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1

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

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discriminant_association
::=

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

[

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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.

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

7/1

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 a general access subtype, a discriminant_constraint
is illegal if there is a place within the immediate scope of the designated
subtype where the designated subtype's view is constrained.

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.

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

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

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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.

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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

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

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Message : Buffer;

Basis : Square(5);

Illegal : Square;