[expr.prim.lambda] - C++20 → C++23

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@@ -1,22 +1,42 @@
 ### Lambda expressions <a id="expr.prim.lambda">[[expr.prim.lambda]]</a>
 ``` bnf
 lambda-expression:
-    lambda-introducer lambda-declaratorₒₚₜ compound-statement
-    lambda-introducer '<' template-parameter-list '>' requires-clauseₒₚₜ 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 requires-clauseₒₚₜ
 ```
 A *lambda-expression* provides a concise way to create a simple function
 object.
@@ -36,29 +56,46 @@ A *lambda-expression* is a prvalue whose result object is called the
 *closure object*.
 [*Note 1*: 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 be one of `mutable`, `constexpr`, or `consteval`.
-[*Note 2*: The trailing *requires-clause* is described in
 [[dcl.decl]]. — *end note*]
-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*]
 A lambda is a *generic lambda* if the *lambda-expression* has any
@@ -66,12 +103,12 @@ generic parameter type placeholders [[dcl.spec.auto]], or if the lambda
 has a *template-parameter-list*.
 [*Example 3*:
 ``` cpp
-int i = [](int i, auto a) { return i; }(3, 4);                  // OK: a generic lambda
-int j = []<class T>(T t, int i) { return i; }(3, 4);            // OK: a generic lambda
 ```
 — *end example*]
 #### Closure types <a id="expr.prim.lambda.closure">[[expr.prim.lambda.closure]]</a>
@@ -110,56 +147,90 @@ and whose *template-parameter-list* consists of the specified
 call operator template is the *requires-clause* immediately following
 `<` *template-parameter-list* `>`, if any. The trailing
 *requires-clause* of the function call operator or operator template is
 the *requires-clause* of the *lambda-declarator*, if any.
-[*Note 2*: The function call operator template for a generic lambda
-might be an abbreviated function template [[dcl.fct]]. — *end note*]
 [*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 [=]() {
       printer(ts ...);
     };
   };
 };
 auto p = vglambda( [](auto v1, auto v2, auto v3)
                    { 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 `consteval`, or it satisfies the
-requirements for a constexpr function [[dcl.constexpr]]. It is an
 immediate function [[dcl.constexpr]] if the corresponding
 *lambda-expression*'s *parameter-declaration-clause* is followed by
 `consteval`.
-[*Note 3*: 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
@@ -170,11 +241,11 @@ struct NonLiteral {
 static_assert(ID(NonLiteral{3}).n == 3);        // error
 ```
 — *end example*]
-[*Example 3*:
 ``` cpp
 auto monoid = [](auto v) { return [=] { return v; }; };
 auto add = [](auto m1) constexpr {
   auto ret = m1();
@@ -199,18 +270,18 @@ static_assert(add(one)(one)() == two());        // error: two() is not a constan
 static_assert(add(one)(one)() == monoid(2)());  // OK
 ```
 — *end example*]
-[*Note 4*:
-The function call operator or operator template may be constrained
 [[temp.constr.decl]] by a *type-constraint* [[temp.param]], a
 *requires-clause* [[temp.pre]], or a trailing *requires-clause*
 [[dcl.decl]].
-[*Example 4*:
 ``` cpp
 template <typename T> concept C1 = ...;
 template <std::size_t N> concept C2 = ...;
 template <typename A, typename B> concept C3 = ...;
@@ -231,27 +302,29 @@ The closure type for a non-generic *lambda-expression* with no
 *lambda-capture* whose constraints (if any) are satisfied 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 on a
-default-constructed instance of the closure type. `F` is a constexpr
-function if the function call operator is a constexpr function and is an
-immediate function if the function call operator is an immediate
-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 5*:
 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
@@ -283,11 +356,11 @@ struct Closure {
 };
 ```
 — *end note*]
-[*Example 5*:
 ``` cpp
 void f1(int (*)(int))   { }
 void f2(char (*)(int))  { }
@@ -299,45 +372,49 @@ void h(char (*)(int))   { }     // #4
 auto glambda = [](auto a) { return a; };
 f1(glambda);                    // OK
 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
 ```
 — *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 on a default-constructed
-instance of the closure type. `F` is a constexpr function if the
-corresponding specialization is a constexpr function and `F` is an
-immediate function if the function call operator template specialization
-is an immediate function.
