Index: common/tech.c
===================================================================
RCS file: /home/freeciv/CVS/freeciv/common/tech.c,v
retrieving revision 1.49
diff -u -r1.49 tech.c
--- common/tech.c	2002/08/07 11:21:48	1.49
+++ common/tech.c	2002/10/10 13:23:42
@@ -221,11 +221,19 @@
 **************************************************************************/
 Tech_Type_id get_next_tech(struct player *pplayer, Tech_Type_id goal)
 {
+  Tech_Type_id result;
+
   if (goal == A_NONE || !tech_exists(goal) ||
       get_invention(pplayer, goal) == TECH_KNOWN) {
     return A_NONE;
   }
-  return (get_next_tech_rec(pplayer, goal));
+
+  result = get_next_tech_rec(pplayer, goal);
+  if(result != A_NONE) {
+    assert(get_invention(pplayer, result) == TECH_REACHABLE);
+  }
+
+  return result;
 }
 
 /**************************************************************************
Index: server/plrhand.c
===================================================================
RCS file: /home/freeciv/CVS/freeciv/server/plrhand.c,v
retrieving revision 1.246
diff -u -r1.246 plrhand.c
--- server/plrhand.c	2002/10/09 14:10:17	1.246
+++ server/plrhand.c	2002/10/10 13:23:43
@@ -55,6 +55,8 @@
                                 struct player *receiver,
                                 enum plr_info_level min_info_level);
 
+static bool choose_goal_tech(struct player *plr);
+
 /**************************************************************************
 ...
 **************************************************************************/
@@ -270,7 +272,7 @@
 
     int saved_bulbs = plr->research.bulbs_researched;
 
-    if (choose_goal_tech(plr) != 0) {
+    if (choose_goal_tech(plr)) {
       notify_player_ex(plr, -1, -1, E_TECH_LEARNED,
 		       _("Game: Learned %s.  "
 			 "Our scientists focus on %s, goal is %s."),
@@ -304,6 +306,9 @@
     }
   }
 
+  assert(is_future_tech(plr->research.researching) || 
+	 get_invention(plr, plr->research.researching) == TECH_REACHABLE);
+
   if (bonus_tech_hack) {
     if (advances[tech_found].bonus_message) {
       notify_player(plr, _("Game: %s"),
@@ -417,11 +422,13 @@
 }
 
 /**************************************************************************
-...
+  Choose the next tech leading to ai.tech_goal and set it in
+  research.researching if found. Update ai.tech_goal if
+  required. Return FALSE if no tech could be found.
 **************************************************************************/
-int choose_goal_tech(struct player *plr)
+static bool choose_goal_tech(struct player *plr)
 {
-  int sub_goal;
+  Tech_Type_id sub_goal;
 
   if (plr->research.bulbs_researched > 0) {
     plr->research.bulbs_researched = 0;
@@ -429,8 +436,10 @@
   if (plr->ai.control) {
     ai_next_tech_goal(plr); /* tech-AI has been changed */
     sub_goal = get_next_tech(plr, plr->ai.tech_goal); /* should be changed */
-  } else sub_goal = get_next_tech(plr, plr->ai.tech_goal);
-  if (sub_goal == 0) {
+  } else {
+    sub_goal = get_next_tech(plr, plr->ai.tech_goal);
+  }
+  if (sub_goal == A_NONE) {
     if (plr->ai.control || plr->research.techs_researched == 1) {
       ai_next_tech_goal(plr);
       sub_goal = get_next_tech(plr, plr->ai.tech_goal);
@@ -439,10 +448,10 @@
     }
   }
 
-  if (sub_goal != 0) {
-    plr->research.researching=sub_goal;
+  if (sub_goal != A_NONE) {
+    plr->research.researching = sub_goal;
   }   
-  return sub_goal;
+  return sub_goal != A_NONE;
 }
 
 
Index: server/plrhand.h
===================================================================
RCS file: /home/freeciv/CVS/freeciv/server/plrhand.h,v
retrieving revision 1.50
diff -u -r1.50 plrhand.h
--- server/plrhand.h	2002/10/09 20:54:20	1.50
+++ server/plrhand.h	2002/10/10 13:23:43
@@ -78,7 +78,6 @@
 void choose_random_tech(struct player *plr);
 void choose_tech(struct player *plr, int tech);
 void choose_tech_goal(struct player *plr, int tech);
-int choose_goal_tech(struct player *plr);
 void get_a_tech(struct player *pplayer, struct player *target);
 
 void send_player_turn_notifications(struct conn_list *dest);