[Ada] Compiler loop on use of a faulty object in an address clause.

[Ada] Compiler loop on use of a faulty object in an address clause.

[Arnaud Charlet]

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This patch fixes a loop in the compiler when an address clause is used to
specify an overlay, and the overlaid object has an illegal object declaration
in which the expression is a premature reference to the object itself.

Compiling p.ads must yield:

  p.ads:3:33: object "Nowhere" cannot be used before end of its declaration

---
with system; use system;

---
with System; use System;
package P is
  Nowhere : constant Address := Nowhere;
  Thing : Integer;
  for Thing'Address use Nowhere;
end;

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

2017-05-02  Ed Schonberg  <schonberg@adacore.com>

	* sem_ch8.adb (Premature_Usage): If the premature usage of
	an entity is as the expression in its own object decaration,
	rewrite the reference as Any_Id to prevent cascaded errors or
	compiler loops when such an entity is used in an address clause.


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Index: sem_ch8.adb
===================================================================
--- sem_ch8.adb	(revision 247461)
+++ sem_ch8.adb	(working copy)
@@ -8562,6 +8562,14 @@
       else
          Error_Msg_N
            ("object& cannot be used before end of its declaration!", N);
+
+         --  If the premature reference appears as the expression in its own
+         --  declaration, rewrite it to prevent compiler loops in subsequent
+         --  uses of this mangled declaration in address clauses.
+
+         if Nkind (Parent (N)) = N_Object_Declaration then
+            Set_Entity (N, Any_Id);
+         end if;
       end if;
    end Premature_Usage;