Annex C (informative) Compatibility [diff]

Change: Removal of register storage-class-specifier.

Rationale: Enable repurposing of deprecated keyword in future revisions of this International Standard.

Effect on original feature: A valid C++ 2014 declaration utilizing the register storage-class-specifier is ill-formed in this International Standard.

The specifier can simply be removed to retain the original meaning.

Change: auto deduction from braced-init-list.

Rationale: More intuitive deduction behavior.

Effect on original feature: Valid C++ 2014 code may fail to compile or may change meaning in this International Standard.

For example:

auto x1{1};         // was std​::​initializer_­list<int>, now int
auto x2{1, 2};      // was std​::​initializer_­list<int>, now ill-formed

Change: Make exception specifications be part of the type system.

Rationale: Improve type-safety.

Effect on original feature: Valid C++ 2014 code may fail to compile or change meaning in this International Standard.

For example:

void g1() noexcept;
void g2();
template<class T> int f(T *, T *);
int x = f(g1, g2);              // ill-formed; previously well-formed

Change: Definition of an aggregate is extended to apply to user-defined types with base classes.

Rationale: To increase convenience of aggregate initialization.

Effect on original feature: Valid C++ 2014 code may fail to compile or produce different results in this International Standard; initialization from an empty initializer list will perform aggregate initialization instead of invoking a default constructor for the affected types.

For example:

struct derived;
struct base {
  friend struct derived;
private:
  base();
};
struct derived : base {};

derived d1{};       // error; the code was well-formed in C++ 2014
derived d2;         // still OK