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3.7.2 Operations of Discriminated Types

If a discriminated type has default_expressions for its discriminants, then unconstrained variables of the type are permitted, and the discriminants of such a variable can be changed by assignment to the variable. For a formal parameter of such a type, an attribute is provided to determine whether the corresponding actual parameter is constrained or unconstrained. 

Static Semantics

For a prefix A that is of a discriminated type (after any implicit dereference), the following attribute is defined: 
Yields the value True if A denotes a constant, a value, a tagged object, or a constrained variable, and False otherwise. 

Erroneous Execution

The execution of a construct is erroneous if the construct has a constituent that is a name denoting a subcomponent that depends on discriminants, and the value of any of these discriminants is changed by this execution between evaluating the name and the last use (within this execution) of the subcomponent denoted by the name

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