[temp.func.order] - C++17 → C++20

Files changed (1) hide show

tmp/tmpf4vs5g58/{from.md → to.md} +91 -19

tmp/tmpf4vs5g58/{from.md → to.md} RENAMED Viewed

@@ -1,49 +1,60 @@
 #### Partial ordering of function templates <a id="temp.func.order">[[temp.func.order]]</a>
 If a function template is overloaded, the use of a function template
-specialization might be ambiguous because template argument deduction (
-[[temp.deduct]]) may associate the function template specialization with
 more than one function template declaration. *Partial ordering* of
 overloaded function template declarations is used in the following
 contexts to select the function template to which a function template
 specialization refers:
 - during overload resolution for a call to a function template
-  specialization ([[over.match.best]]);
 - when the address of a function template specialization is taken;
 - when a placement operator delete that is a function template
   specialization is selected to match a placement operator new (
   [[basic.stc.dynamic.deallocation]], [[expr.new]]);
-- when a friend function declaration ([[temp.friend]]), an explicit
-  instantiation ([[temp.explicit]]) or an explicit specialization (
-  [[temp.expl.spec]]) refers to a function template specialization.
 Partial ordering selects which of two function templates is more
 specialized than the other by transforming each template in turn (see
 next paragraph) and performing template argument deduction using the
 function type. The deduction process determines whether one of the
 templates is more specialized than the other. If so, the more
 specialized template is the one chosen by the partial ordering process.
 To produce the transformed template, for each type, non-type, or
-template template parameter (including template parameter packs (
-[[temp.variadic]]) thereof) synthesize a unique type, value, or class
 template respectively and substitute it for each occurrence of that
 parameter in the function type of the template.
 [*Note 1*: The type replacing the placeholder in the type of the value
 synthesized for a non-type template parameter is also a unique
 synthesized type. — *end note*]
-If only one of the function templates *M* is a non-static member of some
-class *A*, *M* is considered to have a new first parameter inserted in
-its function parameter list. Given cv as the cv-qualifiers of *M* (if
-any), the new parameter is of type “rvalue reference to cv *A*” if the
-optional *ref-qualifier* of *M* is `&&` or if *M* has no *ref-qualifier*
-and the first parameter of the other template has rvalue reference type.
-Otherwise, the new parameter is of type “lvalue reference to cv *A*”.
 [*Note 2*: This allows a non-static member to be ordered with respect
 to a non-member function and for the results to be equivalent to the
 ordering of two equivalent non-members. — *end note*]
@@ -61,11 +72,11 @@ template<class T, class R> int operator*(T&, R&);   // #2
 // template<class R> int operator*(B<A>&, R&);\quad\quad\quad// #1a
 int main() {
   A a;
   B<A> b;
-  b * a;                                            // calls #1a
 }
 ```
 — *end example*]
@@ -102,12 +113,12 @@ void m() {
 — *end example*]
 [*Note 3*:
-Since partial ordering in a call context considers only parameters for
-which there are explicit call arguments, some parameters are ignored
 (namely, function parameter packs, parameters with default arguments,
 and ellipsis parameters).
 [*Example 3*:
@@ -154,14 +165,75 @@ template<class T, class... U> void f(T, U...);          // #1
 template<class T            > void f(T);                // #2
 template<class T, class... U> void g(T*, U...);         // #3
 template<class T            > void g(T);                // #4
 void h(int i) {
-  f(&i);                                                // error: ambiguous
   g(&i);                                                // OK: calls #3
 }
 ```
 — *end example*]
 — *end note*]

