[Ada] Missing error on T'Enum_Rep with no parameter

[Arnaud Charlet]

This patch fixes a bug in which the compiler fails to give an error on
T'Enum_Rep, where T is a type. If X is an object of enumeration type T,
then X'Enum_Rep and T'Enum_Rep(X) are allowed, but not T'Enum_Rep.

The following test must get an error.

enum_val_test.adb:4:21: prefix of "Enum_Rep" attribute must be discrete object

procedure Enum_Val_Test is
   type Color is (Red, Orange, Yellow);
   Current_Index : Color := Orange;
   Rep : Integer := Color'Enum_Rep; -- Illegal!
begin
   null;
end Enum_Val_Test;

Tested on x86_64-pc-linux-gnu, committed on trunk

2017-05-02  Bob Duff  <d...@adacore.com>

        * sem_attr.adb (Attribute_Enum_Rep): Disallow T'Enum_Rep.


Index: sem_attr.adb
===================================================================
--- sem_attr.adb        (revision 247479)
+++ sem_attr.adb        (working copy)
@@ -3763,13 +3763,23 @@
       --------------
 
       when Attribute_Enum_Rep =>
+         --  T'Enum_Rep (X) case
+
          if Present (E1) then
             Check_E1;
             Check_Discrete_Type;
             Resolve (E1, P_Base_Type);
 
-         elsif not Is_Discrete_Type (Etype (P)) then
-            Error_Attr_P ("prefix of % attribute must be of discrete type");
+         --  X'Enum_Rep case.  X must be an object or enumeration literal, and
+         --  it must be of a discrete type.
+
+         elsif not ((Is_Object_Reference (P)
+                       or else (Is_Entity_Name (P)
+                                  and then Ekind (Entity (P)) =
+                                             E_Enumeration_Literal))
+                    and then Is_Discrete_Type (Etype (P)))
+         then
+            Error_Attr_P ("prefix of % attribute must be discrete object");
          end if;
 
          Set_Etype (N, Universal_Integer);