-[*Note 6*: This will result in the implicit instantiation of the
 generic lambda’s body. The instantiated generic lambda’s return type and
 parameter types are required to match the return type and parameter
 types of the pointer to function. — *end note*]
-[*Example 6*:
 ``` 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 7*:
 ``` cpp
 auto Fwd = [](int (*fp)(int), auto a) { return fp(a); };
 auto C = [](auto a) { return a; };
@@ -349,18 +426,14 @@ static_assert(Fwd(NC,3) == 3);  // error
 ```
 — *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 8*:
 ``` cpp
 struct S1 {
   int x, y;
   int operator()(int);
@@ -385,12 +458,12 @@ defaulted default constructor otherwise. It has a defaulted copy
 constructor and a defaulted move constructor [[class.copy.ctor]]. It has
 a deleted copy assignment operator if the *lambda-expression* has a
 *lambda-capture* and defaulted copy and move assignment operators
 otherwise [[class.copy.assign]].
-[*Note 7*: 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
@@ -436,13 +509,12 @@ simple-capture:
 init-capture:
     '...'ₒₚₜ identifier initializer
     '&' '...'ₒₚₜ 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
@@ -475,35 +547,37 @@ A *lambda-expression* shall not have a *capture-default* or
 *simple-capture* in its *lambda-introducer* unless its innermost
 enclosing scope is a block scope [[basic.scope.block]] or it appears
 within a default member initializer and its innermost enclosing scope is
 the corresponding class scope [[basic.scope.class]].
-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 a local entity. The *simple-capture*s `this` and
 `* this` denote the local entity `*this`. An entity that is designated
 by a *simple-capture* is said to be *explicitly captured*.
-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* without ellipsis 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:
 - if the capture is by copy (see below), the non-static data member
   declared for the capture and the variable are treated as two different
   ways of referring to the same object, which has the lifetime of the
   non-static data member, and no additional copy and destruction is
@@ -522,11 +596,14 @@ 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*]
 For the purposes of lambda capture, an expression potentially references
@@ -540,13 +617,13 @@ local entities as follows:
   *id-expression*. — *end note*]
 - A `this` expression potentially references `*this`.
 - A *lambda-expression* potentially references the local entities named
   by its *simple-capture*s.
-If an expression potentially references a local entity within a
-declarative region in which it is odr-usable, and the expression would
-be potentially evaluated if the effect of any enclosing `typeid`
 expressions [[expr.typeid]] were ignored, the entity is said to be
 *implicitly captured* by each intervening *lambda-expression* with an
 associated *capture-default* that does not explicitly capture it. The
 implicit capture of `*this` is deprecated when the *capture-default* is
 `=`; see [[depr.capture.this]].
@@ -557,38 +634,38 @@ implicit capture of `*this` is deprecated when the *capture-default* is
 void f(int, const int (&)[2] = {});         // #1
 void f(const int&, const int (&)[1]);       // #2
 void test() {
   const int x = 17;
   auto g = [](auto a) {
-    f(x);                       // OK: calls #1, does not capture x
   };
   auto g1 = [=](auto a) {
-    f(x);                       // OK: calls #1, captures x
   };
   auto g2 = [=](auto a) {
     int selector[sizeof(a) == 1 ? 1 : 2]{};
-    f(x, selector);             // OK: captures x, might call #1 or #2
   };
   auto g3 = [=](auto a) {
     typeid(a + x);              // captures x regardless of whether a + x is an unevaluated operand
   };
 }
 ```
-Within `g1`, an implementation might optimize away the capture of `x` as
 it is not odr-used.
 — *end example*]
 [*Note 4*:
 The set of captured entities is determined syntactically, and entities
-might be implicitly captured even if the expression denoting a local
-entity is within a discarded statement [[stmt.if]].
