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3.11.1 Completions of Declarations

     {8652/0014} Declarations sometimes come in two parts. {requires a completion} A declaration that requires a second part is said to require completion. {completion (compile-time concept)} The second part is called the completion of the declaration (and of the entity declared), and is either another declaration, a body, or a pragma. A {body} body is a body, an entry_body, or a renaming-as-body (see 8.5.4).
Discussion: Throughout the RM95, there are rules about completions that define the following:
Don't confuse this compile-time concept with the run-time concept of completion defined in 7.6.1.
Note that the declaration of a private type (if limited) can be completed with the declaration of a task type, which is then completed with a body. Thus, a declaration can actually come in three parts.

Name Resolution Rules

   A construct that can be a completion is interpreted as the completion of a prior declaration only if:

Legality Rules

   An implicit declaration shall not have a completion. {requires a completion [distributed]} For any explicit declaration that is specified to require completion, there shall be a corresponding explicit completion.
Discussion: The implicit declarations of predefined operators are not allowed to have a completion. Enumeration literals, although they are subprograms, are not allowed to have a corresponding subprogram_body. That's because the completion rules are described in terms of constructs (subprogram_declarations) and not entities (subprograms). When a completion is required, it has to be explicit; the implicit null package_body that Section 7 talks about cannot serve as the completion of a package_declaration if a completion is required.
   At most one completion is allowed for a given declaration. Additional requirements on completions appear where each kind of completion is defined.
Ramification: A subunit is not a completion; the stub is.
If the completion of a declaration is also a declaration, then that declaration might have a completion, too. For example, a limited private type can be completed with a task type, which can then be completed with a task body. This is not a violation of the ``at most one completion'' rule.
   {completely defined} A type is completely defined at a place that is after its full type definition (if it has one) and after all of its subcomponent types are completely defined. A type shall be completely defined before it is frozen (see 13.14 and 7.3).
Reason: Index types are always completely defined -- no need to mention them. There is no way for a completely defined type to depend on the value of a (still) deferred constant.
88  Completions are in principle allowed for any kind of explicit declaration. However, for some kinds of declaration, the only allowed completion is a pragma Import, and implementations are not required to support pragma Import for every kind of entity.
Discussion: In fact, we expect that implementations will not support pragma Import of things like types -- it's hard to even define the semantics of what it would mean. Therefore, in practice, not every explicit declaration can have a completion. In any case, if an implementation chooses to support pragma Import for, say, types, it can place whatever restrictions on the feature it wants to. For example, it might want the pragma to be a freezing point for the type.
89  There are rules that prevent premature uses of declarations that have a corresponding completion. The Elaboration_Checks of 3.11 prevent such uses at run time for subprograms, protected operations, tasks, and generic units. The rules of 13.14, ``Freezing Rules'' prevent, at compile time, premature uses of other entities such as private types and deferred constants.

Wording Changes from Ada 83

This subclause is new. It is intended to cover all kinds of completions of declarations, be they a body for a spec, a full type for an incomplete or private type, a full constant declaration for a deferred constant declaration, or a pragma Import for any kind of entity.

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