diff --git a/src/library/app_builder.cpp b/src/library/app_builder.cpp
index 73e6d3e9d..0e04f4a0b 100644
--- a/src/library/app_builder.cpp
+++ b/src/library/app_builder.cpp
@@ -587,6 +587,8 @@ struct app_builder::imp {
     }
 
     expr mk_eq_of_heq(expr const & H) {
+        if (is_constant(get_app_fn(H), get_heq_of_eq_name()))
+            return app_arg(H);
         expr p    = m_ctx->relaxed_whnf(m_ctx->infer(H));
         expr A, a, B, b;
         if (!is_heq(p, A, a, B, b)) {
diff --git a/src/library/blast/blast.cpp b/src/library/blast/blast.cpp
index 78a0296e1..9f8e4f2f6 100644
--- a/src/library/blast/blast.cpp
+++ b/src/library/blast/blast.cpp
@@ -731,6 +731,10 @@ public:
         return m_congr_lemma_manager.mk_congr(fn);
     }
 
+    optional<congr_lemma> mk_hcongr_lemma(expr const & fn, unsigned num_args) {
+        return m_congr_lemma_manager.mk_hcongr(fn, num_args);
+    }
+
     optional<congr_lemma> mk_specialized_congr_lemma(expr const & a) {
         return m_congr_lemma_manager.mk_specialized_congr(a);
     }
@@ -1114,6 +1118,11 @@ optional<congr_lemma> mk_congr_lemma(expr const & fn) {
     return g_blastenv->mk_congr_lemma(fn);
 }
 
+optional<congr_lemma> mk_hcongr_lemma(expr const & fn, unsigned num_args) {
+    lean_assert(g_blastenv);
+    return g_blastenv->mk_hcongr_lemma(fn, num_args);
+}
+
 optional<congr_lemma> mk_specialized_congr_lemma(expr const & a) {
     lean_assert(g_blastenv);
     return g_blastenv->mk_specialized_congr_lemma(a);
diff --git a/src/library/blast/blast.h b/src/library/blast/blast.h
index bdd64f1b4..d56c0f40d 100644
--- a/src/library/blast/blast.h
+++ b/src/library/blast/blast.h
@@ -148,6 +148,9 @@ optional<congr_lemma> mk_congr_lemma(expr const & fn);
     and the arguments are taken into account when computing the lemma.
     \pre is_app(a) */
 optional<congr_lemma> mk_specialized_congr_lemma(expr const & a);
+/** \brief Create more general congruence lemma based on heterogeneous equality. */
+optional<congr_lemma> mk_hcongr_lemma(expr const & fn, unsigned num_args);
+
 optional<congr_lemma> mk_rel_iff_congr(expr const & fn);
 optional<congr_lemma> mk_rel_eq_congr(expr const & fn);
 
diff --git a/src/library/blast/congruence_closure.cpp b/src/library/blast/congruence_closure.cpp
index b7dd28dad..4361e38ad 100644
--- a/src/library/blast/congruence_closure.cpp
+++ b/src/library/blast/congruence_closure.cpp
@@ -18,7 +18,7 @@ Author: Leonardo de Moura
 #include "library/blast/options.h"
 
 #ifndef LEAN_DEFAULT_BLAST_CC_HEQ
-#define LEAN_DEFAULT_BLAST_CC_HEQ true
+#define LEAN_DEFAULT_BLAST_CC_HEQ false
 #endif
 
 namespace lean {
@@ -51,9 +51,25 @@ struct congr_lemma_key_eq_fn {
     }
 };
 
+LEAN_THREAD_VALUE(bool, g_heq_based, false);
+
 static list<optional<name>> rel_names_from_arg_kinds(list<congr_arg_kind> const & kinds, name const & R) {
     return map2<optional<name>>(kinds, [&](congr_arg_kind k) {
-            return k == congr_arg_kind::Eq ? optional<name>(R) : optional<name>();
+            switch (k) {
+            case congr_arg_kind::Eq:
+                return optional<name>(R);
+            case congr_arg_kind::HEq:
+                if (g_heq_based && (R == get_eq_name() || R == get_heq_name())) {
+                    /* Remark: we store equality and heterogeneous equality in the same class. */
+                    return optional<name>(get_eq_name());
+                } else {
+                    return optional<name>();
+                }
+            case congr_arg_kind::Fixed: case congr_arg_kind::Cast:
+            case congr_arg_kind::FixedNoParam:
+                return optional<name>();
+            }
+            lean_unreachable();
         });
 }
 