 #### Partial ordering of function templates <a id="temp.func.order">[[temp.func.order]]</a>
 If a function template is overloaded, the use of a function template
+specialization might be ambiguous because template argument deduction
+[[temp.deduct]] may associate the function template specialization with
 more than one function template declaration. *Partial ordering* of
 overloaded function template declarations is used in the following
 contexts to select the function template to which a function template
 specialization refers:
 - during overload resolution for a call to a function template
+  specialization [[over.match.best]];
 - when the address of a function template specialization is taken;
 - when a placement operator delete that is a function template
   specialization is selected to match a placement operator new (
   [[basic.stc.dynamic.deallocation]], [[expr.new]]);
+- when a friend function declaration [[temp.friend]], an explicit
+  instantiation [[temp.explicit]] or an explicit specialization
+  [[temp.expl.spec]] refers to a function template specialization.
 Partial ordering selects which of two function templates is more
 specialized than the other by transforming each template in turn (see
 next paragraph) and performing template argument deduction using the
 function type. The deduction process determines whether one of the
 templates is more specialized than the other. If so, the more
 specialized template is the one chosen by the partial ordering process.
+If both deductions succeed, the partial ordering selects the more
+constrained template (if one exists) as determined below.
 To produce the transformed template, for each type, non-type, or
+template template parameter (including template parameter packs
+[[temp.variadic]] thereof) synthesize a unique type, value, or class
 template respectively and substitute it for each occurrence of that
 parameter in the function type of the template.
 [*Note 1*: The type replacing the placeholder in the type of the value
 synthesized for a non-type template parameter is also a unique
 synthesized type. — *end note*]
+Each function template M that is a member function is considered to have
+a new first parameter of type X(M), described below, inserted in its
+function parameter list. If exactly one of the function templates was
+considered by overload resolution via a rewritten candidate
+[[over.match.oper]] with a reversed order of parameters, then the order
+of the function parameters in its transformed template is reversed. For
+a function template M with cv-qualifiers cv that is a member of a class
+A:
+- The type X(M) is “rvalue reference to cv A” if the optional
+  *ref-qualifier* of M is `&&` or if M has no *ref-qualifier* and the
+  positionally-corresponding parameter of the other transformed template
+  has rvalue reference type; if this determination depends recursively
+  upon whether X(M) is an rvalue reference type, it is not considered to
+  have rvalue reference type.
+- Otherwise, X(M) is “lvalue reference to cv A”.
 [*Note 2*: This allows a non-static member to be ordered with respect
 to a non-member function and for the results to be equivalent to the
 ordering of two equivalent non-members. — *end note*]
 // template<class R> int operator*(B<A>&, R&);\quad\quad\quad// #1a
 int main() {
   A a;
   B<A> b;
+  b * a;                                            // calls #1
 }
 ```
 — *end example*]
 — *end example*]
 [*Note 3*:
+Since, in a call context, such type deduction considers only parameters
+for which there are explicit call arguments, some parameters are ignored
 (namely, function parameter packs, parameters with default arguments,
 and ellipsis parameters).
 [*Example 3*:
 template<class T            > void f(T);                // #2
 template<class T, class... U> void g(T*, U...);         // #3
 template<class T            > void g(T);                // #4
 void h(int i) {
+  f(&i);                                                // OK: calls #2
   g(&i);                                                // OK: calls #3
 }
 ```
 — *end example*]
 — *end note*]
+If deduction against the other template succeeds for both transformed
+templates, constraints can be considered as follows:
+- If their *template-parameter-list*s (possibly including
+  *template-parameter*s invented for an abbreviated function template
+  [[dcl.fct]]) or function parameter lists differ in length, neither
+  template is more specialized than the other.
+- Otherwise:
+  - If exactly one of the templates was considered by overload
+    resolution via a rewritten candidate with reversed order of
+    parameters:
+    - If, for either template, some of the template parameters are not
+      deducible from their function parameters, neither template is more
+      specialized than the other.
+    - If there is either no reordering or more than one reordering of
+      the associated *template-parameter-list* such that
+      - the corresponding *template-parameter*s of the
+        *template-parameter-list*s are equivalent and
+      - the function parameters that positionally correspond between the
+        two templates are of the same type,
+      neither template is more specialized than the other.
+  - Otherwise, if the corresponding *template-parameter*s of the
+    *template-parameter-list*s are not equivalent [[temp.over.link]] or
+    if the function parameters that positionally correspond between the
+    two templates are not of the same type, neither template is more
+    specialized than the other.
+- Otherwise, if the context in which the partial ordering is done is
+  that of a call to a conversion function and the return types of the
+  templates are not the same, then neither template is more specialized
+  than the other.
+- Otherwise, if one template is more constrained than the other
+  [[temp.constr.order]], the more constrained template is more
+  specialized than the other.
+- Otherwise, neither template is more specialized than the other.
+[*Example 6*:
+``` cpp
+template <typename> constexpr bool True = true;
+template <typename T> concept C = True<T>;
+void f(C auto &, auto &) = delete;
+template <C Q> void f(Q &, C auto &);
+void g(struct A *ap, struct B *bp) {
+  f(*ap, *bp);                  // OK: Can use different methods to produce template parameters
+}
+template <typename T, typename U> struct X {};
+template <typename T, C U, typename V> bool operator==(X<T, U>, V) = delete;
+template <C T, C U, C V>               bool operator==(T, X<U, V>);
+void h() {
+  X<void *, int>{} == 0;        // OK: Correspondence of [T, U, V] and [U, V, T]
+}
+```
+— *end example*]

Diff to HTML by rtfpessoa