 (1)
 In addition to the various languagedefined classes of types, types can
be grouped into derivation classes.
Static Semantics
 (2)
 A derived type is derived from its parent type directly; it is derived
indirectly from any type from which its parent type is derived. The
derivation class of types for a type T (also called the class rooted at T) is
the set consisting of T (the root type of the class) and all types derived
from T (directly or indirectly) plus any associated universal or classwide
types (defined below).
 (3)
 Every type is either a specific type, a classwide type, or a universal
type. A specific type is one defined by a type_declaration, a formal_type_declaration, or a full type definition embedded in a declaration for an
object. Classwide and universal types are implicitly defined, to act as
representatives for an entire class of types, as follows:
 (4)
 Classwide types
Classwide types are defined for (and belong to) each derivation class rooted
at a tagged type (see 3.9). Given a subtype S of
a tagged type T, S'Class is the subtype_mark for a corresponding subtype
of the tagged classwide type T'Class. Such types are called ``classwide''
because when a formal parameter is defined to be of a classwide type T'Class,
an actual parameter of any type in the derivation class rooted at T is acceptable
(see 8.6).
 (5)
The set of values for a classwide type T'Class is the discriminated union
of the set of values of each specific type in the derivation class rooted
at T (the tag acts as the implicit discriminant  see
3.9). Classwide types have no primitive subprograms of their own. However,
as explained in 3.9.2, operands of a classwide
type T'Class can be used as part of a dispatching call on a primitive subprogram
of the type T. The only components (including discriminants) of T'Class
that are visible are those of T. If S is a first subtype, then S'Class is
a first subtype.
 (6)
 Universal types
Universal types are defined for (and belong to) the integer, real, and fixed
point classes, and are referred to in this standard as respectively, universal_integer,
universal_real, and universal_fixed. These are analogous to classwide types
for these languagedefined numeric classes. As with classwide types, if
a formal parameter is of a universal type, then an actual parameter of any
type in the corresponding class is acceptable. In addition, a value of a
universal type (including an integer or real numeric_literal) is ``universal''
in that it is acceptable where some particular type in the class is expected
(see 8.6).
 (7)
The set of values of a universal type is the
undiscriminated union of the set of values possible for any
definable type in the associated class. Like classwide
types, universal types have no primitive subprograms of their
own. However, their ``universality'' allows them to be used
as operands with the primitive subprograms of any type in the
corresponding class.
 (8)
 The integer and real numeric classes each have a specific root type in
addition to their universal type, named respectively root_integer and root_real.
 (9)
 A classwide or universal type is said to cover all of the types in its
class. A specific type covers only itself.
 (10)
 A specific type T2 is defined to be a descendant of a type T1 if T2 is
the same as T1, or if T2 is derived (directly or indirectly) from T1. A
classwide type T2'Class is defined to be a descendant of type T1 if T2 is a
descendant of T1. Similarly, the universal types are defined to be
descendants of the root types of their classes. If a type T2 is a descendant
of a type T1, then T1 is called an ancestor of T2. The ultimate ancestor of
a type is the ancestor of the type that is not a descendant of any other
type.
 (11)
 An inherited component (including an inherited discriminant) of a
derived type is inherited from a given ancestor of the type if the
corresponding component was inherited by each derived type in the chain of
derivations going back to the given ancestor.

 (12)
(18) Because operands of a universal type are acceptable to the predefined
operators of any type in their class, ambiguity can result. For universal_integer
and universal_real, this potential ambiguity is resolved by giving a preference
(see 8.6) to the predefined operators of the corresponding
root types (root_integer and root_real, respectively). Hence, in an apparently
ambiguous expression like
(13)
1 + 4 < 7
 (14)
where each of the literals is of type universal_integer, the
predefined operators of root_integer will be preferred over those of
other specific integer types, thereby resolving the ambiguity.
 Email comments, additions, corrections, gripes, kudos, etc. to:
Magnus Kempe  Magnus.Kempe@di.epfl.ch
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