Tweaks to Constraints operators poll functions.

Mainly:
* Make `ED_operator_object_active_editable_ex` properly report poll
  messages on failure.
* Add `ED_operator_object_active_local_editable_posemode_exclusive` for
  bone constraints requiring pure local Object (non-override one).
* General cleanup and adding more poll messages on failures.

source/blender/editors/include/ED_screen.h

@@ -354,6 +354,7 @@ bool ED_operator_uvedit(struct bContext *C);
 bool ED_operator_uvedit_space_image(struct bContext *C);
 bool ED_operator_uvmap(struct bContext *C);
 bool ED_operator_posemode_exclusive(struct bContext *C);
+bool ED_operator_object_active_local_editable_posemode_exclusive(struct bContext *C);
 bool ED_operator_posemode_context(struct bContext *C);
 bool ED_operator_posemode(struct bContext *C);
 bool ED_operator_posemode_local(struct bContext *C);

source/blender/editors/object/object_constraint.c

@@ -696,12 +696,11 @@ static bool edit_constraint_poll_generic(bContext *C,
   Object *ob = (ptr.owner_id) ? (Object *)ptr.owner_id : ED_object_active_context(C);
   bConstraint *con = ptr.data;
 
-  if (!ob) {
-    CTX_wm_operator_poll_msg_set(C, "Context missing active object");
+  if (!ED_operator_object_active_editable_ex(C, ob)) {
     return false;
   }
 
-  if (ID_IS_LINKED(ob) || (ptr.owner_id && ID_IS_LINKED(ptr.owner_id))) {
+  if (ptr.owner_id != NULL && ID_IS_LINKED(ptr.owner_id)) {
     CTX_wm_operator_poll_msg_set(C, "Cannot edit library data");
     return false;
   }
@@ -1746,8 +1745,8 @@ void POSE_OT_constraints_clear(wmOperatorType *ot)
 
   /* callbacks */
   ot->exec = pose_constraints_clear_exec;
-  ot->poll = ED_operator_posemode_exclusive; /* XXX - do we want to ensure there are selected
-                                              * bones too? */
+  /* XXX - do we want to ensure there are selected bones too? */
+  ot->poll = ED_operator_object_active_local_editable_posemode_exclusive;
 }
 
 static int object_constraints_clear_exec(bContext *C, wmOperator *UNUSED(op))
@@ -2480,7 +2479,7 @@ void POSE_OT_ik_clear(wmOperatorType *ot)
 
   /* api callbacks */
   ot->exec = pose_ik_clear_exec;
-  ot->poll = ED_operator_posemode_exclusive;
+  ot->poll = ED_operator_object_active_local_editable_posemode_exclusive;
 
   /* flags */
   ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;

source/blender/editors/screen/screen_ops.c

@@ -358,9 +358,24 @@ bool ED_operator_object_active(bContext *C)
   return ((ob != NULL) && !ed_object_hidden(ob));
 }
 
-bool ED_operator_object_active_editable_ex(bContext *UNUSED(C), const Object *ob)
+bool ED_operator_object_active_editable_ex(bContext *C, const Object *ob)
 {
-  return ((ob != NULL) && !ID_IS_LINKED(ob) && !ed_object_hidden(ob));
+  if (ob == NULL) {
+    CTX_wm_operator_poll_msg_set(C, "Context missing active object");
+    return false;
+  }
+
+  if (ID_IS_LINKED(ob)) {
+    CTX_wm_operator_poll_msg_set(C, "Cannot edit library linked object");
+    return false;
+  }
+
+  if (ed_object_hidden(ob)) {
+    CTX_wm_operator_poll_msg_set(C, "Cannot edit hidden obect");
+    return false;
+  }
+
+  return true;
 }
 
 bool ED_operator_object_active_editable(bContext *C)
@@ -444,28 +459,48 @@ bool ED_operator_editarmature(bContext *C)
 }
 
 /**
- * \brief check for pose mode (no mixed modes)
+ * Check for pose mode (no mixed modes).
  *
- * We want to enable most pose operations in weight paint mode,
- * when it comes to transforming bones, but managing bones layers/groups
- * can be left for pose mode only. (not weight paint mode)
+ * We want to enable most pose operations in weight paint mode, when it comes to transforming
+ * bones, but managing bones layers/groups and their constraints can be left for pose mode only
+ * (not weight paint mode).
  */
-bool ED_operator_posemode_exclusive(bContext *C)
+static bool ed_operator_posemode_exclusive_ex(bContext *C, Object *obact)
 {
-  Object *obact = CTX_data_active_object(C);
-
-  if (obact && !(obact->mode & OB_MODE_EDIT)) {
-    Object *obpose = BKE_object_pose_armature_get(obact);
-    if (obpose != NULL) {
-      if (obact == obpose) {
-        return true;
-      }
+  if (obact != NULL && !(obact->mode & OB_MODE_EDIT)) {
+    if (obact == BKE_object_pose_armature_get(obact)) {
+      return true;
     }
   }
 
+  CTX_wm_operator_poll_msg_set(C, "No object, or not exclusively in pose mode");
   return false;
 }
 
+bool ED_operator_posemode_exclusive(bContext *C)
+{
+  Object *obact = ED_object_active_context(C);
+
+  return ed_operator_posemode_exclusive_ex(C, obact);
+}
+
+/** Object must be editable, fully local (i.e. not an override), and exclusively in Pose mode. */
+bool ED_operator_object_active_local_editable_posemode_exclusive(bContext *C)
+{
+  Object *obact = ED_object_active_context(C);
+
+  if (!ed_operator_posemode_exclusive_ex(C, obact)) {
+    return false;
+  }
+
+  if (ID_IS_OVERRIDE_LIBRARY(obact)) {
+    CTX_wm_operator_poll_msg_set(C, "Object is a local library override");
+    return false;
+  }
+
+  return true;
+}
+
 /* allows for pinned pose objects to be used in the object buttons
  * and the non-active pose object to be used in the 3D view */
 bool ED_operator_posemode_context(bContext *C)