Annex C (informative) Compatibility [diff]

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

C.1.3 [expr]: expressions [diff.cpp23.expr]

1
#
Affected subclause: [expr.arith.conv]
Change: Operations mixing a value of an enumeration type and a value of a different enumeration type or of a floating-point type are no longer valid.

Rationale: Reinforcing type safety.

Effect on original feature: A valid C++ 2023 program that performs operations mixing a value of an enumeration type and a value of a different enumeration type or of a floating-point type is ill-formed.
[Example 1: enum E1 { e }; enum E2 { f }; bool b = e <= 3.7; // ill-formed; previously well-formed int k = f - e; // ill-formed; previously well-formed auto x = true ? e : f; // ill-formed; previously well-formed — end example]
2
#
Affected subclauses: [expr.rel] and [expr.eq]
Change: Comparing two objects of array type is no longer valid.

Rationale: The old behavior was confusing since it compared not the contents of the two arrays, but their addresses.

Effect on original feature: A valid C++ 2023 program directly comparing two array objects is rejected as ill-formed in this document.
[Example 2: int arr1[5]; int arr2[5]; bool same = arr1 == arr2; // ill-formed; previously well-formed bool idem = arr1 == +arr2; // compare addresses bool less = arr1 < +arr2; // compare addresses, unspecified result — end example]
3
#
Affected subclause: [expr.delete]
Change: Calling delete on a pointer to an incomplete class is ill-formed.

Rationale: Reduce undefined behavior.

Effect on original feature: A valid C++ 2023 program that calls delete on an incomplete class type is ill-formed.
[Example 3: struct S; void f(S *p) { delete p; // ill-formed; previously well-formed } struct S {}; — end example]