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4.7 Qualified Expressions

[A qualified_expression is used to state explicitly the type, and to verify the subtype, of an operand that is either an expression or an aggregate. ]


qualified_expression ::= 
   subtype_mark'(expression) | subtype_mark'aggregate

Name Resolution Rules

The operand (the expression or aggregate) shall resolve to be of the type determined by the subtype_mark, or a universal type that covers it. 

Static Semantics

  {AI05-0003-1} [If the operand of a qualified_expression denotes an object, the qualified_expression denotes a constant view of that object.] The nominal subtype of a qualified_expression is the subtype denoted by the subtype_mark.
Proof: {AI05-0003-1} This is stated in 3.3

Dynamic Semantics

{AI12-0100-1} The evaluation of a qualified_expression evaluates the operand (and if of a universal type, converts it to the type determined by the subtype_mark) and checks that its value belongs to the subtype denoted by the subtype_mark. The exception Constraint_Error is raised if this check fails. Furthermore, if predicate checks are enabled for the subtype denoted by the subtype_mark, a check is performed as defined in subclause 3.2.4, “Subtype Predicates” that the value satifies the predicates of the subtype. 
Ramification: This is one of the few contexts in Ada 95 where implicit subtype conversion is not performed prior to a constraint check, and hence no “sliding” of array bounds is provided.
{AI12-0100-1} The effect of a failed predicate check is as defined in 3.2.4; such a check could raise any exception, not just Constraint_Error or Assertion_Error. 
Reason: Implicit subtype conversion is not provided because a qualified_expression with a constrained target subtype is essentially an assertion about the subtype of the operand, rather than a request for conversion. An explicit type_conversion can be used rather than a qualified_expression if subtype conversion is desired.
{AI12-0100-1} We do a predicate check here so that a qualified_expression never allows something that the equivalent type_conversion would not allow. 
23  When a given context does not uniquely identify an expected type, a qualified_expression can be used to do so. In particular, if an overloaded name or aggregate is passed to an overloaded subprogram, it might be necessary to qualify the operand to resolve its type. 


Examples of disambiguating expressions using qualification: 
type Mask is (Fix, Dec, Exp, Signif);
type Code is (Fix, Cla, Dec, Tnz, Sub);
Print (Mask'(Dec));  --  Dec is of type Mask
Print (Code'(Dec));  --  Dec is of type Code 
for J in Code'(Fix) .. Code'(Dec) loop ... -- qualification needed for either Fix or Dec
for J in Code range Fix .. Dec loop ...    -- qualification unnecessary
for J in Code'(Fix) .. Dec loop ...        -- qualification unnecessary for Dec
Dozen'(1 | 3 | 5 | 7 => 2, others => 0) -- see 4.6 

Wording Changes from Ada 2005

{AI05-0003-1} Added a definition of the nominal subtype of a qualified_expression.

Inconsistencies With Ada 2012

{AI12-0100-1} Corrigendum: A qualified_expression now performs a predicate check for the named subtype (if it is enabled). Original Ada 2012 did not include that check (an omission). While this is formally inconsistent (an exception could be raised when none would be raised by original Ada 2012), cases when this could be the case are likely to be rare (the qualified expression would have to have a stricter subtype than the following usage) and the check is more likely to detect bugs than be unexpected. 

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