1

A discriminant_constraint
specifies the values of the discriminants for a given discriminated type.

2

3

discriminant_association ::=

[*discriminant_*selector_name {| *discriminant_*selector_name} =>] expression

[

4

A discriminant_association
is said to be *named* if it has one or more *discriminant_*selector_names;
it is otherwise said to be *positional*. In
a discriminant_constraint,
any positional associations shall precede any named associations.

5

Each selector_name
of a named discriminant_association
shall resolve to denote a discriminant of the subtype being constrained;
the discriminants so named are the *associated
discriminants* of the named association. For a
positional association, the *associated discriminant* is the one
whose discriminant_specification
occurred in the corresponding position in the known_discriminant_part
that defined the discriminants of the subtype being constrained.

6

The expected type for the expression
in a discriminant_association
is that of the associated discriminant(s).

7/2

A discriminant_constraint
is only allowed in a subtype_indication
whose subtype_mark
denotes either an unconstrained discriminated subtype, or an unconstrained
access subtype whose designated subtype is an unconstrained discriminated
subtype. However, in the case of an access subtype, a discriminant_constraint
is illegal if the designated type has a partial view that is constrained
or, for a general access subtype, has default_expressions
for its discriminants. In addition to the places where Legality Rules
normally apply (see 12.3), these rules apply
also in the private part of an instance of a generic unit. In a generic
body, this rule is checked presuming all formal access types of the generic
might be general access types, and all untagged discriminated formal
types of the generic might have default_expressions
for their discriminants.

8

A named discriminant_association
with more than one selector_name
is allowed only if the named discriminants are all of the same type.
A discriminant_constraint
shall provide exactly one value for each discriminant of the subtype
being constrained.

9

The expression
associated with an access discriminant shall be of a type convertible
to the anonymous access type.

10

A discriminant_constraint
is *compatible* with an unconstrained discriminated subtype if each
discriminant value belongs to the subtype of the corresponding discriminant.

11

A composite value *satisfies*
a discriminant constraint if and only if each discriminant of the composite
value has the value imposed by the discriminant constraint.

12

For the elaboration of a discriminant_constraint,
the expressions
in the discriminant_associations
are evaluated in an arbitrary order and converted to the type of the
associated discriminant (which might raise Constraint_Error — see
4.6); the expression
of a named association is evaluated (and converted) once for each associated
discriminant. The result of each evaluation and conversion
is the value imposed by the constraint for the associated discriminant.

NOTES

13

56 The rules of the language ensure that
a discriminant of an object always has a value, either from explicit
or implicit initialization.

14

15

Large : Buffer(200); --* constrained, always 200 characters*

--* (explicit discriminant value)*

Message : Buffer; --* unconstrained, initially 100 characters*

--* (default discriminant value)*

Basis : Square(5); --* constrained, always 5 by 5*

Illegal : Square; --* illegal, a Square has to be constrained*

--

Message : Buffer; --

--

Basis : Square(5); --

Illegal : Square; --

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