P0847R7 Deducing this

Author: jensmaurer

source/basic.tex

@@ -865,6 +865,32 @@
 \end{itemize}
 \end{note}
 
+\pnum
+Two non-static member functions have
+\defnadjx{corresponding}{object parameters}{object parameter} if:
+\begin{itemize}
+\item
+exactly one is an implicit object member function
+with no \grammarterm{ref-qualifier} and
+the types of their object parameters\iref{dcl.fct},
+after removing top-level references,
+are the same, or
+\item
+their object parameters have the same type.
+\end{itemize}
+Two non-static member function templates have
+\defnadjx{corresponding}{object parameters}{object parameter} if:
+\begin{itemize}
+\item
+exactly one is an implicit object member function
+with no \grammarterm{ref-qualifier} and
+the types of their object parameters,
+after removing any references,
+are equivalent, or
+\item
+the types of their object parameters are equivalent.
+\end{itemize}
+
 \pnum
 Two declarations \defn{correspond}
 if they (re)introduce the same name,
@@ -885,25 +911,23 @@
 each declares a function or function template, except when
 \begin{itemize}
 \item
-both declare functions with the same parameter-type-list,
+both declare functions with the same non-object-parameter-type-list,
 \begin{footnote}
 An implicit object parameter\iref{over.match.funcs}
 is not part of the parameter-type-list.
 \end{footnote}
 equivalent\iref{temp.over.link} trailing \grammarterm{requires-clause}s
 (if any, except as specified in \ref{temp.friend}), and,
 if both are non-static members,
-the same \grammarterm{cv-qualifier}s (if any) and
-\grammarterm{ref-qualifier} (if both have one), or
+they have corresponding object parameters, or
 \item
 both declare function templates with equivalent
-parameter-type-lists,
+non-object-parameter-type-lists,
 return types (if any),
 \grammarterm{template-head}s, and
 trailing \grammarterm{requires-clause}s (if any), and,
 if both are non-static members,
-the same \grammarterm{cv-qualifier}s (if any) and
-\grammarterm{ref-qualifier} (if both have one).
+they have corresponding object parameters.
 \end{itemize}
 \end{itemize}
 \begin{note}
@@ -923,16 +947,23 @@
 \begin{codeblock}
 typedef int Int;
 enum E : int { a };
-void f(int);            // \#1
-void f(Int) {}          // defines \#1
-void f(E) {}            // OK: another overload
+void f(int);                    // \#1
+void f(Int) {}                  // defines \#1
+void f(E) {}                    // OK: another overload
 
 struct X {
   static void f();
-  void f() const;       // error: redeclaration
+  void f() const;               // error: redeclaration
   void g();
-  void g() const;       // OK
-  void g() &;           // error: redeclaration
+  void g() const;               // OK
+  void g() &;                   // error: redeclaration
+
+  void h(this X&, int);
+  void h(int) &&;               // OK: another overload
+  void j(this const X&);
+  void j() const&;              // error: redeclaration
+  void k();
+  void k(this X&);              // error: redeclaration
 };
 \end{codeblock}
 \end{example}

source/classes.tex

@@ -997,15 +997,15 @@
 \indextext{member function!volatile}%
 \indextext{member function!const volatile}%
 \begin{note}
-A non-static member function can be declared with
+An implicit object member function can be declared with
 \grammarterm{cv-qualifier}{s}, which affect the type of the \keyword{this}
 pointer\iref{expr.prim.this},
 and/or a \grammarterm{ref-qualifier}\iref{dcl.fct};
 both affect overload resolution\iref{over.match.funcs}
 \end{note}
 
 \pnum
-A non-static member function may be declared
+An implicit object member function may be declared
 virtual\iref{class.virtual} or pure virtual\iref{class.abstract}.
 
 \rSec2[special]{Special member functions}
@@ -2484,7 +2484,7 @@
 \nontermdef{conversion-declarator}\br
     ptr-operator \opt{conversion-declarator}
 \end{bnf}
-shall have no parameters and
+shall have no non-object parameters and
 specifies a conversion from \tcode{X} to
 the type specified by the \grammarterm{conversion-type-id},
 interpreted as a \grammarterm{type-id}\iref{dcl.name}.

