2.177 Pragma Unevaluated_Use_Of_Old

Syntax:

    pragma Unevaluated_Use_Of_Old (Error | Warn | Allow);

This pragma controls the processing of attributes Old and Loop_Entry. If either of these attributes is used in a potentially unevaluated expression (e.g. the then or else parts of an if expression), then normally this usage is considered illegal if the prefix of the attribute is other than an entity name. The language requires this behavior for Old, and GNAT copies the same rule for Loop_Entry.

The reason for this rule is that otherwise, we can have a situation where we save the Old value, and this results in an exception, even though we might not evaluate the attribute. Consider this example:

    package UnevalOld is
       K : Character;
       procedure U (A : String; C : Boolean)  -- ERROR
         with Post => (if C then A(1)'Old = K else True);
    end;

If procedure U is called with a string with a lower bound of 2, and C false, then an exception would be raised trying to evaluate A(1) on entry even though the value would not be actually used.

Although the rule guarantees against this possibility, it is sometimes too restrictive. For example if we know that the string has a lower bound of 1, then we will never raise an exception. The pragma Unevaluated_Use_Of_Old can be used to modify this behavior. If the argument is Error then an error is given (this is the default RM behavior). If the argument is Warn then the usage is allowed as legal but with a warning that an exception might be raised. If the argument is Allow then the usage is allowed as legal without generating a warning.

This pragma may appear as a configuration pragma, or in a declarative part or package specification. In the latter case it applies to uses up to the end of the corresponding statement sequence or sequence of package declarations.