@@ -62,23 +78,26 @@ ext_congr_lemma::ext_congr_lemma(congr_lemma const & H):
     m_congr_lemma(H),
     m_rel_names(rel_names_from_arg_kinds(H.get_arg_kinds(), get_eq_name())),
     m_lift_needed(false),
-    m_fixed_fun(true) {}
-ext_congr_lemma::ext_congr_lemma(name const & R, congr_lemma const & H, bool lift_needed):
+    m_fixed_fun(true),
+    m_heq_based(false) {}
+ext_congr_lemma::ext_congr_lemma(name const & R, congr_lemma const & H, bool lift_needed, bool heq_based):
     m_R(R),
     m_congr_lemma(H),
     m_rel_names(rel_names_from_arg_kinds(H.get_arg_kinds(), get_eq_name())),
     m_lift_needed(lift_needed),
-    m_fixed_fun(true) {}
+    m_fixed_fun(true),
+    m_heq_based(heq_based) {}
 ext_congr_lemma::ext_congr_lemma(name const & R, congr_lemma const & H, list<optional<name>> const & rel_names, bool lift_needed):
     m_R(R),
     m_congr_lemma(H),
     m_rel_names(rel_names),
     m_lift_needed(lift_needed),
-    m_fixed_fun(true) {}
+    m_fixed_fun(true),
+    m_heq_based(false) {}
 
 /* We use the following cache for user-defined lemmas and automatically generated ones. */
 typedef std::unordered_map<congr_lemma_key, optional<ext_congr_lemma>, congr_lemma_key_hash_fn, congr_lemma_key_eq_fn> congr_cache;
-typedef std::tuple<name, expr, expr, expr> cc_todo_entry;
+typedef std::tuple<name, expr, expr, expr, bool> cc_todo_entry;
 
 static expr * g_congr_mark    = nullptr; // dummy congruence proof, it is just a placeholder.
 static expr * g_iff_true_mark = nullptr; // dummy iff_true proof, it is just a placeholder.
@@ -96,13 +115,14 @@ static void clear_todo() {
     get_todo().clear();
 }
 
-static void push_todo(name const & R, expr const & lhs, expr const & rhs, expr const & H) {
-    get_todo().emplace_back(R, lhs, rhs, H);
+static void push_todo(name const & R, expr const & lhs, expr const & rhs, expr const & H, bool heq_proof) {
+    get_todo().emplace_back(R, lhs, rhs, H, heq_proof);
 }
 
 scope_congruence_closure::scope_congruence_closure():
     m_old_cache(g_congr_cache) {
     g_congr_cache = new congr_cache();
+    g_heq_based   = is_standard(env()) && get_blast_cc_heq(ios().get_options());
 }
 
