[Ada] Crash on predicate involving qualified expression in instance

[Pierre-Marie de Rodat <derodat at adacore dot com>]

This patch inhibits the generation of freeze nodes when pre-analyzing the
domain of iteration of an Ada2012 loop that appears as a quantified
expression in a predicate for an array type. This prevents a back-end
abort on an invisible freeze node that would otherwise appear in an
unexpanded code sequence.

The following must compile quietly:

----
with Id_Manager;

package My_Id_Manager is new Id_Manager (Max_Id_Type   => 100_000,
                                         Max_Key_Count => 100);
----
generic
   Max_Id_Type   : Positive;
   Max_Key_Count : Positive;

package Id_Manager is
   type Unique_Id_Type is new Integer range 0 .. Max_Id_Type;

   Undefined_Id : constant Unique_Id_Type := 0;

   type Key_Count is new Integer range 0 .. Max_Key_Count;
   subtype Key_Index is Key_Count range 1 .. Key_Count'Last;

   type Key_Array is array (Key_Index range <>) of Unique_Id_Type
     with Predicate => Key_Array'First = 1;

   type Id_Manager_State (Capacity : Key_Count) is private;

   procedure Display_Objects (TheObject : Id_Manager_State);

private
   type Id_Manager_State (Capacity : Key_Count) is record
      Id_Key   : Key_Array (1 .. Capacity) := (others => Undefined_Id);
      Key_Size : Key_Count                 := 0;
   end record;
end Id_Manager;
----
package body Id_Manager is
   procedure Display_Objects (TheObject : Id_Manager_State) is
   begin
      for Item of TheObject.Id_Key loop
         null;
      end loop;
   end Display_Objects;
end Id_Manager;

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

2018-05-23  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_ch5.adb (Preanalyze_Range): The pre-analysis of the domain of
	iteration of an Ada2012 loop is performed to determine the type of the
	domain, but full analysis is performed once the loop is rewritten as a
	while-loop during expansion. The pre-analysis suppresses expansion; it
	must also suppress the generation of freeze nodes, which may otherwise
	appear in the wrong scope before rewritting.

--- gcc/ada/sem_ch5.adb
+++ gcc/ada/sem_ch5.adb
@@ -4076,6 +4076,17 @@ package body Sem_Ch5 is
       Full_Analysis := False;
       Expander_Mode_Save_And_Set (False);
 
+      --  In addition to the above we must ecplicity suppress the
+      --  generation of freeze nodes which might otherwise be generated
+      --  during resolution of the range (e.g. if given by an attribute
+      --  that will freeze its prefix).
+
+      Set_Must_Not_Freeze (R_Copy);
+
+      if Nkind (R_Copy) = N_Attribute_Reference then
+         Set_Must_Not_Freeze (Prefix (R_Copy));
+      end if;
+
       Analyze (R_Copy);
 
       if Nkind (R_Copy) in N_Subexpr and then Is_Overloaded (R_Copy) then