[Ada] More precise propagation of Size attribute in generic instances

GNATprove analyzer for SPARK code depends on the frontend to accurately
propagate the known value of Size attribute. This was not done for
formal type parameters in generic instantiations. Now fixed.

There is no impact on compilation.

2019-08-21  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_ch3.adb (Analyze_Subtype_Declaration): Inherit RM_Size
	field for formal type parameters in generic instantiations.

From-SVN: r274788

gcc/ada/ChangeLog

 2019-08-21  Yannick Moy  <moy@adacore.com>
 
+	* sem_ch3.adb (Analyze_Subtype_Declaration): Inherit RM_Size
+	field for formal type parameters in generic instantiations.
+
+2019-08-21  Yannick Moy  <moy@adacore.com>
+
 	* sem_spark.adb: Update references to the SPARK RM.
 
 2019-08-21  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/sem_ch3.adb

@@ -5555,6 +5555,14 @@ package body Sem_Ch3 is
             =>
                Set_Ekind                (Id, E_Record_Subtype);
 
+               --  Subtype declarations introduced for formal type parameters
+               --  in generic instantiations should inherit the Size value of
+               --  the type they rename.
+
+               if Present (Generic_Parent_Type (N)) then
+                  Set_RM_Size           (Id, RM_Size (T));
+               end if;
+
                if Ekind (T) = E_Record_Subtype
                  and then Present (Cloned_Subtype (T))
                then