 scope_congruence_closure::~scope_congruence_closure() {
@@ -136,6 +156,7 @@ void congruence_closure::mk_entry_core(name const & R, expr const & e, bool to_p
     n.m_interpreted  = interpreted;
     n.m_constructor  = constructor;
     n.m_to_propagate = to_propagate;
+    n.m_heq_proofs   = false;
     n.m_mt           = m_gmt;
     m_entries.insert(eqc_key(R, e), n);
     if (R != get_eq_name()) {
@@ -146,7 +167,8 @@ void congruence_closure::mk_entry_core(name const & R, expr const & e, bool to_p
             expr it = n->m_next;
             while (it != e) {
                 if (m_entries.find(eqc_key(R, it))) {
-                    push_todo(R, e, it, *g_lift_mark);
+                    bool heq_proof = false;
+                    push_todo(R, e, it, *g_lift_mark, heq_proof);
                     break;
                 }
                 auto it_n = m_entries.find(eqc_key(get_eq_name(), it));
@@ -332,7 +354,8 @@ static optional<ext_congr_lemma> mk_relation_congr_lemma(name const & R, expr co
             if (R == get_iff_name()) {
                 if (optional<congr_lemma> cgr = mk_rel_iff_congr(fn)) {
                     auto child_rel_names = rel_names_from_arg_kinds(cgr->get_arg_kinds(), const_name(fn));
-                    return optional<ext_congr_lemma>(R, *cgr, child_rel_names, false);
+                    bool lift_needed = false;
+                    return optional<ext_congr_lemma>(R, *cgr, child_rel_names, lift_needed);
                 }
             }
             if (optional<congr_lemma> cgr = mk_rel_eq_congr(fn)) {
@@ -352,13 +375,40 @@ static optional<ext_congr_lemma> mk_ext_specialized_congr_lemma(name const & R,
     if (!eq_congr)
         return optional<ext_congr_lemma>();
     ext_congr_lemma res1(*eq_congr);
-    // If all arguments are Eq kind, then we can use generic congr axiom and consider equality for the function.
+    /* If all arguments are Eq kind, then we can use generic congr axiom and consider equality for the function. */
     if (eq_congr->all_eq_kind())
         res1.m_fixed_fun = false;
     if (R == get_eq_name())
         return optional<ext_congr_lemma>(res1);
     bool lift_needed = true;
-    return optional<ext_congr_lemma>(R, *eq_congr, lift_needed);
+    bool heq_based   = false;
+    return optional<ext_congr_lemma>(R, *eq_congr, lift_needed, heq_based);
+}
+
+/* Automatically generated congruence lemma based on heterogeneous equality. */
+static optional<ext_congr_lemma> mk_hcongr_lemma(name const & R, expr const & fn, unsigned nargs) {
+    optional<congr_lemma> eq_congr = mk_hcongr_lemma(fn, nargs);
+    if (!eq_congr)
+        return optional<ext_congr_lemma>();
+    ext_congr_lemma res1(*eq_congr);
+    /* If all arguments are Eq kind, then we can use generic congr axiom and consider equality for the function. */
+    if (eq_congr->all_eq_kind())
+        res1.m_fixed_fun = false;
+    if (R == get_eq_name() || R == get_heq_name())
+        return optional<ext_congr_lemma>(res1);
+    /* If R is not equality (=) nor heterogeneous equality (==),
+       we try to lift, but we can only lift if the congruence lemma produces an equality. */
+    expr type = eq_congr->get_type();
+    while (is_pi(type)) type = binding_body(type);
+    lean_assert(is_eq(type) || is_heq(type));
+    if (is_heq(type)) {
+        /* We cannot lift heterogeneous equality. */
+        return optional<ext_congr_lemma>();
+    } else {
+        bool heq_based   = !eq_congr->all_eq_kind() || is_heq(type);
+        bool lift_needed = true;
+        return optional<ext_congr_lemma>(R, *eq_congr, lift_needed, heq_based);
+    }
 }
 
