[Ada] Fix compiler abort on invalid discriminant constraint

This patch fixes a compiler abort on a discriminant constraint when the
constraint is a subtype indication.

2018-05-22  Patrick Bernardi  <bernardi@adacore.com>

gcc/ada/

	* sem_ch3.adb (Build_Discriminant_Constraints): Raise an error if the
	user tries to use a subtype indication as a discriminant constraint.

gcc/testsuite/

	* gnat.dg/discr50.adb: New testcase.

From-SVN: r260525

gcc/ada/ChangeLog

+2018-05-22  Patrick Bernardi  <bernardi@adacore.com>
+
+	* sem_ch3.adb (Build_Discriminant_Constraints): Raise an error if the
+	user tries to use a subtype indication as a discriminant constraint.
+
 2018-05-22  Ed Schonberg  <schonberg@adacore.com>
 
 	* exp_ch4.ads, exp_ch4.adb, exp_util.adb, expander.adb: Remove mention

gcc/ada/sem_ch3.adb

@@ -9877,6 +9877,12 @@ package body Sem_Ch3 is
               ("a range is not a valid discriminant constraint", Constr);
             Discr_Expr (D) := Error;
 
+         elsif Nkind (Constr) = N_Subtype_Indication then
+            Error_Msg_N
+              ("a subtype indication is not a valid discriminant constraint",
+               Constr);
+            Discr_Expr (D) := Error;
+
          else
             Process_Discriminant_Expression (Constr, Discr);
             Discr_Expr (D) := Constr;

gcc/testsuite/ChangeLog

+2018-05-22  Patrick Bernardi  <bernardi@adacore.com>
+
+	* gnat.dg/discr50.adb: New testcase.
+
 2018-05-22  Ed Schonberg  <schonberg@adacore.com>
 
 	* gnat.dg/discr49.adb, gnat.dg/discr49_rec1.adb,

gcc/testsuite/gnat.dg/discr50.adb

+--  { dg-do compile }
+
+procedure Discr50 is
+   type My_Record (D : Integer) is record
+      A : Integer;
+   end record;
+
+   B : My_Record (Positive range 1 .. 10);  -- { dg-error "a subtype indication is not a valid discriminant constraint" }
+begin
+   null;
+end Discr50;