
An index_constraint determines the range of possible values for every
index of an array subtype, and thereby the corresponding array bounds.
Syntax

index_constraint ::= (discrete_range {, discrete_range})

discrete_range ::= discrete_subtype_indication  range
Name Resolution Rules

The type of a discrete_range is the type of the subtype defined by the
subtype_indication, or the type of the range. For an index_constraint,
each discrete_range shall resolve to be of the type of the corresponding
index.
Legality Rules

An index_constraint shall appear only in a subtype_indication whose
subtype_mark denotes either an unconstrained array subtype, or an
unconstrained access subtype whose designated subtype is an
unconstrained array subtype; in either case, the index_constraint shall
provide a discrete_range for each index of the array type.
Static Semantics

A discrete_range defines a range whose bounds are given by the range, or
by the range of the subtype defined by the subtype_indication.
Dynamic Semantics

An index_constraint is compatible with an unconstrained array subtype if
and only if the index range defined by each discrete_range is
compatible, See section 3.5 Scalar Types, with the corresponding index subtype. If any of
the discrete_ranges defines a null range, any array thus constrained is
a null array, having no components. An array value satisfies an
index_constraint if at each index position the array value and the
index_constraint have the same index bounds.

The elaboration of an index_constraint consists of the evaluation of the
discrete_range(s), in an arbitrary order. The evaluation of a
discrete_range consists of the elaboration of the subtype_indication or
the evaluation of the range.
NOTES

(43) The elaboration of a subtype_indication consisting of a
subtype_mark followed by an index_constraint checks the compatibility of
the index_constraint with the subtype_mark, See section 3.2.2 Subtype Declarations.

(44) Even if an array value does not satisfy the index constraint of an
array subtype, Constraint_Error is not raised on conversion to the array
subtype, so long as the length of each dimension of the array value and
the array subtype match. See section 4.6 Type Conversions.
Examples

Examples of array declarations including an index constraint:

Board : Matrix(1 .. 8, 1 .. 8);  See section 3.6 Array Types
Rectangle : Matrix(1 .. 20, 1 .. 30);
Inverse : Matrix(1 .. N, 1 .. N);  N need not be static

Filter : Bit_Vector(0 .. 31);

Example of array declaration with a constrained array subtype:

My_Schedule : Schedule;
 all arrays of type Schedule have the same bounds

Example of record type with a component that is an array:

type Var_Line(Length : Natural) is
record
Image : String(1 .. Length);
end record;

Null_Line : Var_Line(0);  Null_Line.Image is a null array