 optional<ext_congr_lemma> mk_ext_congr_lemma(name const & R, expr const & e) {
@@ -369,33 +419,53 @@ optional<ext_congr_lemma> mk_ext_congr_lemma(name const & R, expr const & e) {
     auto it1 = g_congr_cache->find(key1);
     if (it1 != g_congr_cache->end())
         return it1->second;
-    /* Check if (g := fn+specialization prefix) is in the cache */
-    unsigned prefix_sz  = get_specialization_prefix_size(fn, nargs);
-    unsigned rest_nargs = nargs - prefix_sz;
-    expr g = e;
-    for (unsigned i = 0; i < rest_nargs; i++) g = app_fn(g);
-    congr_lemma_key key2(R, g, rest_nargs);
-    auto it2 = g_congr_cache->find(key2);
-    if (it2 != g_congr_cache->end())
-        return it2->second;
-    /* Check if there is user defined lemma for (R, fn, nargs).
-       Remark: specialization prefix is irrelevan for used defined congruence lemmas. */
-    if (auto lemma = mk_ext_user_congr_lemma(R, fn, nargs)) {
-        g_congr_cache->insert(mk_pair(key1, lemma));
-        return lemma;
-    }
-    /* Try automatically generated lemma for equivalence relation over iff/eq */
-    if (auto lemma = mk_relation_congr_lemma(R, fn, nargs)) {
-        g_congr_cache->insert(mk_pair(key1, lemma));
-        return lemma;
-    }
-    /* Try automatically generated specialized congruence lemma */
-    if (auto lemma = mk_ext_specialized_congr_lemma(R, e)) {
-        if (prefix_sz == 0)
+    if (g_heq_based) {
+        /* Check if there is user defined lemma for (R, fn, nargs).
+           Remark: specialization prefix is irrelevan for used defined congruence lemmas. */
+        auto lemma = mk_ext_user_congr_lemma(R, fn, nargs);
+        /* Try automatically generated lemma for equivalence relation over iff/eq */
+        if (!lemma) lemma = mk_relation_congr_lemma(R, fn, nargs);
+        /* Try automatically generated congruence lemma with support for heterogeneous equality. */
+        if (!lemma) lemma = mk_hcongr_lemma(R, fn, nargs);
+
+        if (lemma) {
+            /* succeeded */
             g_congr_cache->insert(mk_pair(key1, lemma));
-        else
-            g_congr_cache->insert(mk_pair(key2, lemma));
-        return lemma;
+            return lemma;
+        }
+    } else {
+        lean_assert(!g_heq_based);
+        /* When heterogeneous equality support is disabled, we use specialization prefix +
+           congruence lemmas that take care of subsingletons */
+
+        /* Check if (g := fn+specialization prefix) is in the cache */
+        unsigned prefix_sz  = get_specialization_prefix_size(fn, nargs);
+        unsigned rest_nargs = nargs - prefix_sz;
+        expr g = e;
+        for (unsigned i = 0; i < rest_nargs; i++) g = app_fn(g);
+        congr_lemma_key key2(R, g, rest_nargs);
+        auto it2 = g_congr_cache->find(key2);
+        if (it2 != g_congr_cache->end())
+            return it2->second;
+        /* Check if there is user defined lemma for (R, fn, nargs).
+           Remark: specialization prefix is irrelevan for used defined congruence lemmas. */
+        if (auto lemma = mk_ext_user_congr_lemma(R, fn, nargs)) {
+            g_congr_cache->insert(mk_pair(key1, lemma));
+            return lemma;
+        }
+        /* Try automatically generated lemma for equivalence relation over iff/eq */
+        if (auto lemma = mk_relation_congr_lemma(R, fn, nargs)) {
+            g_congr_cache->insert(mk_pair(key1, lemma));
+            return lemma;
+        }
+        /* Try automatically generated specialized congruence lemma */
+        if (auto lemma = mk_ext_specialized_congr_lemma(R, e)) {
+            if (prefix_sz == 0)
+                g_congr_cache->insert(mk_pair(key1, lemma));
+            else
+                g_congr_cache->insert(mk_pair(key2, lemma));
+            return lemma;
+        }
     }
     /* cache failure */
     g_congr_cache->insert(mk_pair(key1, optional<ext_congr_lemma>()));
@@ -496,7 +566,6 @@ int congruence_closure::congr_key_cmp::operator()(congr_key const & k1, congr_ke
                 lean_assert(*it1); lean_assert(*it2);
                 switch (head(*it2)) {
                 case congr_arg_kind::HEq:
-                    lean_unreachable();
                 case congr_arg_kind::Eq:
                     lean_assert(head(*it1));
                     r = g_cc->compare_root(*head(*it1), args1[i], args2[i]);
@@ -560,7 +629,6 @@ auto congruence_closure::mk_congr_key(ext_congr_lemma const & lemma, expr const
                 lean_assert(*it1); lean_assert(*it2);
                 switch (head(*it2)) {
                 case congr_arg_kind::HEq:
-                    lean_unreachable();
                 case congr_arg_kind::Eq:
                     lean_assert(head(*it1));
                     h = hash(h, get_root(*head(*it1), args[i]).hash());
@@ -601,7 +669,8 @@ void congruence_closure::check_iff_true(congr_key const & k) {
     if (lhs != rhs)
         return;
     // Add e <-> true
-    push_todo(get_iff_name(), e, mk_true(), *g_iff_true_mark);
+    bool heq_proof = false;
+    push_todo(get_iff_name(), e, mk_true(), *g_iff_true_mark, heq_proof);
 }
 
 void congruence_closure::add_congruence_table(ext_congr_lemma const & lemma, expr const & e) {
@@ -615,7 +684,8 @@ void congruence_closure::add_congruence_table(ext_congr_lemma const & lemma, exp
         new_entry.m_cg_root = old_k->m_expr;
         m_entries.insert(k, new_entry);
         // 2. Put new equivalence in the TODO queue
-        push_todo(lemma.m_R, e, old_k->m_expr, *g_congr_mark);
+        bool heq_proof = false; // TODO(Leo): fix this
+        push_todo(lemma.m_R, e, old_k->m_expr, *g_congr_mark, heq_proof);
     } else {
         m_congruences.insert(k);
     }
@@ -635,8 +705,11 @@ static bool is_logical_app(expr const & n) {
          (const_name(fn) == get_ite_name() && is_prop(n)));
 }
 
-void congruence_closure::internalize_core(name const & R, expr const & e, bool toplevel, bool to_propagate) {
+void congruence_closure::internalize_core(name R, expr const & e, bool toplevel, bool to_propagate) {
     lean_assert(closed(e));
+    if (g_heq_based && R == get_heq_name())
+        R = get_eq_name();
+
     // we allow metavariables after partitions have been frozen
     if (has_expr_metavar(e) && !m_froze_partitions)
         return;
@@ -804,7 +877,8 @@ void congruence_closure::propagate_no_confusion_eq(expr const & e1, expr const &
 