source/declarations.tex

@@ -3440,10 +3440,10 @@
 
 \begin{bnf}
 \nontermdef{parameter-declaration}\br
-    \opt{attribute-specifier-seq} decl-specifier-seq declarator\br
-    \opt{attribute-specifier-seq} decl-specifier-seq declarator \terminal{=} initializer-clause\br
-    \opt{attribute-specifier-seq} decl-specifier-seq \opt{abstract-declarator}\br
-    \opt{attribute-specifier-seq} decl-specifier-seq \opt{abstract-declarator} \terminal{=} initializer-clause
+    \opt{attribute-specifier-seq} \opt{\keyword{this}} decl-specifier-seq declarator\br
+    \opt{attribute-specifier-seq} \opt{\keyword{this}} decl-specifier-seq declarator \terminal{=} initializer-clause\br
+    \opt{attribute-specifier-seq} \opt{\keyword{this}} decl-specifier-seq \opt{abstract-declarator}\br
+    \opt{attribute-specifier-seq} \opt{\keyword{this}} decl-specifier-seq \opt{abstract-declarator} \terminal{=} initializer-clause
 \end{bnf}
 
 The optional \grammarterm{attribute-specifier-seq} in a \grammarterm{parameter-declaration}
@@ -3552,6 +3552,66 @@
 \end{codeblock}
 \end{example}
 
+\pnum
+An \defn{explicit-object-parameter-declaration} is
+a \grammarterm{parameter-declaration} with a \keyword{this} specifier.
+An explicit-object-parameter-declaration shall appear only as
+the first \grammarterm{parameter-declaration} of
+a \grammarterm{parameter-declaration-list} of either:
+\begin{itemize}
+\item
+a \grammarterm{member-declarator}
+that declares a member function\iref{class.mem}, or
+\item
+a \grammarterm{lambda-declarator}\iref{expr.prim.lambda}.
+\end{itemize}
+A \grammarterm{member-declarator} with an explicit-object-parameter-declaration
+shall not include
+a \grammarterm{ref-qualifier} or a \grammarterm{cv-qualifier-seq} and
+shall not be declared \keyword{static} or \keyword{virtual}.
+\begin{example}
+\begin{codeblock}
+struct C {
+  void f(this C& self);
+  template <typename Self> void g(this Self&& self, int);
+
+  void h(this C) const;         // error: \tcode{const} not allowed here
+};
+
+void test(C c) {
+  c.f();                        // OK, calls \tcode{C::f}
+  c.g(42);                      // OK, calls \tcode{C::g<C\&>}
+  std::move(c).g(42);           // OK, calls \tcode{C::g<C>}
+}
+\end{codeblock}
+\end{example}
+
+\pnum
+A function parameter declared with an explicit-object-parameter-declaration
+is an \defnadj{explicit object}{parameter}.
+An explicit object parameter shall not be
+a function parameter pack\iref{temp.variadic}.
+An \defnadj{explicit object}{member function} is a non-static member function
+with an explicit object parameter.
+An \defnadj{implicit object}{member function} is a non-static member function
+without an explicit object parameter.
+
+\pnum
+The \defnadj{object}{parameter} of a non-static member function is either
+the explicit object parameter or
+the implicit object parameter\iref{over.match.funcs}.
+
+\pnum
+A \defnadj{non-object}{parameter} is a function parameter
+that is not the explicit object parameter.
+The \defn{non-object-parameter-type-list} of a member function is
+the parameter-type-list of that function with the explicit object parameter,
+if any, omitted.
+\begin{note}
+The non-object-parameter-type-list consists of
+the adjusted types of all the non-object parameters.
+\end{note}
+
 \pnum
 A function type with a \grammarterm{cv-qualifier-seq} or a
 \grammarterm{ref-qualifier} (including a type named by
@@ -6383,16 +6443,16 @@
 \tcode{std::coroutine_traits<R, P$_1$, $\dotsc$, P$_n$>::promise_type},
 where
 \tcode{R} is the return type of the function, and
-$\tcode{P}_1 \dotsc \tcode{P}_n$ are the sequence of types of the function parameters,
-preceded by the type of the implicit object parameter\iref{over.match.funcs}
+$\tcode{P}_1 \dotsc \tcode{P}_n$ are the sequence of types of the non-object function parameters,
+preceded by the type of the object parameter\iref{dcl.fct}
 if the coroutine is a non-static member function.
 The promise type shall be a class type.
 
 \pnum
 In the following, $\tcode{p}_i$ is an lvalue of type $\tcode{P}_i$,
 where
-$\tcode{p}_1$ denotes \tcode{*this} and
-$\tcode{p}_{i+1}$ denotes the $i^\textrm{th}$ function parameter
+$\tcode{p}_1$ denotes the object parameter and
+$\tcode{p}_{i+1}$ denotes the $i^\textrm{th}$ non-object function parameter
 for a non-static member function, and
 $\tcode{p}_i$ denotes
 the $i^\textrm{th}$ function parameter otherwise.

source/expressions.tex

@@ -1194,7 +1194,7 @@
 
 \pnum
 \indextext{\idxcode{this}}%
-The keyword \keyword{this} names a pointer to the object for which a non-static member
+The keyword \keyword{this} names a pointer to the object for which an implicit object member
 function\iref{class.mfct.non-static} is invoked or a non-static data member's
 initializer\iref{class.mem} is evaluated.
 