 [*Example 5*:
 ``` cpp
 template<bool B>
@@ -604,12 +681,12 @@ void f(int n) {
 — *end example*]
 — *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*.
 [*Note 5*: As a consequence, if a *lambda-expression* explicitly
 captures an entity that is not odr-usable, the program is ill-formed
 [[basic.def.odr]]. — *end note*]
@@ -619,26 +696,26 @@ captures an entity that is not odr-usable, the program is ill-formed
 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 odr-usable due to intervening lambda m3
           int x = n;            // error: n is odr-used but not odr-usable due to intervening lambda m3
-          x += m;               // OK: m implicitly captured by m4 and explicitly captured by m3
           x += i;               // error: i is odr-used but not odr-usable
                                 // due to intervening function and class scopes
-          x += f;               // OK: this captured implicitly by m4 and explicitly by m3
         };
       };
     }
   };
 }
@@ -701,11 +778,11 @@ 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 7*: An *id-expression* that is not an odr-use refers to the
 original entity, never to a member of the closure type. However, such an
@@ -721,12 +798,12 @@ the closure type.
 ``` 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
   };
 }
 ```
@@ -774,13 +851,15 @@ auto h(int &r) {
 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 11*:
 The nested *lambda-expression*s and invocations below will output
@@ -821,12 +900,11 @@ 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* containing an ellipsis is a pack expansion
 [[temp.variadic]]. An *init-capture* containing an ellipsis is a pack
-expansion that introduces an *init-capture* pack [[temp.variadic]] whose
-declarative region is the *lambda-expression*’s *compound-statement*.
 [*Example 12*:
 ``` cpp
 template<class... Args>

 ### Lambda expressions <a id="expr.prim.lambda">[[expr.prim.lambda]]</a>
+#### General <a id="expr.prim.lambda.general">[[expr.prim.lambda.general]]</a>
 ``` bnf
 lambda-expression:
+    lambda-introducer attribute-specifier-seqₒₚₜ lambda-declarator compound-statement
+    lambda-introducer '<' template-parameter-list '>' requires-clauseₒₚₜ attribute-specifier-seqₒₚₜ
+       lambda-declarator compound-statement
 ```
 ``` bnf
 lambda-introducer:
     '[' lambda-captureₒₚₜ ']'
 ```
 ``` bnf
 lambda-declarator:
+ lambda-specifier-seq noexcept-specifierₒₚₜ attribute-specifier-seqₒₚₜ trailing-return-typeₒₚₜ
+ noexcept-specifier attribute-specifier-seqₒₚₜ trailing-return-typeₒₚₜ
+    trailing-return-typeₒₚₜ
+    '(' parameter-declaration-clause ')' lambda-specifier-seqₒₚₜ noexcept-specifierₒₚₜ attribute-specifier-seqₒₚₜ
+       trailing-return-typeₒₚₜ requires-clauseₒₚₜ
+```
+``` bnf
+lambda-specifier:
+    consteval
+    constexpr
+    mutable
+    static
+```
+``` bnf
+lambda-specifier-seq:
+    lambda-specifier
+    lambda-specifier lambda-specifier-seq
 ```
 A *lambda-expression* provides a concise way to create a simple function
 object.
 *closure object*.
 [*Note 1*: A closure object behaves like a function object
 [[function.objects]]. — *end note*]
+An ambiguity can arise because a *requires-clause* can end in an
+*attribute-specifier-seq*, which collides with the
+*attribute-specifier-seq* in *lambda-expression*. In such cases, any
+attributes are treated as *attribute-specifier-seq* in
+*lambda-expression*.
+[*Note 2*: Such ambiguous cases cannot have valid semantics because the
+constraint expression would not have type `bool`. — *end note*]
+A *lambda-specifier-seq* shall contain at most one of each
+*lambda-specifier* and shall not contain both `constexpr` and
+`consteval`. If the *lambda-declarator* contains an explicit object
+parameter [[dcl.fct]], then no *lambda-specifier* in the
+*lambda-specifier-seq* shall be `mutable` or `static`. The
+*lambda-specifier-seq* shall not contain both `mutable` and `static`. If
+the *lambda-specifier-seq* contains `static`, there shall be no
+*lambda-capture*.