 /* Remark: If added_prop is not none, then it contains the proposition provided to ::add.
    We use it here to avoid an unnecessary propagation back to the current_state. */
-void congruence_closure::add_eqv_step(name const & R, expr e1, expr e2, expr const & H, optional<expr> const & added_prop) {
+void congruence_closure::add_eqv_step(name const & R, expr e1, expr e2, expr const & H, optional<expr> const & added_prop,
+                                      bool heq_proof) {
     auto n1 = m_entries.find(eqc_key(R, e1));
     auto n2 = m_entries.find(eqc_key(R, e2));
     if (!n1 || !n2)
@@ -888,6 +962,8 @@ void congruence_closure::add_eqv_step(name const & R, expr e1, expr e2, expr con
     new_r1.m_next  = r2->m_next;
     new_r2.m_next  = r1->m_next;
     new_r2.m_size += r1->m_size;
+    if (heq_proof)
+        new_r2.m_heq_proofs = true;
     m_entries.insert(eqc_key(R, e1_root), new_r1);
     m_entries.insert(eqc_key(R, e2_root), new_r2);
     lean_assert(check_invariant());
@@ -916,7 +992,8 @@ void congruence_closure::add_eqv_step(name const & R, expr e1, expr e2, expr con
                 bool propagate_back = false;
                 mk_entry(R2, e1, propagate_back);
                 mk_entry(R2, e2, propagate_back);
-                push_todo(R2, e1, e2, *g_lift_mark);
+                bool heq_proof = false;
+                push_todo(R2, e1, e2, *g_lift_mark, heq_proof);
             }
         }
     }
@@ -957,15 +1034,16 @@ void congruence_closure::add_eqv_step(name const & R, expr e1, expr e2, expr con
 void congruence_closure::process_todo(optional<expr> const & added_prop) {
     auto & todo = get_todo();
     while (!todo.empty()) {
-        name R; expr lhs, rhs, H;
-        std::tie(R, lhs, rhs, H) = todo.back();
+        name R; expr lhs, rhs, H; bool heq_proof;
+        std::tie(R, lhs, rhs, H, heq_proof) = todo.back();
         todo.pop_back();
-        add_eqv_step(R, lhs, rhs, H, added_prop);
+        add_eqv_step(R, lhs, rhs, H, added_prop, heq_proof);
     }
 }
 
-void congruence_closure::add_eqv_core(name const & R, expr const & lhs, expr const & rhs, expr const & H, optional<expr> const & added_prop) {
-    push_todo(R, lhs, rhs, H);
+void congruence_closure::add_eqv_core(name const & R, expr const & lhs, expr const & rhs, expr const & H,
+                                      optional<expr> const & added_prop, bool heq_proof) {
+    push_todo(R, lhs, rhs, H, heq_proof);
     process_todo(added_prop);
 }
 
@@ -975,9 +1053,15 @@ void congruence_closure::add_eqv(name const & R, expr const & lhs, expr const &
     flet<congruence_closure *> set_cc(g_cc, this);
     clear_todo();
     bool toplevel = false; bool to_propagate = false;
-    internalize_core(R, lhs, toplevel, to_propagate);
-    internalize_core(R, rhs, toplevel, to_propagate);
-    add_eqv_core(R, lhs, rhs, H, none_expr());
+    bool heq_proof = false;
+    name _R = R;
+    if (g_heq_based && R == get_heq_name()) {
+        heq_proof = true;
+        _R = get_eq_name();
+    }
+    internalize_core(_R, lhs, toplevel, to_propagate);
+    internalize_core(_R, rhs, toplevel, to_propagate);
+    add_eqv_core(_R, lhs, rhs, H, none_expr(), heq_proof);
 }
 
