fix(library/unifier): missing justification when updating choice constraints

The bug was not producing incorrect results, but really bad error
messages.
See: empty.lean.expected.out

src/library/unifier.cpp

@@ -442,7 +442,8 @@ struct unifier_fn {
         if (m_num_steps > m_config.m_max_steps)
             throw exception(sstream() << "unifier maximum number of steps (" << m_config.m_max_steps << ") exceeded, " <<
                             "the maximum number of steps can be increased by setting the option unifier.max_steps " <<
-                            "(remark: the unifier uses higher order unification and unification-hints, which may trigger non-termination");
+                            "(remark: the unifier uses higher order unification and unification-hints, " <<
+                            "which may trigger non-termination");
         m_num_steps++;
     }
 
@@ -1025,7 +1026,7 @@ struct unifier_fn {
                     set_conflict(c.get_justification());
                     return false;
                 }
-                if (!process_constraints(cs))
+                if (!process_constraints(cs, c.get_justification()))
                     return false;
                 type = *t;
                 m_type_map.insert(m, mk_pair(type, justification()));
@@ -1038,7 +1039,7 @@ struct unifier_fn {
                 // and store the new type in m_type_map
                 jst  = mk_composite1(jst, type_jst.second);
                 type = type_jst.first;
-                c    = update_justification(c, jst);
+                c    = update_justification(c, mk_composite1(c.get_justification(), jst));
                 m_type_map.insert(m, mk_pair(type, jst));
             }
             unsigned cidx = add_cnstr(c, cnstr_group::ClassInstance);

tests/lean/cls_err.lean

@@ -0,0 +1,14 @@
+import logic
+
+inductive H [class] (A : Type) :=
+mk : A → H A
+
+definition foo {A : Type} {h : H A} : A :=
+H.rec (λa, a) h
+
+section
+  parameter A : Type
+  parameter h : H A
+  definition tst : A :=
+  foo
+end

tests/lean/cls_err.lean.expected.out

@@ -0,0 +1,2 @@
+cls_err.lean:13:2: error: failed to synthesize placeholder
+A : Type ⊢ H A

tests/lean/empty.lean.expected.out

@@ -1,11 +1,2 @@
-empty.lean:6:25: error: type error in placeholder assigned to
-  Empty
-placeholder has type
-  Type.{1}
-but is expected to have type
-  Type.{?M_1}
-the assignment was attempted when trying to solve
-    type mismatch at definition 'v2', has type
-      Empty
-    but is expected to have type
-      Empty
+empty.lean:6:25: error: failed to synthesize placeholder
+ ⊢ nonempty Empty