+[*Note 3*: The trailing *requires-clause* is described in
 [[dcl.decl]]. — *end note*]
+If a *lambda-declarator* does not include a
+*parameter-declaration-clause*, it is as if `()` were inserted at the
+start of the *lambda-declarator*. If the *lambda-declarator* does not
+include a *trailing-return-type*, it is considered to be `-> auto`.
+[*Note 4*: In that case, the return type is deduced from `return`
+statements as described in [[dcl.spec.auto]]. — *end note*]
 [*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*]
 A lambda is a *generic lambda* if the *lambda-expression* has any
 has a *template-parameter-list*.
 [*Example 3*:
 ``` cpp
+int i = [](int i, auto a) { return i; }(3, 4);                  // OK, a generic lambda
+int j = []<class T>(T t, int i) { return i; }(3, 4);            // OK, a generic lambda
 ```
 — *end example*]
 #### Closure types <a id="expr.prim.lambda.closure">[[expr.prim.lambda.closure]]</a>
 call operator template is the *requires-clause* immediately following
 `<` *template-parameter-list* `>`, if any. The trailing
 *requires-clause* of the function call operator or operator template is
 the *requires-clause* of the *lambda-declarator*, if any.
+[*Note 2*: The function call operator template for a generic lambda can
+be an abbreviated function template [[dcl.fct]]. — *end note*]
 [*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 [=]() {
       printer(ts ...);
     };
   };
 };
 auto p = vglambda( [](auto v1, auto v2, auto v3)
                    { std::cout << v1 << v2 << v3; } );
+auto q = p(1, 'a', 3.14);                                       // OK, outputs 1a3.14
+q();                                                            // OK, outputs 1a3.14
+auto fact = [](this auto self, int n) -> int {                  // OK, explicit object parameter
+  return (n <= 1) ? 1 : n * self(n-1);
+};
+std::cout << fact(5);                                           // OK, outputs 120
+```
+— *end example*]
+Given a lambda with a *lambda-capture*, the type of the explicit object
+parameter, if any, of the lambda’s function call operator (possibly
+instantiated from a function call operator template) shall be either:
+- the closure type,
+- a class type derived from the closure type, or
+- a reference to a possibly cv-qualified such type.
+[*Example 2*:
+``` cpp
+struct C {
+  template <typename T>
+  C(T);
+};
+void func(int i) {
+  int x = [=](this auto&&) { return i; }();     // OK
+  int y = [=](this C) { return i; }();          // error
+  int z = [](this C) { return 42; }();          // OK
+}
 ```
 — *end example*]
+The function call operator or operator template is a static member
+function or static member function template [[class.static.mfct]] if the
+*lambda-expression*’s *parameter-declaration-clause* is followed by
+`static`. Otherwise, it is a non-static member function or member
+function template [[class.mfct.non.static]] that is declared `const`
+[[class.mfct.non.static]] if and only if the *lambda-expression*’s
+*parameter-declaration-clause* is not followed by `mutable` and the
+*lambda-declarator* does not contain an explicit object parameter. 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. An *attribute-specifier-seq* in a
+*lambda-expression* preceding a *lambda-declarator* appertains to 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
+`consteval`, or it is constexpr-suitable [[dcl.constexpr]]. It is an
 immediate function [[dcl.constexpr]] if the corresponding
 *lambda-expression*'s *parameter-declaration-clause* is followed by
 `consteval`.
+[*Example 3*:
 ``` cpp
 auto ID = [](auto a) { return a; };
 static_assert(ID(3) == 3);                      // OK
 static_assert(ID(NonLiteral{3}).n == 3);        // error
 ```
 — *end example*]
+[*Example 4*:
 ``` cpp
 auto monoid = [](auto v) { return [=] { return v; }; };
 auto add = [](auto m1) constexpr {
   auto ret = m1();
 static_assert(add(one)(one)() == monoid(2)());  // OK
 ```
 — *end example*]
+[*Note 3*:
+The function call operator or operator template can be constrained
 [[temp.constr.decl]] by a *type-constraint* [[temp.param]], a
 *requires-clause* [[temp.pre]], or a trailing *requires-clause*
 [[dcl.decl]].
