n*n;
       int j = 40;
       auto m3 = [this,m] {
         auto m4 = [&,j] {       // error: j not captured by m3
-          int x = n;            // error: n implicitly captured by m4
- // but not captured by m3
-          x += m;               // OK: m implicitly captured by m4
-                                // and explicitly captured by m3
           x += i;               // error: i is outside of the reaching scope
-          x += f;               // OK: this captured implicitly by m4
-                                // and explicitly by m3
         };
       };
     }
   };
 }
 ```
 A *lambda-expression* appearing in a default argument shall not
 implicitly or explicitly capture any entity.
 ``` cpp
 void f2() {
   int i = 1;
   void g1(int = ([i]{ return i; })());          // ill-formed
   void g2(int = ([i]{ return 0; })());          // ill-formed
@@ -409,38 +605,105 @@ void f2() {
   void g4(int = ([=]{ return 0; })());          // OK
   void g5(int = ([]{ return sizeof i; })());    // OK
 }
 ```
-An entity is *captured by copy* if it is implicitly captured and the
-*capture-default* is `=` or if it is explicitly captured with a capture
-that is not of the form `&` *identifier* or `&` *identifier*
-*initializer*. For each entity captured by copy, an unnamed non-static
-data member is declared in the closure type. The declaration order of
-these members is unspecified. The type of such a data member is the type
-of the corresponding captured entity if the entity is not a reference to
-an object, or the referenced type otherwise. If the captured entity is a
-reference to a function, the corresponding data member is also a
-reference to a function. A member of an anonymous union shall not be
-captured by copy.
 An entity is *captured by reference* if it is implicitly or explicitly
 captured but not captured by copy. It is unspecified whether additional
 unnamed non-static data members are declared in the closure type for
-entities captured by reference. A member of an anonymous union shall not
-be captured by reference.
 If a *lambda-expression* `m2` captures an entity and that entity is
 captured by an immediately enclosing *lambda-expression* `m1`, then
 `m2`’s capture is transformed as follows:
 - if `m1` captures the entity by copy, `m2` captures the corresponding
   non-static data member of `m1`’s closure type;
 - if `m1` captures the entity by reference, `m2` captures the same
   entity captured by `m1`.
-the nested lambda expressions and invocations below will output
 `123234`.
 ``` cpp
 int a = 1, b = 1, c = 1;
 auto m1 = [a, &b, &c]() mutable {
@@ -454,86 +717,63 @@ auto m1 = [a, &b, &c]() mutable {
 a = 2; b = 2; c = 2;
 m1();
 std::cout << a << b << c;
 ```
-Every *id-expression* within the *compound-statement* of a
-*lambda-expression* that is an odr-use ([[basic.def.odr]]) of an entity
-captured by copy is transformed into an access to the corresponding
-unnamed data member of the closure type. An *id-expression* that is not
-an odr-use refers to the original entity, never to a member of the
-closure type. Furthermore, such an *id-expression* does not cause the
-implicit capture of the entity. If `this` is captured, each odr-use of
-`this` is transformed into an access to the corresponding unnamed data
-member of the closure type, cast ([[expr.cast]]) to the type of `this`.
-The cast ensures that the transformed expression is a prvalue.
-``` cpp
-void f(const int*);
-void g() {
-  const int N = 10;
-  [=] {
-    int arr[N];             // OK: not an odr-use, refers to automatic variable
-    f(&N);                  // OK: causes N to be captured; &N points to the
-                            // corresponding member of the closure type
-  };
-}
-```
 Every occurrence of `decltype((x))` where `x` is a possibly
 parenthesized *id-expression* that names an entity of automatic storage
 duration is treated as if `x` were transformed into an access to a
 corresponding data member of the closure type that would have been
 declared if `x` were an odr-use of the denoted entity.
 ``` cpp
 void f3() {
   float x, &r = x;
   [=] {                     // x and r are not captured (appearance in a decltype operand is not an odr-use)
     decltype(x) y1;         // y1 has type float
-    decltype((x)) y2 = y1;  // y2 has type float const& because this lambda
-                            // is not mutable and x is an lvalue
     decltype(r) r1 = y1;    // r1 has type float& (transformation not considered)
     decltype((r)) r2 = y2;  // r2 has type float const&
   };
 }
 ```
-The closure type associated with a *lambda-expression* has a deleted (
-[[dcl.fct.def.delete]]) default constructor and a deleted copy
-assignment operator. It has an implicitly-declared copy constructor (
-[[class.copy]]) and may have an implicitly-declared move constructor (
-[[class.copy]]). The copy/move constructor is implicitly defined in the
-same way as any other implicitly declared copy/move constructor would be
-implicitly defined.
