diff --git a/src/frontends/lean/decl_attributes.cpp b/src/frontends/lean/decl_attributes.cpp
index f05fe22c0..480d07486 100644
--- a/src/frontends/lean/decl_attributes.cpp
+++ b/src/frontends/lean/decl_attributes.cpp
@@ -114,8 +114,6 @@ void decl_attributes::parse(buffer<name> const & ns, parser & p) {
             m_is_quasireducible = true;
             p.next();
         } else if (p.curr_is_token(get_class_tk())) {
-            if (m_def_only)
-                throw parser_error("invalid '[class]' attribute, definitions cannot be marked as classes", pos);
             m_is_class = true;
             p.next();
         } else if (p.curr_is_token(get_multiple_instances_tk())) {
diff --git a/src/library/class.cpp b/src/library/class.cpp
index 74f464453..e51056163 100644
--- a/src/library/class.cpp
+++ b/src/library/class.cpp
@@ -151,9 +151,7 @@ template class scoped_ext<class_config>;
 typedef scoped_ext<class_config> class_ext;
 
 static void check_class(environment const & env, name const & c_name) {
-    declaration c_d = env.get(c_name);
-    if (c_d.is_definition())
-        throw exception(sstream() << "invalid class, '" << c_name << "' is a definition");
+    env.get(c_name);
 }
 
 static void check_is_class(environment const & env, name const & c_name) {
@@ -182,12 +180,25 @@ void get_classes(environment const & env, buffer<name> & classes) {
         });
 }
 
+bool is_class(environment const & env, name const & c) {
+    class_state const & s = class_ext::get_state(env);
+    return s.m_instances.contains(c);
+}
+
+type_checker_ptr mk_class_type_checker(environment const & env, name_generator && ngen, bool conservative) {
+    auto pred = conservative ? mk_not_reducible_pred(env) : mk_irreducible_pred(env);
+    class_state s = class_ext::get_state(env);
+    return mk_type_checker(env, std::move(ngen), [=](name const & n) {
+            return s.m_instances.contains(n) || pred(n);
+        });
+}
+
 static name * g_tmp_prefix = nullptr;
 environment add_instance(environment const & env, name const & n, unsigned priority, bool persistent) {
     declaration d = env.get(n);
     expr type = d.get_type();
     name_generator ngen(*g_tmp_prefix);
-    auto tc = mk_type_checker(env, ngen.mk_child());
+    auto tc = mk_class_type_checker(env, ngen.mk_child(), false);
     while (true) {
         type = tc->whnf(type).first;
         if (!is_pi(type))
@@ -213,11 +224,6 @@ bool try_multiple_instances(environment const & env, name const & n) {
     return s.try_multiple_instances(n);
 }
 
-bool is_class(environment const & env, name const & c) {
-    class_state const & s = class_ext::get_state(env);
-    return s.m_instances.contains(c);
-}
-
 bool is_instance(environment const & env, name const & i) {
     class_state const & s = class_ext::get_state(env);
     return s.is_instance(i);
@@ -229,7 +235,7 @@ list<name> get_class_instances(environment const & env, name const & c) {
 }
 
 /** \brief If the constant \c e is a class, return its name */
-optional<name> constant_is_ext_class(environment const & env, expr const & e) {
+static optional<name> constant_is_ext_class(environment const & env, expr const & e) {
     name const & cls_name = const_name(e);
     if (is_class(env, cls_name)) {
         return optional<name>(cls_name);
@@ -243,7 +249,7 @@ optional<name> constant_is_ext_class(environment const & env, expr const & e) {
     l_false:  \c type is not a class.
     l_undef:  procedure did not establish whether \c type is a class or not.
 */
-lbool is_quick_ext_class(type_checker const & tc, expr const & type, name & result) {
+static lbool is_quick_ext_class(type_checker const & tc, expr const & type, name & result) {
     environment const & env = tc.env();
     expr const * it         = &type;
     while (true) {
@@ -287,7 +293,7 @@ lbool is_quick_ext_class(type_checker const & tc, expr const & type, name & resu
 }
 
