pragma Extensions_Allowed (On | Off);
This configuration pragma enables or disables the implementation extension mode (the use of Off as a parameter cancels the effect of the -gnatX command switch).
In extension mode, the latest version of the Ada language is implemented (currently Ada 2012), and in addition a small number of GNAT specific extensions are recognized as follows:
Constrained attribute is permitted for objects of
generic types. The result indicates if the corresponding actual
is constrained.