 void congruence_closure::add(hypothesis_idx hidx) {
@@ -1013,30 +1097,53 @@ void congruence_closure::add(expr const & type, expr const & proof) {
     if (is_equivalence_relation_app(p, R, lhs, rhs)) {
         if (is_neg) {
             bool toplevel = true; bool to_propagate = false;
+            bool heq_proof = false;
             internalize_core(get_iff_name(), p, toplevel, to_propagate);
-            add_eqv_core(get_iff_name(), p, mk_false(), mk_iff_false_intro(proof), some_expr(type));
+            add_eqv_core(get_iff_name(), p, mk_false(), mk_iff_false_intro(proof), some_expr(type), heq_proof);
         } else {
-            bool toplevel = false; bool to_propagate = false;
+            bool toplevel  = false; bool to_propagate = false;
+            bool heq_proof = false;
+            if (g_heq_based && R == get_heq_name()) {
+                /* When heterogeneous equality support is enabled, we
+                   store equality and heterogeneous equality are stored in the same equivalence classes. */
+                R = get_eq_name();
+                heq_proof = true;
+            }
             internalize_core(R, lhs, toplevel, to_propagate);
             internalize_core(R, rhs, toplevel, to_propagate);
-            add_eqv_core(R, lhs, rhs, proof, some_expr(type));
+            add_eqv_core(R, lhs, rhs, proof, some_expr(type), heq_proof);
         }
     } else if (is_prop(p)) {
         bool toplevel = true; bool to_propagate = false;
+        bool heq_proof = false;
         internalize_core(get_iff_name(), p, toplevel, to_propagate);
         if (is_neg) {
-            add_eqv_core(get_iff_name(), p, mk_false(), mk_iff_false_intro(proof), some_expr(type));
+            add_eqv_core(get_iff_name(), p, mk_false(), mk_iff_false_intro(proof), some_expr(type), heq_proof);
         } else {
-            add_eqv_core(get_iff_name(), p, mk_true(), mk_iff_true_intro(proof), some_expr(type));
+            add_eqv_core(get_iff_name(), p, mk_true(), mk_iff_true_intro(proof), some_expr(type), heq_proof);
         }
     }
 }
 
+bool congruence_closure::has_heq_proofs(expr const & root) const {
+    lean_assert(m_entries.find(eqc_key(get_eq_name(), root)));
+    lean_assert(m_entries.find(eqc_key(get_eq_name(), root))->m_root == root);
+    return m_entries.find(eqc_key(get_eq_name(), root))->m_heq_proofs;
+}
+
 bool congruence_closure::is_eqv(name const & R, expr const & e1, expr const & e2) const {
-    auto n1 = m_entries.find(eqc_key(R, e1));
+    name R_norm = R;
+    if (g_heq_based && R == get_heq_name()) {
+        R_norm = get_eq_name();
+    }
+    auto n1 = m_entries.find(eqc_key(R_norm, e1));
     if (!n1) return false;
-    auto n2 = m_entries.find(eqc_key(R, e2));
+    auto n2 = m_entries.find(eqc_key(R_norm, e2));
     if (!n2) return false;
+    /* Remark: this method assumes that is_eqv is invoked with type correct parameters.
+       An eq class may contain equality and heterogeneous equality proofs when g_heq_based
+       is enabled. When this happens, the answer is correct only if e1 and e2 have the same type.
+    */
     return n1->m_root == n2->m_root;
 }
 
@@ -1168,11 +1275,20 @@ expr congruence_closure::mk_proof(name const & R, expr const & lhs, expr const &
     }
 }
 
-static expr flip_proof(name const & R, expr const & H, bool flipped) {
-    if (!flipped || H == *g_congr_mark || H == *g_iff_true_mark || H == *g_lift_mark) {
+static expr flip_proof(name const & R, expr const & H, bool flipped, bool has_heq_proofs) {
+    if (H == *g_congr_mark || H == *g_iff_true_mark || H == *g_lift_mark) {
         return H;
     } else {
-        return get_app_builder().mk_symm(R, H);
+        auto & b = get_app_builder();
+        expr new_H = H;
+        if (has_heq_proofs && is_eq(relaxed_whnf(infer_type(new_H)))) {
+            new_H = b.mk_heq_of_eq(new_H);
+        }
+        if (!flipped) {
+            return new_H;
+        } else {
+            return b.mk_symm(R, new_H);
+        }
     }
 }
 
@@ -1182,26 +1298,33 @@ static expr mk_trans(name const & R, optional<expr> const & H1, expr const & H2)
 