 /** \brief Full/Expensive test for \c is_ext_class */
-optional<name> is_full_ext_class(type_checker & tc, expr type) {
+static optional<name> is_full_ext_class(type_checker & tc, expr type) {
     type = tc.whnf(type).first;
     if (is_pi(type)) {
         return is_full_ext_class(tc, instantiate(binding_body(type), mk_local(tc.mk_fresh_name(), binding_domain(type))));
diff --git a/src/library/class.h b/src/library/class.h
index 7149f0327..fb67ae289 100644
--- a/src/library/class.h
+++ b/src/library/class.h
@@ -5,8 +5,10 @@ Released under Apache 2.0 license as described in the file LICENSE.
 Author: Leonardo de Moura
 */
 #pragma once
-
+#include "library/util.h"
 namespace lean {
+/** \brief Create type checker that treats classes as opaque constants */
+type_checker_ptr mk_class_type_checker(environment const & env, name_generator && ngen, bool conservative);
 /** \brief Add a new 'class' to the environment (if it is not already declared) */
 environment add_class(environment const & env, name const & n, bool persistent = true);
 /** \brief Add a new 'class instance' to the environment with default priority. */
diff --git a/src/library/class_instance_synth.cpp b/src/library/class_instance_synth.cpp
index 79f73b9e7..f89bc7472 100644
--- a/src/library/class_instance_synth.cpp
+++ b/src/library/class_instance_synth.cpp
@@ -113,10 +113,7 @@ struct class_instance_context {
         m_trace_instances = get_class_trace_instances(ios.get_options());
         m_max_depth       = get_class_instance_max_depth(ios.get_options());
         m_conservative    = get_class_conservative(ios.get_options());
-        if (m_conservative)
-            m_tc = mk_type_checker(env, m_ngen.mk_child(), UnfoldReducible);
-        else
-            m_tc = mk_type_checker(env, m_ngen.mk_child());
+        m_tc              = mk_class_type_checker(env, m_ngen.mk_child(), m_conservative);
         options opts      = m_ios.get_options();
         opts              = opts.update_if_undef(get_pp_purify_metavars_name(), false);
         opts              = opts.update_if_undef(get_pp_implicit_name(), true);
diff --git a/tests/lean/bad_class.lean.expected.out b/tests/lean/bad_class.lean.expected.out
index 7c8f87591..e69de29bb 100644
--- a/tests/lean/bad_class.lean.expected.out
+++ b/tests/lean/bad_class.lean.expected.out
@@ -1,2 +0,0 @@
-bad_class.lean:4:0: error: invalid class, 'foo.subsingleton' is a definition
-bad_class.lean:6:0: error: 'eq' is not a class
diff --git a/tests/lean/run/679a.lean b/tests/lean/run/679a.lean
new file mode 100644
index 000000000..89f28a518
--- /dev/null
+++ b/tests/lean/run/679a.lean
@@ -0,0 +1,22 @@
+import data.finset
+open bool nat list finset
+
+attribute finset [class]
+
+definition fin_nat [instance] : finset nat :=
+to_finset [0]
+
+definition fin_bool [instance] : finset bool :=
+to_finset [tt, ff]
+
+definition tst (A : Type) [s : finset A] : finset A :=
+s
+
+example : tst nat = to_finset [0] :=
+rfl
+
+example : tst bool = to_finset [ff, tt] :=
+dec_trivial
+
+example : tst bool = to_finset [tt, ff] :=
+rfl
diff --git a/tests/lean/run/679b.lean b/tests/lean/run/679b.lean
new file mode 100644
index 000000000..c43356af9
--- /dev/null
+++ b/tests/lean/run/679b.lean
@@ -0,0 +1,22 @@
+import data.finset
+open bool nat list finset
+
+definition fset [class] (A : Type) := finset A
+
+definition fin_nat [instance] : fset nat :=
+to_finset [0]
+
+definition fin_bool [instance] : fset bool :=
+to_finset [tt, ff]
+
+definition tst (A : Type) [s : fset A] : finset A :=
+s
+
+example : tst nat = to_finset [0] :=
+rfl
+
+example : tst bool = to_finset [ff, tt] :=
+dec_trivial
+
+example : tst bool = to_finset [tt, ff] :=
+rfl