diff --git a/src/library/class_instance_resolution.cpp b/src/library/class_instance_resolution.cpp
index 3f985745c..fc0d3dd6c 100644
--- a/src/library/class_instance_resolution.cpp
+++ b/src/library/class_instance_resolution.cpp
@@ -160,6 +160,67 @@ struct cienv {
         virtual void pop() { m_cienv.m_state = m_stack.back(); m_stack.pop_back(); }
         virtual void commit() { m_stack.pop_back(); }
 
+        static bool has_meta_arg(expr e) {
+            while (is_app(e)) {
+                if (is_meta(app_arg(e)))
+                    return true;
+                e = app_fn(e);
+            }
+            return false;
+        }
+
+        /** IF \c e is of the form (f ... (?m t_1 ... t_n) ...) where ?m is an unassigned
+            metavariable whose type is a type class, and (?m t_1 ... t_n) must be synthesized
+            by type class resolution, then we return ?m.
+            Otherwise, we return none */
+        optional<expr> find_unsynth_metavar(expr const & e) {
+            if (!has_meta_arg(e))
+                return none_expr();
+            buffer<expr> args;
+            expr const & fn = get_app_args(e, args);
+            expr type       = m_cienv.infer_type(fn);
+            unsigned i      = 0;
+            while (i < args.size()) {
+                type = m_cienv.whnf(type);
+                if (!is_pi(type))
+                    return none_expr();
+                expr const & arg = args[i];
+                if (binding_info(type).is_inst_implicit() && is_meta(arg)) {
+                    expr const & m = get_app_fn(arg);
+                    if (is_mvar(m)) {
+                        expr m_type = instantiate_uvars_mvars(infer_metavar(m));
+                        if (!has_expr_metavar_relaxed(m_type)) {
+                            return some_expr(m);
+                        }
+                    }
+                }
+                type = instantiate(binding_body(type), arg);
+                i++;
+            }
+            return none_expr();
+        }
+
+        bool mk_instance(expr const & m) {
+            lean_assert(m);
+            std::cout << "\n\nFOUND CANDIDATE: " << m << "\n\n";
+            // TODO(Leo)
+            return false;
+        }
+
+        virtual bool on_is_def_eq_failure(expr const & e1, expr const & e2) {
+            if (is_app(e1) && is_app(e2)) {
+                if (auto m1 = find_unsynth_metavar(e1)) {
+                    if (mk_instance(*m1))
+                        return true;
+                }
+                if (auto m2 = find_unsynth_metavar(e2)) {
+                    if (mk_instance(*m2))
+                        return true;
+                }
+            }
+            return false;
+        }
+
         virtual bool ignore_universe_def_eq(level const & l1, level const & l2) const {
             if (is_meta(l1) || is_meta(l2)) {
                 // The unifier may invoke this module before universe metavariables in the class
diff --git a/src/library/type_inference.cpp b/src/library/type_inference.cpp
index 0bb5270ee..a714ffdd8 100644
--- a/src/library/type_inference.cpp
+++ b/src/library/type_inference.cpp
@@ -498,14 +498,18 @@ bool type_inference::is_def_eq_binding(expr e1, expr e2) {
                           instantiate_rev(e2, subst.size(), subst.data()));
 }
 
-bool type_inference::is_def_eq_args(expr t, expr s) {
-    while (is_app(t) && is_app(s)) {
-        if (!is_def_eq_core(app_arg(t), app_arg(s)))
+bool type_inference::is_def_eq_args(expr const & e1, expr const & e2) {
+    lean_assert(is_app(e1) && is_app(e2));
+    buffer<expr> args1, args2;
+    get_app_args(e1, args1);
+    get_app_args(e2, args2);
+    if (args1.size() != args2.size())
+        return false;
+    for (unsigned i = 0; i < args1.size(); i++) {
+        if (!is_def_eq_core(args1[i], args2[i]))
             return false;
-        t = app_fn(t);
-        s = app_fn(s);
     }
-    return !is_app(t) && !is_app(s);
+    return true;
 }
 
 bool type_inference::is_def_eq_eta(expr const & e1, expr const & e2) {
@@ -745,7 +749,10 @@ bool type_inference::is_def_eq_core(expr const & t, expr const & s) {
     if (is_def_eq_proof_irrel(t_n, s_n))
         return true;
 
-    return false;
+    if (on_is_def_eq_failure(t_n, s_n))
+        return is_def_eq_core(t_n, s_n);
+    else
+        return false;
 }
 
 bool type_inference::is_def_eq(expr const & e1, expr const & e2) {
diff --git a/src/library/type_inference.h b/src/library/type_inference.h
index ee9de4069..b62df50da 100644
--- a/src/library/type_inference.h
+++ b/src/library/type_inference.h
@@ -42,7 +42,7 @@ class type_inference {
 
     lbool quick_is_def_eq(expr const & e1, expr const & e2);
     bool is_def_eq_core(expr const & e1, expr const & e2);
-    bool is_def_eq_args(expr t, expr s);
+    bool is_def_eq_args(expr const & e1, expr const & e2);
     bool is_def_eq_binding(expr e1, expr e2);
     bool is_def_eq_eta(expr const & e1, expr const & e2);
     bool is_def_eq_proof_irrel(expr const & e1, expr const & e2);
@@ -141,6 +141,11 @@ public:
     /** \brief Keep the changes since last push */
     virtual void commit() = 0;
 
+    /** \brief This method is invoked before failure.
+        The "customer" may try to assign unassigned mvars in the given expression.
+        The result is true to indicate that some metavariable has been assigned. */
+    virtual bool on_is_def_eq_failure(expr const &, expr const &) { return false; }
+
     bool is_assigned(level const & u) const { return get_assignment(u) != nullptr; }
     bool is_assigned(expr const & m) const { return get_assignment(m) != nullptr; }