 optional<expr> congruence_closure::get_eqv_proof(name const & R, expr const & e1, expr const & e2) const {
     app_builder & b = get_app_builder();
+    name R_key = R; // We use R_key to access the equivalence class data
+    if (g_heq_based && R == get_heq_name()) {
+        R_key = get_eq_name();
+    }
     if (has_expr_metavar(e1) || has_expr_metavar(e2)) return none_expr();
     if (is_def_eq(e1, e2))
         return some_expr(b.lift_from_eq(R, b.mk_eq_refl(e1)));
-    auto n1 = m_entries.find(eqc_key(R, e1));
+    auto n1 = m_entries.find(eqc_key(R_key, e1));
     if (!n1) return none_expr();
-    auto n2 = m_entries.find(eqc_key(R, e2));
+    auto n2 = m_entries.find(eqc_key(R_key, e2));
     if (!n2) return none_expr();
     if (n1->m_root != n2->m_root) return none_expr();
+    bool heq_proofs = R_key == get_eq_name() && has_heq_proofs(n1->m_root);
+    // R_trans is the relation we use to build the transitivity proofs
+    name R_trans = heq_proofs ? get_heq_name() : R_key;
     // 1. Retrieve "path" from e1 to root
     buffer<expr> path1, Hs1;
     rb_tree<expr, expr_quick_cmp> visited;
     expr it1 = e1;
     while (true) {
         visited.insert(it1);
-        auto it1_n = m_entries.find(eqc_key(R, it1));
+        auto it1_n = m_entries.find(eqc_key(R_key, it1));
         lean_assert(it1_n);
         if (!it1_n->m_target)
             break;
         path1.push_back(*it1_n->m_target);
-        Hs1.push_back(flip_proof(R, *it1_n->m_proof, it1_n->m_flipped));
+        Hs1.push_back(flip_proof(R_trans, *it1_n->m_proof, it1_n->m_flipped, heq_proofs));
         it1 = *it1_n->m_target;
     }
     lean_assert(it1 == n1->m_root);
@@ -1212,11 +1335,11 @@ optional<expr> congruence_closure::get_eqv_proof(name const & R, expr const & e1
     while (true) {
         if (visited.contains(it2))
             break; // found common
-        auto it2_n = m_entries.find(eqc_key(R, it2));
+        auto it2_n = m_entries.find(eqc_key(R_key, it2));
         lean_assert(it2_n);
         lean_assert(it2_n->m_target);
         path2.push_back(it2);
-        Hs2.push_back(flip_proof(R, *it2_n->m_proof, !it2_n->m_flipped));
+        Hs2.push_back(flip_proof(R_trans, *it2_n->m_proof, !it2_n->m_flipped, heq_proofs));
         it2 = *it2_n->m_target;
     }
     // it2 is the common element...
@@ -1238,16 +1361,20 @@ optional<expr> congruence_closure::get_eqv_proof(name const & R, expr const & e1
     optional<expr> pr;
     expr lhs = e1;
     for (unsigned i = 0; i < path1.size(); i++) {
-        pr  = mk_trans(R, pr, mk_proof(R, lhs, path1[i], Hs1[i]));
+        pr  = mk_trans(R_trans, pr, mk_proof(R, lhs, path1[i], Hs1[i]));
         lhs = path1[i];
     }
     unsigned i = Hs2.size();
     while (i > 0) {
         --i;
-        pr  = mk_trans(R, pr, mk_proof(R, lhs, path2[i], Hs2[i]));
+        pr  = mk_trans(R_trans, pr, mk_proof(R, lhs, path2[i], Hs2[i]));
         lhs = path2[i];
     }
     lean_assert(pr);
+    if (heq_proofs && R == get_eq_name())
+        pr = b.mk_eq_of_heq(*pr);
+    else if (!heq_proofs && R == get_heq_name())
+        pr = b.mk_heq_of_eq(*pr);
     return pr;
 }
 
diff --git a/src/library/blast/congruence_closure.h b/src/library/blast/congruence_closure.h
index 2cefc31ae..ce0cc2f9c 100644
--- a/src/library/blast/congruence_closure.h
+++ b/src/library/blast/congruence_closure.h
@@ -48,6 +48,10 @@ class congruence_closure {
         unsigned       m_to_propagate:1; // must be propagated back to state when in equivalence class containing true/false
         unsigned       m_interpreted:1;  // true if the node should be viewed as an abstract value
         unsigned       m_constructor:1;  // true if head symbol is a constructor
+        /* m_heq_proofs == true iff some proofs in the equivalence class are based on heterogeneous equality.
+           This flag is only used when option blast.cc.heq is set to true.
+           Moreover, we represent equality and heterogeneous equality in a single equivalence class. */
+        unsigned       m_heq_proofs:1;
         unsigned       m_size;           // number of elements in the equivalence class, it is meaningless if 'e' != m_root
         unsigned       m_mt;
         // The field m_mt is used to implement the mod-time optimization introduce by the Simplify theorem prover.
@@ -117,7 +121,7 @@ class congruence_closure {
     void update_non_eq_relations(name const & R);
 
     void register_to_propagate(expr const & e);
-    void internalize_core(name const & R, expr const & e, bool toplevel, bool to_propagate);
+    void internalize_core(name R, expr const & e, bool toplevel, bool to_propagate);
     void process_todo(optional<expr> const & added_prop);
 
