1 A subtype_declaration declares a subtype of some previously declared type, as defined by a subtype_indication.
2 subtype_declaration ::= subtype defining_identifier is subtype_indication;
3/2 subtype_indication ::= [null_exclusion] subtype_mark [constraint]
4 subtype_mark ::= subtype_name
4.a Ramification: Note that name includes attribute_reference; thus, S'Base can be used as a subtype_mark.
4.b Reason: We considered changing subtype_mark to subtype_name. However, existing users are used to the word "mark," so we're keeping it.
5 constraint ::= scalar_constraint | composite_constraint
6 scalar_constraint ::= range_constraint | digits_constraint | delta_constraint
7 composite_constraint ::= index_constraint | discriminant_constraint
8 A subtype_mark shall resolve to denote a subtype. {determines (a type by a subtype_mark)} The type determined by a subtype_mark is the type of the subtype denoted by the subtype_mark.
8.a Ramification: Types are never directly named; all subtype_marks denote subtypes — possibly an unconstrained (base) subtype, but never the type. When we use the term anonymous type we really mean a type with no namable subtypes.
9 {elaboration (subtype_declaration) [partial]} The elaboration of a subtype_declaration consists of the elaboration of the subtype_indication. {elaboration (subtype_indication) [partial]} The elaboration of a subtype_indication creates a new subtype. If the subtype_indication does not include a constraint, the new subtype has the same (possibly null) constraint as that denoted by the subtype_mark. The elaboration of a subtype_indication that includes a constraint proceeds as follows:
10 The constraint is first elaborated.
11 {Range_Check [partial]} {check, language-defined (Range_Check)} A check is then made that the constraint is compatible with the subtype denoted by the subtype_mark.
11.a Ramification: The checks associated with constraint compatibility are all Range_Checks. Discriminant_Checks and Index_Checks are associated only with checks that a value satisfies a constraint.
12 The condition imposed by a constraint is the condition obtained after elaboration of the constraint. {compatibility (constraint with a subtype) [distributed]} The rules defining compatibility are given for each form of constraint in the appropriate subclause. These rules are such that if a constraint is compatible with a subtype, then the condition imposed by the constraint cannot contradict any condition already imposed by the subtype on its values. {Constraint_Error (raised by failure of run-time check)} The exception Constraint_Error is raised if any check of compatibility fails.
12.a To be honest: The condition imposed by a constraint is named after it — a range_constraint imposes a range constraint, etc.
12.b Ramification: A range_constraint causes freezing of its type. Other constraints do not.
NOTES
13 (4) A scalar_constraint may be applied to a subtype of an appropriate scalar type (see §3.5, §3.5.9, and §J.3), even if the subtype is already constrained. On the other hand, a composite_constraint may be applied to a composite subtype (or an access-to-composite subtype) only if the composite subtype is unconstrained (see §3.6.1 and §3.7.1).
14 Examples of subtype declarations:
15/2
subtype Rainbow is Color range Red .. Blue; -- see §3.2.1
subtype Red_Blue is Rainbow;
subtype Int is Integer;
subtype Small_Int is Integer range -10 .. 10;
subtype Up_To_K is Column range 1 .. K; -- see §3.2.1
subtype Square is Matrix(1 .. 10, 1 .. 10); -- see §3.6
subtype Male is Person(Sex => M); -- see §3.10.1
subtype Binop_Ref is not null Binop_Ptr; -- see §3.10
15.a {incompatibilities with Ada 83} In Ada 95, all range_constraints cause freezing of their type. Hence, a type-related representation item for a scalar type has to precede any range_constraints whose type is the scalar type.
15.b Subtype_marks allow only subtype names now, since types are never directly named. There is no need for RM83-3.3.2(3), which says a subtype_mark can denote both the type and the subtype; in Ada 95, you denote an unconstrained (base) subtype if you want, but never the type.
15.c The syntactic category type_mark is now called subtype_mark, since it always denotes a subtype.
15.d/2 {extensions to Ada 95} An optional null_exclusion can be used in a subtype_indication. This is described in §3.10