-The closure type associated with a *lambda-expression* has an
-implicitly-declared destructor ([[class.dtor]]).
 When the *lambda-expression* is evaluated, the entities that are
 captured by copy are used to direct-initialize each corresponding
 non-static data member of the resulting closure object, and the
 non-static data members corresponding to the *init-capture*s are
 initialized as indicated by the corresponding *initializer* (which may
 be copy- or direct-initialization). (For array members, the array
 elements are direct-initialized in increasing subscript order.) These
 initializations are performed in the (unspecified) order in which the
-non-static data members are declared. This ensures that the destructions
-will occur in the reverse order of the constructions.
-If an entity is implicitly or explicitly captured by reference, invoking
-the function call operator of the corresponding *lambda-expression*
-after the lifetime of the entity has ended is likely to result in
-undefined behavior.
 A *simple-capture* followed by an ellipsis is a pack expansion (
 [[temp.variadic]]). An *init-capture* followed by an ellipsis is
 ill-formed.
 ``` cpp
 template<class... Args>
 void f(Args... args) {
   auto lm = [&, args...] { return g(args...); };
   lm();
 }
 ```

 ### Lambda expressions <a id="expr.prim.lambda">[[expr.prim.lambda]]</a>
 ``` bnf
 lambda-expression:
     lambda-introducer lambda-declaratorₒₚₜ compound-statement
 ```
 ``` bnf
 lambda-introducer:
     '[' lambda-captureₒₚₜ ']'
 ```
 ``` bnf
 lambda-declarator:
+    '(' parameter-declaration-clause ')' decl-specifier-seqₒₚₜ
+ noexcept-specifierₒₚₜ attribute-specifier-seqₒₚₜ trailing-return-typeₒₚₜ
 ```
+Lambda expressions provide a concise way to create simple function
+objects.
+[*Example 1*:
+``` cpp
+#include <algorithm>
+#include <cmath>
+void abssort(float* x, unsigned N) {
+  std::sort(x, x + N, [](float a, float b) { return std::abs(a) < std::abs(b); });
+}
+```
+— *end example*]
+A *lambda-expression* is a prvalue whose result object is called the
+*closure object*. A *lambda-expression* shall not appear in an
+unevaluated operand (Clause  [[expr]]), in a *template-argument*, in an
+*alias-declaration*, in a typedef declaration, or in the declaration of
+a function or function template outside its function body and default
+arguments.
+[*Note 1*: The intention is to prevent lambdas from appearing in a
+signature. — *end note*]
+[*Note 2*: A closure object behaves like a function object (
+[[function.objects]]). — *end note*]
+In the *decl-specifier-seq* of the *lambda-declarator*, each
+*decl-specifier* shall either be `mutable` or `constexpr`.
+If a *lambda-expression* does not include a *lambda-declarator*, it is
+as if the *lambda-declarator* were `()`. The lambda return type is
+`auto`, which is replaced by the type specified by the
+*trailing-return-type* if provided and/or deduced from `return`
+statements as described in  [[dcl.spec.auto]].
+[*Example 2*:
+``` cpp
+auto x1 = [](int i){ return i; };     // OK: return type is int
+auto x2 = []{ return { 1, 2 }; };     // error: deducing return type from braced-init-list
+int j;
+auto x3 = []()->auto&& { return j; }; // OK: return type is int&
+```
+— *end example*]
+#### Closure types <a id="expr.prim.lambda.closure">[[expr.prim.lambda.closure]]</a>
+The type of a *lambda-expression* (which is also the type of the closure
+object) is a unique, unnamed non-union class type, called the *closure
+type*, whose properties are described below.
+The closure type is declared in the smallest block scope, class scope,
+or namespace scope that contains the corresponding *lambda-expression*.
