1

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

1.a

The rules in this clause are intentionally parallel
to those given in Record Aggregates.

2

3

discriminant_association ::=

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

[

4

{*named discriminant
association*} A discriminant_association
is said to be *named* if it has one or more *discriminant_*selector_names;
{*positional discriminant association*}
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;
{*associated discriminants (of a named
discriminant_association)*} the discriminants
so named are the *associated discriminants* of the named association.
{*associated discriminants (of a positional
discriminant_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

{*expected type (discriminant_association
expression)* [partial]} The expected type
for the expression
in a discriminant_association
is that of the associated discriminant(s).

7/2

{*8652/0008*}
{*AI95-00168-01*}
{*AI95-00363-01*}
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 a
general 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. {*generic contract
issue* [partial]} there
is a place within the immediate scope of the designated subtype where
the designated subtype's view is constrained.

7.a.1/2

7.a/2

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. {*convertible
(required)* [partial]}

9.a

10

{*compatibility (discriminant
constraint with a subtype)* [partial]} A
discriminant_constraint
is *compatible* with an unconstrained discriminated subtype if each
discriminant value belongs to the subtype of the corresponding discriminant.

10.a

10.b

11

{*satisfies (a discriminant
constraint)* [partial]} 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

{*elaboration (discriminant_constraint)*
[partial]} 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. {*implicit subtype conversion
(discriminant values)* [partial]} The result
of each evaluation and conversion is the value imposed by the constraint
for the associated discriminant.

12.a

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.

13.a

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; --

15.a

{*inconsistencies with Ada 83*} Dependent
compatibility checks are no longer performed on subtype declaration.
Instead they are deferred until object creation (see 3.3.1).
This is upward compatible for a program that does not raise Constraint_Error.

15.b

Everything in RM83-3.7.2(7-12), which specifies
the initial values for discriminants, is now redundant with 3.3.1, 6.4.1,
8.5.1, and 12.4. Therefore, we don't repeat it here. Since the material
is largely intuitive, but nevertheless complicated to state formally,
it doesn't seem worth putting it in a "NOTE."

15.c/2

{*8652/0008*}
{*AI95-00168-01*}
{*AI95-00363-01*}
{*incompatibilities with Ada 95*} The
Corrigendum added a restriction on discriminant_constraints
for general access subtypes. Such constraints are prohibited if the designated
type can be treated as constrained somewhere in the program. Ada 2005
goes further and prohibits such discriminant_constraints
if the designated type has (or might have, in the case of a formal type)
defaults for its discriminants. The use of general access subtypes is
rare, and this eliminates a boatload of problems that required many restrictions
on the use of aliased objects and components (now lifted). Similarly,
Ada 2005 prohibits discriminant_constraints
on any access type whose designated type has a partial view that is constrained.
Such a type will not be constrained in the heap to avoid privacy problems.
Again, the use of such subtypes is rare (they can only happen within
the package and its child units).

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