     int compare_symm(name const & R, expr lhs1, expr rhs1, expr lhs2, expr rhs2) const;
@@ -138,14 +142,16 @@ class congruence_closure {
     void update_mt(name const & R, expr const & e);
     expr mk_iff_false_intro(expr const & proof);
     expr mk_iff_true_intro(expr const & proof);
-    void add_eqv_step(name const & R, expr e1, expr e2, expr const & H, optional<expr> const & added_prop);
-    void add_eqv_core(name const & R, expr const & lhs, expr const & rhs, expr const & H, optional<expr> const & added_prop);
+    void add_eqv_step(name const & R, expr e1, expr e2, expr const & H, optional<expr> const & added_prop, bool heq_proof);
+    void add_eqv_core(name const & R, expr const & lhs, expr const & rhs, expr const & H, optional<expr> const & added_prop, bool heq_proof);
     void propagate_no_confusion_eq(expr const & e1, expr const & e2);
 
     expr mk_congr_proof_core(name const & R, expr const & lhs, expr const & rhs) const;
     expr mk_congr_proof(name const & R, expr const & lhs, expr const & rhs) const;
     expr mk_proof(name const & R, expr const & lhs, expr const & rhs, expr const & H) const;
 
+    bool has_heq_proofs(expr const & root) const;
+
     void trace_eqc(name const & R, expr const & e) const;
 public:
     void initialize();
@@ -255,8 +261,10 @@ struct ext_congr_lemma {
     /* If m_fixed_fun is false, then we build equivalences for functions, and use generic congr lemma, and ignore m_congr_lemma.
        That is, even the function can be treated as an Eq argument. */
     unsigned             m_fixed_fun:1;
+    /* If m_uses_heq is true, then lemma is based on heterogeneous equality. */
+    unsigned             m_heq_based:1;
     ext_congr_lemma(congr_lemma const & H);
-    ext_congr_lemma(name const & R, congr_lemma const & H, bool lift_needed);
+    ext_congr_lemma(name const & R, congr_lemma const & H, bool lift_needed, bool heq_based);
     ext_congr_lemma(name const & R, congr_lemma const & H, list<optional<name>> const & rel_names, bool lift_needed);
 
     name const & get_relation() const { return m_R; }
diff --git a/tests/lean/run/blast_cc_heq1.lean b/tests/lean/run/blast_cc_heq1.lean
new file mode 100644
index 000000000..c817f76b4
--- /dev/null
+++ b/tests/lean/run/blast_cc_heq1.lean
@@ -0,0 +1,32 @@
+set_option blast.strategy "cc"
+set_option blast.cc.heq true -- make sure heterogeneous congruence lemmas are enabled
+
+example (a b c : Prop) : a = b → b = c → (a ↔ c) :=
+by blast
+
+example (a b c : Prop) : a = b → b == c → (a ↔ c) :=
+by blast
+
+example (a b c : nat) : a == b → b = c → a == c :=
+by blast
+
+example (a b c : nat) : a == b → b = c → a = c :=
+by blast
+
+example (a b c d : nat) : a == b → b == c → c == d → a = d :=
+by blast
+
+example (a b c d : nat) : a == b → b = c → c == d → a = d :=
+by blast
+
+example (a b c : Prop) : a = b → b = c → (a ↔ c) :=
+by blast
+
+example (a b c : Prop) : a == b → b = c → (a ↔ c) :=
+by blast
+
+example (a b c d : Prop) : a == b → b == c → c == d → (a ↔ d) :=
+by blast
+
+definition foo (a b c d : Prop) : a == b → b = c → c == d → (a ↔ d) :=
+by blast
diff --git a/tests/lean/run/blast_cc_heq2.lean b/tests/lean/run/blast_cc_heq2.lean
new file mode 100644
index 000000000..0bccb9a62
--- /dev/null
+++ b/tests/lean/run/blast_cc_heq2.lean
@@ -0,0 +1,11 @@
+set_option blast.strategy "cc"
+set_option blast.cc.heq true -- make sure heterogeneous congruence lemmas are enabled
+
+example (a b c : nat) (f : nat → nat) : a == b → b = c → f a == f c :=
+by blast
+
+example (a b c : nat) (f : nat → nat) : a == b → b = c → f a = f c :=
+by blast
+
+example (a b c d : nat) (f : nat → nat) : a == b → b = c → c == f d → f a = f (f d) :=
+by blast