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6.1.1 Preconditions and Postconditions

1/4
For a noninstance subprogram, a generic subprogram, or an entry, the following language-defined aspects may be specified with an aspect_specification (see 13.1.1):
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Pre
This aspect specifies a specific precondition for a callable entity; it shall be specified by an expression, called a specific precondition expression. If not specified for an entity, the specific precondition expression for the entity is the enumeration literal True.
3/3
Pre'Class
This aspect specifies a class-wide precondition for an operation of a tagged type and its descendants; it shall be specified by an expression, called a class-wide precondition expression. If not specified for an entity, then if no other class-wide precondition applies to the entity, the class-wide precondition expression for the entity is the enumeration literal True.
4/3
Post
This aspect specifies a specific postcondition for a callable entity; it shall be specified by an expression, called a specific postcondition expression. If not specified for an entity, the specific postcondition expression for the entity is the enumeration literal True.
5/3
Post'Class
This aspect specifies a class-wide postcondition for an operation of a tagged type and its descendants; it shall be specified by an expression, called a class-wide postcondition expression. If not specified for an entity, the class-wide postcondition expression for the entity is the enumeration literal True.

Name Resolution Rules

6/3
The expected type for a precondition or postcondition expression is any boolean type.
7/4
Within the expression for a Pre'Class or Post'Class aspect for a primitive subprogram S of a tagged type T, a name that denotes a formal parameter (or S'Result) of type T is interpreted as though it had a (notional) type NT that is a formal derived type whose ancestor type is T, with directly visible primitive operations. Similarly, a name that denotes a formal access parameter (or S'Result) of type access-to-T is interpreted as having type access-to-NT. The result of this interpretation is that the only operations that can be applied to such names are those defined for such a formal derived type.
8/3
For an attribute_reference with attribute_designator Old, if the attribute reference has an expected type or shall resolve to a given type, the same applies to the prefix; otherwise, the prefix shall be resolved independently of context.

Legality Rules

9/3
The Pre or Post aspect shall not be specified for an abstract subprogram or a null procedure. Only the Pre'Class and Post'Class aspects may be specified for such a subprogram.
10/3
If a type T has an implicitly declared subprogram P inherited from a parent type T1 and a homograph (see 8.3) of P from a progenitor type T2, and
11/3
the corresponding primitive subprogram P1 of type T1 is neither null nor abstract; and
12/3
the class-wide precondition expression True does not apply to P1 (implicitly or explicitly); and
13/3
there is a class-wide precondition expression that applies to the corresponding primitive subprogram P2 of T2 that does not fully conform to any class-wide precondition expression that applies to P1
14/3
then:
15/3
If the type T is abstract, the implicitly declared subprogram P is abstract.
16/3
Otherwise, the subprogram P requires overriding and shall be overridden with a nonabstract subprogram.
17/3
If a renaming of a subprogram or entry S1 overrides an inherited subprogram S2, then the overriding is illegal unless each class-wide precondition expression that applies to S1 fully conforms to some class-wide precondition expression that applies to S2 and each class-wide precondition expression that applies to S2 fully conforms to some class-wide precondition expression that applies to S1.
17.1/4
  Pre'Class shall not be specified for an overriding primitive subprogram of a tagged type T unless the Pre'Class aspect is specified for the corresponding primitive subprogram of some ancestor of T.
17.2/4
  In addition to the places where Legality Rules normally apply (see 12.3), these rules also apply in the private part of an instance of a generic unit. 

Static Semantics

18/4
If a Pre'Class or Post'Class aspect is specified for a primitive subprogram S of a tagged type T, or such an aspect defaults to True, then a corresponding expression also applies to the corresponding primitive subprogram S of each descendant of T. The corresponding expression is constructed from the associated expression as follows:
18.1/4
References to formal parameters of S (or to S itself) are replaced with references to the corresponding formal parameters of the corresponding inherited or overriding subprogram S (or to the corresponding subprogram S itself).
18.2/4
  The primitive subprogram S is illegal if it is not abstract and the corresponding expression for a Pre'Class or Post'Class aspect would be illegal.
19/3
If performing checks is required by the Pre, Pre'Class, Post, or Post'Class assertion policies (see 11.4.2) in effect at the point of a corresponding aspect specification applicable to a given subprogram or entry, then the respective precondition or postcondition expressions are considered enabled.
20/3
An expression is potentially unevaluated if it occurs within:
21/3
any part of an if_expression other than the first condition;
22/3
a dependent_expression of a case_expression;
22.1/4
a predicate of a quantified_expression;
23/3
the right operand of a short-circuit control form; or
24/3
a membership_choice other than the first of a membership operation. 
25/3
For a prefix X that denotes an object of a nonlimited type, the following attribute is defined: 
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X'Old
Each X'Old in a postcondition expression that is enabled denotes a constant that is implicitly declared at the beginning of the subprogram body, entry body, or accept statement.
26.1/4
The implicitly declared entity denoted by each occurrence of X'Old is declared as follows:
26.2/4
If X is of an anonymous access type defined by an access_definition A then 
26.3/4
X'Old : constant A := X;
26.4/4
If X is of a specific tagged type T then 
26.5/4
anonymous : constant T'Class := T'Class(X);
X'Old : T renames T(anonymous);
26.6/4
where the name X'Old denotes the object renaming. 
26.7/4
Otherwise
26.8/4
X'Old : constant S := X;
26.9/4
where S is the nominal subtype of X. This includes the case where the type of S is an anonymous array type or a universal type. 
26.10/4
The nominal subtype of X'Old is as implied by the above definitions. The expected type of the prefix of an Old attribute is that of the attribute. Similarly, if an Old attribute shall resolve to be of some type, then the prefix of the attribute shall resolve to be of that type.
27/3
Reference to this attribute is only allowed within a postcondition expression. The prefix of an Old attribute_reference shall not contain a Result attribute_reference, nor an Old attribute_reference, nor a use of an entity declared within the postcondition expression but not within prefix itself (for example, the loop parameter of an enclosing quantified_expression). The prefix of an Old attribute_reference that is potentially unevaluated shall statically denote an entity.
28/3
For a prefix F that denotes a function declaration, the following attribute is defined:
29/3
F'Result
Within a postcondition expression for function F, denotes the result object of the function. The type of this attribute is that of the function result except within a Post'Class postcondition expression for a function with a controlling result or with a controlling access result. For a controlling result, the type of the attribute is T'Class, where T is the function result type. For a controlling access result, the type of the attribute is an anonymous access type whose designated type is T'Class, where T is the designated type of the function result type.
30/3
Use of this attribute is allowed only within a postcondition expression for F. 