+[*Example 5*:
 ``` cpp
 template <typename T> concept C1 = ...;
 template <std::size_t N> concept C2 = ...;
 template <typename A, typename B> concept C3 = ...;
 *lambda-capture* whose constraints (if any) are satisfied 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. If the function call operator is a static
+member function, then the value returned by this conversion function is
+the address of the function call operator. Otherwise, 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 on a default-constructed instance of the closure type. `F`
+is a constexpr function if the function call operator is a constexpr
+function and is an immediate function if the function call operator is
+an immediate 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 4*:
 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
 };
 ```
 — *end note*]
+[*Example 6*:
 ``` cpp
 void f1(int (*)(int))   { }
 void f2(char (*)(int))  { }
 auto glambda = [](auto a) { return a; };
 f1(glambda);                    // OK
 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
 ```
 — *end example*]
+If the function call operator template is a static member function
+template, then the value returned by any given specialization of this
+conversion function template is the address of the corresponding
+function call operator template specialization. Otherwise, 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 on a default-constructed instance of
+the closure type. `F` is a constexpr function if the corresponding
+specialization is a constexpr function and `F` is an immediate function
+if the function call operator template specialization is an immediate
+function.
+[*Note 5*: This will result in the implicit instantiation of the
 generic lambda’s body. The instantiated generic lambda’s return type and
 parameter types are required to match the return type and parameter
 types of the pointer to function. — *end note*]
+[*Example 7*:
 ``` 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 8*:
 ``` cpp
 auto Fwd = [](int (*fp)(int), auto a) { return fp(a); };
 auto C = [](auto a) { return a; };
 ```
 — *end example*]
 The *lambda-expression*’s *compound-statement* yields the
+*function-body* [[dcl.fct.def]] of the function call operator, but it is
+not within the scope of the closure type.
+[*Example 9*:
 ``` cpp
 struct S1 {
   int x, y;
   int operator()(int);
 constructor and a defaulted move constructor [[class.copy.ctor]]. It has
 a deleted copy assignment operator if the *lambda-expression* has a
 *lambda-capture* and defaulted copy and move assignment operators
 otherwise [[class.copy.assign]].
+[*Note 6*: These special member functions are implicitly defined as
+usual, which can result in them being 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
 init-capture:
     '...'ₒₚₜ identifier initializer
     '&' '...'ₒₚₜ identifier initializer
 ```
+The body of a *lambda-expression* may refer to local entities 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
 *simple-capture* in its *lambda-introducer* unless its innermost
 enclosing scope is a block scope [[basic.scope.block]] or it appears
 within a default member initializer and its innermost enclosing scope is
 the corresponding class scope [[basic.scope.class]].
+The *identifier* in a *simple-capture* shall denote a local entity
+[[basic.lookup.unqual]], [[basic.pre]]. The *simple-capture*s `this` and
 `* this` denote the local entity `*this`. An entity that is designated
 by a *simple-capture* is said to be *explicitly captured*.
+If an *identifier* in a *capture* appears as the *declarator-id* of a
+parameter of the *lambda-declarator*’s *parameter-declaration-clause* or
+as the name of a template parameter of the *lambda-expression*’s
+*template-parameter-list*, the program is ill-formed.