@@ -1213,8 +1213,10 @@
 between the optional
 \grammarterm{cv-qualifier-seq} and the end of the \grammarterm{function-definition},
 \grammarterm{member-declarator}, or \grammarterm{declarator}. It shall not appear within
-the declaration of a static member function of the current class (although its type and value category
-are defined within a static member function as they are within a non-static
+the declaration of either
+a static member function or an explicit object member function
+of the current class (although its type and value category
+are defined within such member functions as they are within an implicit object
 member function).
 \begin{note}
 This is because declaration matching does not
@@ -1450,7 +1452,7 @@
 the type of a class member access expression\iref{expr.ref}
 naming the non-static data member
 that would be declared for such a capture
-in the closure object of
+in the object parameter\iref{dcl.fct} of the function call operator of
 the innermost such intervening \grammarterm{lambda-expression}.
 \begin{note}
 If that \grammarterm{lambda-expression} is not declared \keyword{mutable},
@@ -1688,6 +1690,10 @@
 In the \grammarterm{decl-specifier-seq} of the \grammarterm{lambda-declarator},
 each \grammarterm{decl-specifier}
 shall be one of \keyword{mutable}, \keyword{constexpr}, or \keyword{consteval}.
+If the \grammarterm{lambda-declarator} contains
+an explicit object parameter\iref{dcl.fct},
+then no \grammarterm{decl-specifier} in the \grammarterm{decl-specifier-seq}
+shall be \keyword{mutable}.
 \begin{note}
 The trailing \grammarterm{requires-clause} is described in \ref{dcl.decl}.
 \end{note}
@@ -1792,14 +1798,51 @@
                    { std::cout << v1 << v2 << v3; } );
 auto q = p(1, 'a', 3.14);                                       // OK: outputs \tcode{1a3.14}
 q();                                                            // OK: outputs \tcode{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{codeblock}
+\end{example}
+
+\pnum
+Given a lambda with a \grammarterm{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:
+\begin{itemize}
+\item
+the closure type,
+\item
+a class type derived from the closure type, or
+\item
+a reference to a possibly cv-qualified such type.
+\end{itemize}
+\begin{example}
+\begin{codeblock}
+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{codeblock}
 \end{example}
 
 \pnum
 The function call operator or operator template is declared
 \keyword{const}~(\ref{class.mfct.non-static}) if and only if the
 \grammarterm{lambda-expression}'s \grammarterm{parameter-declaration-clause} is not
-followed by \keyword{mutable}. It is neither virtual nor declared \tcode{volatile}. Any
+followed by \keyword{mutable} and
+the \grammarterm{lambda-declarator} does not contain
+an explicit object parameter.
+It is neither virtual nor declared \tcode{volatile}. Any
 \grammarterm{noexcept-specifier} specified on a \grammarterm{lambda-expression}
 applies to the corresponding function call operator or operator template.
 An \grammarterm{attribute-specifier-seq} in a \grammarterm{lambda-declarator} appertains
@@ -3073,6 +3116,10 @@
 When a function is called, each parameter\iref{dcl.fct} is
 initialized~(\ref{dcl.init}, \ref{class.copy.ctor}) with
 its corresponding argument.
+If the function is an explicit object member function and
+there is an implied object argument\iref{over.call.func},
+the list of provided arguments is preceded by the implied object argument
+for the purposes of this correspondence.
 If there is no corresponding argument,
 the default argument for the parameter is used.
 \begin{example}
@@ -3081,7 +3128,7 @@
 int x = f<int>();               // error: no argument for second function parameter
 \end{codeblock}
 \end{example}
-If the function is a non-static member
+If the function is an implicit object member
 function, the \keyword{this} parameter of the function\iref{expr.prim.this}
 is initialized with a pointer to the object of the call, converted
 as if by an explicit type conversion\iref{expr.cast}.
@@ -4328,11 +4375,13 @@
 \begin{itemize}
 \item
 If the operand is a \grammarterm{qualified-id} naming a non-static or variant member \tcode{m}
-of some class \tcode{C}, the result has type ``pointer to member
+of some class \tcode{C}, other than an explicit object member function, the result has type ``pointer to member
 of class \tcode{C} of type \tcode{T}'' and designates \tcode{C::m}.
 \item
 Otherwise, the result has type ``pointer to \tcode{T}'' and points to
 the designated object\iref{intro.memory} or function\iref{basic.compound}.
+If the operand names an explicit object member function\iref{dcl.fct},
+the operand shall be a \grammarterm{qualified-id}.
 \begin{note}
 In particular, taking the address of a variable of type ``\cv{}~\tcode{T}''
 yields a pointer of type ``pointer to \cv{}~\tcode{T}''.