Dynamic Semantics

31/3
Upon a call of the subprogram or entry, after evaluating any actual parameters, precondition checks are performed as follows:
32/3
The specific precondition check begins with the evaluation of the specific precondition expression that applies to the subprogram or entry, if it is enabled; if the expression evaluates to False, Assertions.Assertion_Error is raised; if the expression is not enabled, the check succeeds.
33/3
The class-wide precondition check begins with the evaluation of any enabled class-wide precondition expressions that apply to the subprogram or entry. If and only if all the class-wide precondition expressions evaluate to False, Assertions.Assertion_Error is raised.
34/3
The precondition checks are performed in an arbitrary order, and if any of the class-wide precondition expressions evaluate to True, it is not specified whether the other class-wide precondition expressions are evaluated. The precondition checks and any check for elaboration of the subprogram body are performed in an arbitrary order. It is not specified whether in a call on a protected operation, the checks are performed before or after starting the protected action. For an entry call, the checks are performed prior to checking whether the entry is open.
35/3
Upon successful return from a call of the subprogram or entry, prior to copying back any by-copy in out or out parameters, the postcondition check is performed. This consists of the evaluation of any enabled specific and class-wide postcondition expressions that apply to the subprogram or entry. If any of the postcondition expressions evaluate to False, then Assertions.Assertion_Error is raised. The postcondition expressions are evaluated in an arbitrary order, and if any postcondition expression evaluates to False, it is not specified whether any other postcondition expressions are evaluated. The postcondition check, and any constraint or predicate checks associated with in out or out parameters are performed in an arbitrary order.
35.1/4
  For a call to a task entry, the postcondition check is performed before the end of the rendezvous; for a call to a protected operation, the postcondition check is performed before the end of the protected action of the call. The postcondition check for any call is performed before the finalization of any implicitly-declared constants associated (as described above) with Old attribute_references but after the finalization of any other entities whose accessibility level is that of the execution of the callable construct. 
36/3
If a precondition or postcondition check fails, the exception is raised at the point of the call; the exception cannot be handled inside the called subprogram or entry. Similarly, any exception raised by the evaluation of a precondition or postcondition expression is raised at the point of call.
37/4
For any call to a subprogram or entry S (including dispatching calls), the checks that are performed to verify specific precondition expressions and specific and class-wide postcondition expressions are determined by those for the subprogram or entry actually invoked. Note that the class-wide postcondition expressions verified by the postcondition check that is part of a call on a primitive subprogram of type T includes all class-wide postcondition expressions originating in any progenitor of T, even if the primitive subprogram called is inherited from a type T1 and some of the postcondition expressions do not apply to the corresponding primitive subprogram of T1. Any operations within a class-wide postcondition expression that were resolved as primitive operations of the (notional) formal derived type NT, are in the evaluation of the postcondition bound to the corresponding operations of the type identified by the controlling tag of the call on S. This applies to both dispatching and non-dispatching calls on S.
38/4
The class-wide precondition check for a call to a subprogram or entry S consists solely of checking the class-wide precondition expressions that apply to the denoted callable entity (not necessarily to the one that is invoked). Any operations within such an expression that were resolved as primitive operations of the (notional) formal derived type NT are in the evaluation of the precondition bound to the corresponding operations of the type identified by the controlling tag of the call on S. This applies to both dispatching and non-dispatching calls on S.
39/3
For a call via an access-to-subprogram value, all precondition and postcondition checks performed are determined by the subprogram or entry denoted by the prefix of the Access attribute reference that produced the value. 
NOTES
40/3
5  A precondition is checked just before the call. If another task can change any value that the precondition expression depends on, the precondition need not hold within the subprogram or entry body. 

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