+[*Note 1*: This determines the set of namespaces and classes associated
+with the closure type ([[basic.lookup.argdep]]). The parameter types of
+a *lambda-declarator* do not affect these associated namespaces and
+classes. — *end note*]
+The closure type is not an aggregate type ([[dcl.init.aggr]]). An
+implementation may define the closure type differently from what is
+described below provided this does not alter the observable behavior of
+the program other than by changing:
 - the size and/or alignment of the closure type,
 - whether the closure type is trivially copyable (Clause  [[class]]),
 - whether the closure type is a standard-layout class (Clause
   [[class]]), or
 - whether the closure type is a POD class (Clause  [[class]]).
 An implementation shall not add members of rvalue reference type to the
 closure type.
 The closure type for a non-generic *lambda-expression* has a public
 inline function call operator ([[over.call]]) whose parameters and
 return type are described by the *lambda-expression*’s
 *parameter-declaration-clause* and *trailing-return-type* respectively.
 For a generic lambda, the closure type has a public inline function call
 *lambda-expression*'s *trailing-return-type* and
 *parameter-declaration-clause* by replacing each occurrence of `auto` in
 the *decl-specifier*s of the *parameter-declaration-clause* with the
 name of the corresponding invented *template-parameter*.
+[*Example 1*:
 ``` cpp
 auto glambda = [](auto a, auto&& b) { return a < b; };
 bool b = glambda(3, 3.14);                             // OK
 auto vglambda = [](auto printer) {
   return [=](auto&& ... ts) {                          // OK: ts is a function parameter pack
     printer(std::forward<decltype(ts)>(ts)...);
     return [=]() {
                    { std::cout << v1 << v2 << v3; } );
 auto q = p(1, 'a', 3.14);                              // OK: outputs 1a3.14
 q();                                                   // OK: outputs 1a3.14
 ```
+— *end example*]
+The function call operator or operator template is declared `const` (
 [[class.mfct.non-static]]) if and only if the *lambda-expression*’s
 *parameter-declaration-clause* is not followed by `mutable`. It is
+neither virtual nor declared `volatile`. Any *noexcept-specifier*
 specified on a *lambda-expression* applies to the corresponding function
 call operator or operator template. An *attribute-specifier-seq* in a
 *lambda-declarator* appertains to the type of the corresponding function
+call operator or operator template. The function call operator or any
+given operator template specialization is a constexpr function if either
+the corresponding *lambda-expression*'s *parameter-declaration-clause*
+is followed by `constexpr`, or it satisfies the requirements for a
+constexpr function ([[dcl.constexpr]]).
+[*Note 2*: Names referenced in the *lambda-declarator* are looked up in
+the context in which the *lambda-expression* appears. — *end note*]
+[*Example 2*:
+``` cpp
+auto ID = [](auto a) { return a; };
+static_assert(ID(3) == 3); // OK
+struct NonLiteral {
+  NonLiteral(int n) : n(n) { }
+  int n;
+};
+static_assert(ID(NonLiteral{3}).n == 3); // ill-formed
+```
+— *end example*]
+[*Example 3*:
+``` cpp
+auto monoid = [](auto v) { return [=] { return v; }; };
+auto add = [](auto m1) constexpr {
+  auto ret = m1();
+  return [=](auto m2) mutable {
+    auto m1val = m1();
+    auto plus = [=](auto m2val) mutable constexpr
+                   { return m1val += m2val; };
+    ret = plus(m2());
+    return monoid(ret);
+  };
+};
+constexpr auto zero = monoid(0);
+constexpr auto one = monoid(1);
+static_assert(add(one)(zero)() == one()); // OK
+// Since two below is not declared constexpr, an evaluation of its constexpr member function call operator
+// cannot perform an lvalue-to-rvalue conversion on one of its subobjects (that represents its capture)
+// in a constant expression.
+auto two = monoid(2);
+assert(two() == 2); // OK, not a constant expression.
+static_assert(add(one)(one)() == two()); // ill-formed: two() is not a constant expression
+static_assert(add(one)(one)() == monoid(2)()); // OK
+```
+— *end example*]
 The closure type for a non-generic *lambda-expression* with no
+*lambda-capture* has a conversion function to pointer to function with
+C++language linkage ([[dcl.link]]) having the same parameter and return
+types as the closure type’s function call operator. The conversion is to
+“pointer to `noexcept` function” if the function call operator has a
+non-throwing exception specification. The value returned by this
+conversion function is the address of a function `F` that, when invoked,
+has the same effect as invoking the closure type’s function call
+operator. `F` is a constexpr function if the function call operator is a
+constexpr function. For a generic lambda with no *lambda-capture*, the
+closure type has a conversion function template to pointer to function.
