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3.10.1 Incomplete Type Declarations

There are no particular limitations on the designated type of an access type. In particular, the type of a component of the designated type can be another access type, or even the same access type. This permits mutually dependent and recursive access types. An incomplete_type_declaration can be used to introduce a type to be used as a designated type, while deferring its full definition to a subsequent full_type_declaration.
       incomplete_type_declaration ::= type defining_identifier [discriminant_part];
Legality Rules
An incomplete_type_declaration requires a completion, which shall be a full_type_declaration. If the incomplete_type_declaration occurs immediately within either the visible part of a package_specification or a declarative_part, then the full_type_declaration shall occur later and immediately within this visible part or declarative_part. If the incomplete_type_declaration occurs immediately within the private part of a given package_specification, then the full_type_declaration shall occur later and immediately within either the private part itself, or the declarative_part of the corresponding package_body.
If an incomplete_type_declaration has a known_discriminant_part, then a full_type_declaration that completes it shall have a fully conforming (explicit) known_discriminant_part (see 6.3.1). If an incomplete_type_declaration has no discriminant_part (or an unknown_discriminant_part), then a corresponding full_type_declaration is nevertheless allowed to have discriminants, either explicitly, or inherited via derivation.
The only allowed uses of a name that denotes an incomplete_type_declaration are as follows:
A dereference (whether implicit or explicit -- see 4.1) shall not be of an incomplete type.
Static Semantics
An incomplete_type_declaration declares an incomplete type and its first subtype; the first subtype is unconstrained if a known_discriminant_part appears.
Dynamic Semantics
The elaboration of an incomplete_type_declaration has no effect.

Example of a recursive type:
       type Cell;  --  incomplete type declaration
       type Link is access Cell;
       type Cell is
             Value  : Integer;
             Succ   : Link;
             Pred   : Link;
          end record;
       Head   : Link  := new Cell'(0, null, null);
       Next   : Link  := Head.Succ;
Examples of mutually dependent access types:
       type Person(<>);    -- incomplete type declaration
       type Car;           -- incomplete type declaration
       type Person_Name is access Person;
       type Car_Name    is access all Car;
       type Car is
             Number  : Integer;
             Owner   : Person_Name;
          end record;
       type Person(Sex : Gender) is
             Name     : String(1 .. 20);
             Birth    : Date;
             Age      : Integer range 0 .. 130;
             Vehicle  : Car_Name;
             case Sex is
                when M => Wife           : Person_Name(Sex => F);
                when F => Husband        : Person_Name(Sex => M);
             end case;
          end record;
       My_Car, Your_Car, Next_Car : Car_Name := new Car;  -- see 4.8
       George : Person_Name := new Person(M);
       George.Vehicle := Your_Car;

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