Annex C (informative) Compatibility [diff]

C.1 C++ and ISO C++ 2023 [diff.cpp23]

C.1.4 [temp]: templates [diff.cpp23.temp]

1
#
Affected subclause: [temp.constr]
Change: Some atomic constraints become fold expanded constraints.

Rationale: Permit the subsumption of fold expressions.

Effect on original feature: Valid C++ 2023 code may become ill-formed.
[Example 1: template <typename ...V> struct A; struct S { static constexpr int compare(const S&) { return 1; } }; template <typename ...T, typename ...U> void f(A<T ...> *, A<U ...> *) requires (T::compare(U{}) && ...); // was well-formed (atomic constraint of type bool), // now ill-formed (results in an atomic constraint of type int) void g(A<S, S> *ap) { f(ap, ap); } — end example]
2
#
Affected subclause: [temp.deduct.call]
Change: Template argument deduction from overload sets succeeds in more cases.

Rationale: Allow consideration of constraints to disambiguate overload sets used as parameters in function calls.

Effect on original feature: Valid C++ 2023 code may become ill-formed.
[Example 2: template <typename T> void f(T &&, void (*)(T &&)); void g(int &); // #1 inline namespace A { void g(short &&); // #2 } inline namespace B { void g(short &&); // #3 } void q() { int x; f(x, g); // ill-formed; previously well-formed, deducing T = int& }
There is no change to the applicable deduction rules for the individual g candidates: Type deduction from #1 does not succeed; type deductions from #2 and #3 both succeed.
— end example]