+The conversion function template has the same invented
 *template-parameter-list*, and the pointer to function has the same
 parameter types, as the function call operator template. The return type
 of the pointer to function shall behave as if it were a
 *decltype-specifier* denoting the return type of the corresponding
+function call operator template specialization.
+[*Note 3*:
+If the generic lambda has no *trailing-return-type* or the
+*trailing-return-type* contains a placeholder type, return type
+deduction of the corresponding function call operator template
+specialization has to be done. The corresponding specialization is that
+instantiation of the function call operator template with the same
+template arguments as those deduced for the conversion function
+template. Consider the following:
 ``` cpp
 auto glambda = [](auto a) { return a; };
 int (*fp)(int) = glambda;
 ```
   template<class T> operator fptr_t<T>() const
     { return &lambda_call_operator_invoker; }
 };
 ```
+— *end note*]
+[*Example 4*:
 ``` cpp
 void f1(int (*)(int))   { }
 void f2(char (*)(int))  { }
 void g(int (*)(int))    { }  // #1
 g(glambda);   // error: ambiguous
 h(glambda);   // OK: calls #3 since it is convertible from ID
 int& (*fpi)(int*) = [](auto* a) -> auto& { return *a; }; // OK
 ```
+— *end example*]
 The value returned by any given specialization of this conversion
+function template is the address of a function `F` that, when invoked,
 has the same effect as invoking the generic lambda’s corresponding
+function call operator template specialization. `F` is a constexpr
+function if the corresponding specialization is a constexpr function.
+[*Note 4*: This will result in the implicit instantiation of the
+generic lambda’s body. The instantiated generic lambda’s return type and
+parameter types shall match the return type and parameter types of the
+pointer to function. — *end note*]
+[*Example 5*:
 ``` cpp
 auto GL = [](auto a) { std::cout << a; return a; };
 int (*GL_int)(int) = GL;  // OK: through conversion function template
 GL_int(3);                // OK: same as GL(3)
 ```
+— *end example*]
+The conversion function or conversion function template is public,
+constexpr, non-virtual, non-explicit, const, and has a non-throwing
+exception specification ([[except.spec]]).
+[*Example 6*:
+``` cpp
+auto Fwd = [](int (*fp)(int), auto a) { return fp(a); };
+auto C = [](auto a) { return a; };
+static_assert(Fwd(C,3) == 3); // OK
+// No specialization of the function call operator template can be constexpr (due to the local static).
+auto NC = [](auto a) { static int s; return a; };
+static_assert(Fwd(NC,3) == 3); // ill-formed
+```
+— *end example*]
 The *lambda-expression*’s *compound-statement* yields the
 *function-body* ([[dcl.fct.def]]) of the function call operator, but
 for purposes of name lookup ([[basic.lookup]]), determining the type
 and value of `this` ([[class.this]]) and transforming *id-expression*s
 referring to non-static class members into class member access
 expressions using `(*this)` ([[class.mfct.non-static]]), the
 *compound-statement* is considered in the context of the
 *lambda-expression*.
+[*Example 7*:
 ``` cpp
 struct S1 {
   int x, y;
   int operator()(int);
   void f() {
     };
   }
 };
 ```
+— *end example*]
 Further, a variable `__func__` is implicitly defined at the beginning of
 the *compound-statement* of the *lambda-expression*, with semantics as
 described in  [[dcl.fct.def.general]].
+The closure type associated with a *lambda-expression* has no default
+constructor and a deleted copy assignment operator. It has a defaulted
+copy constructor and a defaulted move constructor ([[class.copy]]).
+[*Note 5*: These special member functions are implicitly defined as
+usual, and might therefore be defined as deleted. — *end note*]
+The closure type associated with a *lambda-expression* has an
+implicitly-declared destructor ([[class.dtor]]).
+A member of a closure type shall not be explicitly instantiated (
+[[temp.explicit]]), explicitly specialized ([[temp.expl.spec]]), or
+named in a `friend` declaration ([[class.friend]]).
