This patch fixes a bug in which a spurious error is given on an
aggregate of a type derived from a subtype with a constrained
discriminant.

2019-08-14  Bob Duff  <duff@adacore.com>

gcc/ada/

	* exp_aggr.adb (Init_Hidden_Discriminants): Avoid processing the
	wrong discriminant, which could be of the wrong type.

gcc/testsuite/

	* gnat.dg/discr57.adb: New testcase.

From-SVN: r274458