P3247R0: Deprecate the notion of trivial types

Introduction

The core language defines "trivial types", and in particular, trivial class types, even though that concept is used mainly for the definition of the library type trait is_trivial. This proposal deprecates the type trait is_trivial and moves the definition of "trivial type" next to the new location of the specification of the type trait.

This proposal follows the footsteps of "POD", deprecated with P0767R1 (November, 2017).

This proposal partially addresses core issue 1808.

Wording changes

Remove from 6.9.1 [basic.types.general] paragraph 9:

[...] Scalar types, trivial class types (11.2 [class.prop]), arrays of such types, and cv-qualified versions of these types are collectively called trivial types. [...]

Change in 11.2 [class.prop] paragraph 2:

A trivial class is a class that is trivially copyable and has one or more eligible default constructors (11.4.5.2), all of which are trivial. [Note 1: In particular, a trivially copyable or trivial class does not have virtual functions or virtual base classes. — end note]

Change in 11.2 [class.prop] paragraph 7:

[Example 2 :

struct N {   // neither trivial trivially copyable nor standard-layout
  int i;
  int j;
  virtual ~N();
};

struct T {  // trivial trivially copyable but not standard-layout
  int i;
private:
  int j;
};

struct SL {  // standard-layout but not trivial trivially copyable
  int i;
  int j;
  ~SL();
};

struct POD {  // both trivial trivially copyable and standard-layout
  int i;
  int j;
};

— end example]

Change in 11.9.5 [class.cdtor] paragraph 1:

extern X xobj;
int* p3 = &xobj.i;    // OK, X is a trivial class has no non-trivial constructors
X xobj;

Remove from 23.3.5.4 [meta.unary.prop]:

template<class T> struct is_trivial;

T is a trivial type (6.9.1 [basic.types.general])

remove_all_extents_t<T> shall be a complete type or cv void.

Change in 24.7.3.4.4 [mdspan.layout.policy.overview] paragraphs 1 and 2:

Each of layout_left, layout_right, and layout_stride , as well as each specialization of layout_left_padded and layout_right_padded, meets the layout mapping policy requirements and is a trivial trivially copyable type. Furthermore, default-initialization of an object of such a type invokes a trivial default constructor.

Each specialization of layout_left_padded and layout_right_padded meets the layout mapping policy requirements and is a trivial type.

Change in D.14 [depr.meta.types]:

namespace std {
  template<class T> struct is_trivial;
  template<class T> constexpr bool is_trivial_v = is_trivial<T>::value;
  template<class T> struct is_pod;
  template<class T> constexpr bool is_pod_v = is_pod<T>::value;
...
}

The behavior of a program...

A trivial class is a class that is trivially copyable and where default-initialization of an object of such a class type invokes a trivial default constructor (11.4.5.2 [class.default.ctor]). [Note: In particular, a trivial class does not have virtual functions or virtual base classes. — end note] A trivial type is a scalar type, a trivial class, an array of such a type, or a cv-qualified version of one of these types.

A POD class is a class that ...

  template<class T> struct is_trivial;

Preconditions: remove_all_extents_t<T> shall be a complete type or cv void.

Remarks: is_trivial<T> is a Cpp17UnaryTypeTrait (21.3.2 [meta.rqmts]) with a base characteristic of true_type if T is a trivial type, and false_type otherwise.

[Note: It is unspecified whether a closure type (7.5.5.2 [expr.prim.lambda.closure]) is a trivial type. — end note]

  template<class T> struct is_pod;