+#### Captures <a id="expr.prim.lambda.capture">[[expr.prim.lambda.capture]]</a>
+``` bnf
+lambda-capture:
+    capture-default
+    capture-list
+    capture-default ',' capture-list
+```
+``` bnf
+capture-default:
+    '&'
+    '='
+```
+``` bnf
+capture-list:
+    capture '...'ₒₚₜ
+    capture-list ',' capture '...'ₒₚₜ
+```
+``` bnf
+capture:
+    simple-capture
+    init-capture
+```
+``` bnf
+simple-capture:
+    identifier
+    '&' identifier
+    'this'
+    '* this'
+```
+``` bnf
+init-capture:
+    identifier initializer
+    '&' identifier initializer
+```
+The body of a *lambda-expression* may refer to variables with automatic
+storage duration and the `*this` object (if any) of enclosing block
+scopes by capturing those entities, as described below.
 If a *lambda-capture* includes a *capture-default* that is `&`, no
 identifier in a *simple-capture* of that *lambda-capture* shall be
 preceded by `&`. If a *lambda-capture* includes a *capture-default* that
 is `=`, each *simple-capture* of that *lambda-capture* shall be of the
+form “`&` *identifier*” or “`* this`”.
+[*Note 1*: The form `[&,this]` is redundant but accepted for
+compatibility with ISO C++14. — *end note*]
+Ignoring appearances in *initializer*s of *init-capture*s, an identifier
+or `this` shall not appear more than once in a *lambda-capture*.
+[*Example 1*:
 ``` cpp
 struct S2 { void f(int i); };
 void S2::f(int i) {
   [&, i]{ };        // OK
   [&, &i]{ };       // error: i preceded by & when & is the default
+  [=, *this]{ };    // OK
   [=, this]{ };     // error: this when = is the default
   [i, i]{ };        // error: i repeated
+  [this, *this]{ }; // error: this appears twice
 }
 ```
+— *end example*]
 A *lambda-expression* whose smallest enclosing scope is a block scope (
 [[basic.scope.block]]) is a *local lambda expression*; any other
 *lambda-expression* shall not have a *capture-default* or
 *simple-capture* in its *lambda-introducer*. The *reaching scope* of a
 local lambda expression is the set of enclosing scopes up to and
+including the innermost enclosing function and its parameters.
+[*Note 2*: This reaching scope includes any intervening
+*lambda-expression*s. — *end note*]
 The *identifier* in a *simple-capture* is looked up using the usual
 rules for unqualified name lookup ([[basic.lookup.unqual]]); each such
 lookup shall find an entity. An entity that is designated by a
 *simple-capture* is said to be *explicitly captured*, and shall be
+`*this` (when the *simple-capture* is “`this`” or “`* this`”) or a
+variable with automatic storage duration declared in the reaching scope
+of the local lambda expression.
+If an *identifier* in a *simple-capture* appears as the *declarator-id*
+of a parameter of the *lambda-declarator*'s
+*parameter-declaration-clause*, the program is ill-formed.
+[*Example 2*:
+``` cpp
+void f() {
+  int x = 0;
+  auto g = [x](int x) { return 0; }    // error: parameter and simple-capture have the same name
+}
+```
+— *end example*]
 An *init-capture* behaves as if it declares and explicitly captures a
 variable of the form “`auto` *init-capture* `;`” whose declarative
 region is the *lambda-expression*’s *compound-statement*, except that:
   non-static data member, and no additional copy and destruction is
   performed, and
 - if the capture is by reference, the variable’s lifetime ends when the
   closure object’s lifetime ends.
+[*Note 3*: This enables an *init-capture* like “`x = std::move(x)`”;
+the second “`x`” must bind to a declaration in the surrounding
+context. — *end note*]
+[*Example 3*:
 ``` cpp
 int x = 4;
 auto y = [&r = x, x = x+1]()->int {
             r += 2;
             return x+2;
          }();  // Updates ::x to 6, and initializes y to 7.
+auto z = [a = 42](int a) { return 1; } // error: parameter and local variable have the same name
 ```
+— *end example*]
 A *lambda-expression* with an associated *capture-default* that does not
+explicitly capture `*this` or a variable with automatic storage duration
 (this excludes any *id-expression* that has been found to refer to an
 *init-capture*'s associated non-static data member), is said to
+*implicitly capture* the entity (i.e., `*this` or a variable) if the
 *compound-statement*:
+- odr-uses ([[basic.def.odr]]) the entity (in the case of a variable),
+- odr-uses ([[basic.def.odr]]) `this` (in the case of the object
+  designated by `*this`), or
 - names the entity in a potentially-evaluated expression (
   [[basic.def.odr]]) where the enclosing full-expression depends on a
   generic lambda parameter declared within the reaching scope of the
   *lambda-expression*.