 [*Example 2*:
 ``` cpp
 void f() {
   int x = 0;
+  auto g = [x](int x) { return 0; };    // error: parameter and capture have the same name
+  auto h = [y = 0]<typename y>(y) { return 0; };    // error: template parameter and capture
+                                                    // have the same name
 }
 ```
 — *end example*]
+An *init-capture* inhabits the lambda scope [[basic.scope.lambda]] of
+the *lambda-expression*. An *init-capture* without ellipsis behaves as
+if it declares and explicitly captures a variable of the form “`auto`
+*init-capture* `;`”, except that:
 - if the capture is by copy (see below), the non-static data member
   declared for the capture and the variable are treated as two different
   ways of referring to the same object, which has the lifetime of the
   non-static data member, and no additional copy and destruction is
 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 conceptual local variable have the same name
+auto counter = [i=0]() mutable -> decltype(i) {     // OK, returns int
+  return i++;
+};
 ```
 — *end example*]
 For the purposes of lambda capture, an expression potentially references
   *id-expression*. — *end note*]
 - A `this` expression potentially references `*this`.
 - A *lambda-expression* potentially references the local entities named
   by its *simple-capture*s.
+If an expression potentially references a local entity within a scope in
+which it is odr-usable [[basic.def.odr]], and the expression would be
+potentially evaluated if the effect of any enclosing `typeid`
 expressions [[expr.typeid]] were ignored, the entity is said to be
 *implicitly captured* by each intervening *lambda-expression* with an
 associated *capture-default* that does not explicitly capture it. The
 implicit capture of `*this` is deprecated when the *capture-default* is
 `=`; see [[depr.capture.this]].
 void f(int, const int (&)[2] = {});         // #1
 void f(const int&, const int (&)[1]);       // #2
 void test() {
   const int x = 17;
   auto g = [](auto a) {
+    f(x);                       // OK, calls #1, does not capture x
   };
   auto g1 = [=](auto a) {
+    f(x);                       // OK, calls #1, captures x
   };
   auto g2 = [=](auto a) {
     int selector[sizeof(a) == 1 ? 1 : 2]{};
+    f(x, selector);             // OK, captures x, can call #1 or #2
   };
   auto g3 = [=](auto a) {
     typeid(a + x);              // captures x regardless of whether a + x is an unevaluated operand
   };
 }
 ```
+Within `g1`, an implementation can optimize away the capture of `x` as
 it is not odr-used.
 — *end example*]
 [*Note 4*:
 The set of captured entities is determined syntactically, and entities
+are implicitly captured even if the expression denoting a local entity
+is within a discarded statement [[stmt.if]].
 [*Example 5*:
 ``` cpp
 template<bool B>
 — *end example*]
 — *end note*]
 An entity is *captured* if it is captured explicitly or implicitly. An
+entity captured by a *lambda-expression* is odr-used [[term.odr.use]] by
+the *lambda-expression*.
 [*Note 5*: As a consequence, if a *lambda-expression* explicitly
 captures an entity that is not odr-usable, the program is ill-formed
 [[basic.def.odr]]. — *end note*]
 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 odr-usable due to intervening lambda m3
           int x = n;            // error: n is odr-used but not odr-usable due to intervening lambda m3
+          x += m;               // OK, m implicitly captured by m4 and explicitly captured by m3
           x += i;               // error: i is odr-used but not odr-usable
                                 // due to intervening function and class scopes
+          x += f;               // OK, this captured implicitly by m4 and explicitly by m3
         };
       };
     }
   };
 }
 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 [[term.odr.use]] of an entity
 captured by copy is transformed into an access to the corresponding
 unnamed data member of the closure type.
 [*Note 7*: An *id-expression* that is not an odr-use refers to the
 original entity, never to a member of the closure type. However, such an
 ``` 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
   };
 }
 ```
 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` is not
+ `mutable`, the non-static data member is considered to be
+  const-qualified.
+- If `m1` captures the entity by reference, `m2` captures the same
   entity captured by `m1`.
 [*Example 11*:
 The nested *lambda-expression*s and invocations below will output
 corresponding *lambda-expression* after the lifetime of the entity has
 ended is likely to result in undefined behavior. — *end note*]
 A *simple-capture* containing an ellipsis is a pack expansion
 [[temp.variadic]]. An *init-capture* containing an ellipsis is a pack
+expansion that declares an *init-capture* pack [[temp.variadic]].
 [*Example 12*:
 ``` cpp
 template<class... Args>

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