-- output:
-- $ ./obj/main
-- Original.
-- $ ./obj2/main
-- Replacement.

-- test.gpr
project Test is
   for Source_Dirs use ("src");
   for Main use ("main.adb");
   for Object_Dir use "obj";
end Test;
-- test2.gpr
project Test2 extends "test" is
   for Source_Dirs use ("replacement");
   for Source_Files use ("foo.adb");
   for Object_Dir use "obj2";
end Test2;

-- src/foo.adb
with Ada.Text_IO;

package body Foo is
   procedure Foobar is
   begin
      Ada.Text_IO.Put_Line ("Original.");
   end Foobar;
end Foo;
-- src/foo.ads
package Foo is
   procedure Foobar;
end Foo;
-- src/main.adb
with Foo;

procedure Main is
begin
   Foo.Foobar;
end Main;

-- replacement/foo.adb
with Ada.Text_IO;

package body Foo is
   procedure Foobar is
   begin
      Ada.Text_IO.Put_Line ("Replacement.");
   end Foobar;
end Foo;