+[*Example 4*:
 ``` cpp
 void f(int, const int (&)[2] = {})    { }   // #1
 void f(const int&, const int (&)[1])  { }   // #2
 void test() {
   const int x = 17;
     f(x, selector);             // OK: is a dependent expression, so captures x
   };
 }
 ```
+— *end example*]
 All such implicitly captured entities shall be declared within the
+reaching scope of the lambda expression.
+[*Note 4*: The implicit capture of an entity by a nested
+*lambda-expression* can cause its implicit capture by the containing
+*lambda-expression* (see below). Implicit odr-uses of `this` can result
+in implicit capture. — *end note*]
 An entity is *captured* if it is captured explicitly or implicitly. An
 entity captured by a *lambda-expression* is odr-used (
 [[basic.def.odr]]) in the scope containing the *lambda-expression*. If
+`*this` is captured by a local lambda expression, its nearest enclosing
 function shall be a non-static member function. If a *lambda-expression*
 or an instantiation of the function call operator template of a generic
 lambda odr-uses ([[basic.def.odr]]) `this` or a variable with automatic
 storage duration from its reaching scope, that entity shall be captured
 by the *lambda-expression*. If a *lambda-expression* captures an entity
 and that entity is not defined or captured in the immediately enclosing
 lambda expression or function, the program is ill-formed.
+[*Example 5*:
 ``` cpp
 void f1(int i) {
   int const N = 20;
   auto m1 = [=]{
     int const M = 30;
     auto m2 = [i]{
       int x[N][M];              // OK: N and M are not odr-used
+      x[0][0] = i;              // OK: i is explicitly captured by m2 and implicitly captured by m1
     };
   };
   struct s1 {
     int f;
     void work(int n) {
       int m = n*n;
       int j = 40;
       auto m3 = [this,m] {
         auto m4 = [&,j] {       // error: j not captured by m3
+          int x = n;            // error: n implicitly captured by m4 but not captured by m3
+          x += m;               // OK: m implicitly captured by m4 and explicitly captured by m3
           x += i;               // error: i is outside of the reaching scope
+          x += f;               // OK: this captured implicitly by m4 and explicitly by m3
         };
       };
     }
   };
 }
+struct s2 {
+  double ohseven = .007;
+  auto f() {
+    return [this] {
+      return [*this] {
+          return ohseven;       // OK
+      }
+    }();
+  }
+  auto g() {
+    return [] {
+      return [*this] { };       // error: *this not captured by outer lambda-expression
+    }();
+  }
+};
 ```
+— *end example*]
 A *lambda-expression* appearing in a default argument shall not
 implicitly or explicitly capture any entity.
+[*Example 6*:
 ``` cpp
 void f2() {
   int i = 1;
   void g1(int = ([i]{ return i; })());          // ill-formed
   void g2(int = ([i]{ return 0; })());          // ill-formed
   void g4(int = ([=]{ return 0; })());          // OK
   void g5(int = ([]{ return sizeof i; })());    // OK
 }
 ```
+— *end example*]
+An entity is *captured by copy* if
+- it is implicitly captured, the *capture-default* is `=`, and the
+  captured entity is not `*this`, or
+- it is explicitly captured with a capture that is not of the form
+  `this`, `&` *identifier*, or `&` *identifier* *initializer*.
+For each entity captured by copy, an unnamed non-static data member is
+declared in the closure type. The declaration order of these members is
+unspecified. The type of such a data member is the referenced type if
+the entity is a reference to an object, an lvalue reference to the
+referenced function type if the entity is a reference to a function, or
+the type of the corresponding captured entity otherwise. A member of an
+anonymous union shall not be captured by copy.
+Every *id-expression* within the *compound-statement* of a
+*lambda-expression* that is an odr-use ([[basic.def.odr]]) of an entity
+captured by copy is transformed into an access to the corresponding
+unnamed data member of the closure type.
+[*Note 5*: An *id-expression* that is not an odr-use refers to the
+original entity, never to a member of the closure type. Furthermore,
+such an *id-expression* does not cause the implicit capture of the
+entity. — *end note*]
+If `*this` is captured by copy, each odr-use of `this` is transformed
+into a pointer to the corresponding unnamed data member of the closure
+type, cast ([[expr.cast]]) to the type of `this`.
