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7.3.2 Type Invariants

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For a private type or private extension, the following language-defined aspects may be specified with an aspect_specification (see 13.1.1):
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Type_Invariant

This aspect shall be specified by an expression, called an invariant expression. Type_Invariant may be specified on a private_type_declaration, on a private_extension_declaration, or on a full_type_declaration that declares the completion of a private type or private extension.
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Type_Invariant'Class

This aspect shall be specified by an expression, called an invariant expression. Type_Invariant'Class may be specified on a private_type_declaration or a private_extension_declaration.

Name Resolution Rules

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The expected type for an invariant expression is any boolean type.
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Within an invariant expression, the identifier of the first subtype of the associated type denotes the current instance of the type. Within an invariant expression associated with type T, the type of the current instance is T for the Type_Invariant aspect and T'Class for the Type_Invariant'Class aspect.

Legality Rules

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The Type_Invariant'Class aspect shall not be specified for an untagged type. The Type_Invariant aspect shall not be specified for an abstract type.

Static Semantics

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If the Type_Invariant aspect is specified for a type T, then the invariant expression applies to T.
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If the Type_Invariant'Class aspect is specified for a tagged type T, then the invariant expression applies to all descendants of T.

Dynamic Semantics

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If one or more invariant expressions apply to a type T, then an invariant check is performed at the following places, on the specified object(s):
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After successful default initialization of an object of type T, the check is performed on the new object;
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After successful conversion to type T, the check is performed on the result of the conversion;
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For a view conversion, outside the immediate scope of T, that converts from a descendant of T (including T itself) to an ancestor of type T (other than T itself), a check is performed on the part of the object that is of type T:
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after assigning to the view conversion; and
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after successful return from a call that passes the view conversion as an in out or out parameter.
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After a successful call on the Read or Input stream attribute of the type T, the check is performed on the object initialized by the stream attribute;
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An invariant is checked upon successful return from a call on any subprogram or entry that: 
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is declared within the immediate scope of type T (or by an instance of a generic unit, and the generic is declared within the immediate scope of type T), and
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is visible outside the immediate scope of type T or overrides an operation that is visible outside the immediate scope of T, and
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has a result with a part of type T, or one or more parameters with a part of type T, or an access to variable parameter whose designated type has a part of type T.
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The check is performed on each such part of type T
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 If performing checks is required by the Invariant or Invariant'Class assertion policies (see 11.4.2) in effect at the point of corresponding aspect specification applicable to a given type, then the respective invariant expression is considered enabled.
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 The invariant check consists of the evaluation of each enabled invariant expression that applies to T, on each of the objects specified above. If any of these evaluate to False, Assertions.Assertion_Error is raised at the point of the object initialization, conversion, or call. If a given call requires more than one evaluation of an invariant expression, either for multiple objects of a single type or for multiple types with invariants, the evaluations are performed in an arbitrary order, and if one of them evaluates to False, it is not specified whether the others are evaluated. Any invariant check is performed prior to copying back any by-copy in out or out parameters. Invariant checks, any postcondition check, and any constraint or predicate checks associated with in out or out parameters are performed in an arbitrary order.
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 The invariant checks performed on a call are determined by the subprogram or entry actually invoked, whether directly, as part of a dispatching call, or as part of a call through an access-to-subprogram value.
NOTES
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13  For a call of a primitive subprogram of type NT that is inherited from type T, the specified checks of the specific invariants of both the types NT and T are performed. For a call of a primitive subprogram of type NT that is overridden for type NT, the specified checks of the specific invariants of only type NT are performed.

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