+[*Note 6*: The cast ensures that the transformed expression is a
+prvalue. — *end note*]
+An *id-expression* within the *compound-statement* of a
+*lambda-expression* that is an odr-use of a reference captured by
+reference refers to the entity to which the captured reference is bound
+and not to the captured reference.
+[*Note 7*: The validity of such captures is determined by the lifetime
+of the object to which the reference refers, not by the lifetime of the
+reference itself. — *end note*]
+[*Example 7*:
+``` cpp
+void f(const int*);
+void g() {
+  const int N = 10;
+  [=] {
+    int arr[N];     // OK: not an odr-use, refers to automatic variable
+    f(&N);          // OK: causes N to be captured; &N points to
+                    // the corresponding member of the closure type
+  };
+}
+auto h(int &r) {
+  return [&] {
+    ++r;            // Valid after h returns if the lifetime of the
+                    // object to which r is bound has not ended
+  };
+}
+```
+— *end example*]
 An entity is *captured by reference* if it is implicitly or explicitly
 captured but not captured by copy. It is unspecified whether additional
 unnamed non-static data members are declared in the closure type for
+entities captured by reference. If declared, such non-static data
+members shall be of literal type.
+[*Example 8*:
+``` cpp
+// The inner closure type must be a literal type regardless of how reference captures are represented.
+static_assert([](int n) { return [&n] { return ++n; }(); }(3) == 4);
+```
+— *end example*]
+A bit-field or a member of an anonymous union shall not be captured by
+reference.
 If a *lambda-expression* `m2` captures an entity and that entity is
 captured by an immediately enclosing *lambda-expression* `m1`, then
 `m2`’s capture is transformed as follows:
 - if `m1` captures the entity by copy, `m2` captures the corresponding
   non-static data member of `m1`’s closure type;
 - if `m1` captures the entity by reference, `m2` captures the same
   entity captured by `m1`.
+[*Example 9*:
+The nested lambda expressions and invocations below will output
 `123234`.
 ``` cpp
 int a = 1, b = 1, c = 1;
 auto m1 = [a, &b, &c]() mutable {
 a = 2; b = 2; c = 2;
 m1();
 std::cout << a << b << c;
 ```
+— *end example*]
 Every occurrence of `decltype((x))` where `x` is a possibly
 parenthesized *id-expression* that names an entity of automatic storage
 duration is treated as if `x` were transformed into an access to a
 corresponding data member of the closure type that would have been
 declared if `x` were an odr-use of the denoted entity.
+[*Example 10*:
 ``` cpp
 void f3() {
   float x, &r = x;
   [=] {                     // x and r are not captured (appearance in a decltype operand is not an odr-use)
     decltype(x) y1;         // y1 has type float
+    decltype((x)) y2 = y1;  // y2 has type float const& because this lambda is not mutable and x is an lvalue
     decltype(r) r1 = y1;    // r1 has type float& (transformation not considered)
     decltype((r)) r2 = y2;  // r2 has type float const&
   };
 }
 ```
+— *end example*]
 When the *lambda-expression* is evaluated, the entities that are
 captured by copy are used to direct-initialize each corresponding
 non-static data member of the resulting closure object, and the
 non-static data members corresponding to the *init-capture*s are
 initialized as indicated by the corresponding *initializer* (which may
 be copy- or direct-initialization). (For array members, the array
 elements are direct-initialized in increasing subscript order.) These
 initializations are performed in the (unspecified) order in which the
+non-static data members are declared.
+[*Note 8*: This ensures that the destructions will occur in the reverse
+order of the constructions. — *end note*]
+[*Note 9*: If a non-reference entity is implicitly or explicitly
+captured by reference, invoking the function call operator of the
+corresponding *lambda-expression* after the lifetime of the entity has
+ended is likely to result in undefined behavior. — *end note*]
 A *simple-capture* followed by an ellipsis is a pack expansion (
 [[temp.variadic]]). An *init-capture* followed by an ellipsis is
 ill-formed.
+[*Example 11*:
 ``` cpp
 template<class... Args>
 void f(Args... args) {
   auto lm = [&, args...] { return g(args...); };
   lm();
 }
 ```
+— *end example*]

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