diff --git a/src/frontends/lean/frontend.cpp b/src/frontends/lean/frontend.cpp
index 74d6a23c6..0fd492fdd 100644
--- a/src/frontends/lean/frontend.cpp
+++ b/src/frontends/lean/frontend.cpp
@@ -403,7 +403,7 @@ operator_info frontend::find_op_for(expr const & n, bool unicode) const {
         return r;
     } else {
         object const & obj = find_object(const_name(n));
-        if (obj && obj.is_builtin() && obj.get_name() == n)
+        if (obj && obj.is_builtin() && obj.get_name() == const_name(n))
             return to_ext(m_env).find_op_for(obj.get_value(), unicode);
         else
             return r;
diff --git a/src/frontends/lean/operator_info.h b/src/frontends/lean/operator_info.h
index 686b1dc92..3daef1d30 100644
--- a/src/frontends/lean/operator_info.h
+++ b/src/frontends/lean/operator_info.h
@@ -46,7 +46,7 @@ public:
 
     friend void swap(operator_info & o1, operator_info & o2) { std::swap(o1.m_ptr, o2.m_ptr); }
 
-    operator bool() const { return m_ptr != nullptr; }
+    explicit operator bool() const { return m_ptr != nullptr; }
 
     friend operator_info infix(name const & op, unsigned precedence);
     friend operator_info infixl(name const & op, unsigned precedence);
diff --git a/src/frontends/lean/pp.cpp b/src/frontends/lean/pp.cpp
index e0ee6b92d..282869e46 100644
--- a/src/frontends/lean/pp.cpp
+++ b/src/frontends/lean/pp.cpp
@@ -1121,12 +1121,12 @@ class pp_fn {
     }
 
     bool uses_prefix(expr const & e, name const & prefix) {
-        return find(e, [&](expr const & e) {
-                return
-                    (is_constant(e) && is_prefix_of(prefix, const_name(e)))   ||
-                    (is_abstraction(e) && is_prefix_of(prefix, abst_name(e))) ||
-                    (is_let(e) && is_prefix_of(prefix, let_name(e)));
-            });
+        return static_cast<bool>(find(e, [&](expr const & e) {
+                    return
+                        (is_constant(e) && is_prefix_of(prefix, const_name(e)))   ||
+                        (is_abstraction(e) && is_prefix_of(prefix, abst_name(e))) ||
+                        (is_let(e) && is_prefix_of(prefix, let_name(e)));
+                }));
     }
 
     name find_unused_prefix(expr const & e) {
diff --git a/src/kernel/context.h b/src/kernel/context.h
index 92058bc4b..f9436d926 100644
--- a/src/kernel/context.h
+++ b/src/kernel/context.h
@@ -39,7 +39,7 @@ public:
     context_entry const & lookup(unsigned vidx) const;
     std::pair<context_entry const &, context> lookup_ext(unsigned vidx) const;
     bool empty() const { return is_nil(m_list); }
-    operator bool() const { return !empty(); }
+    explicit operator bool() const { return !empty(); }
     unsigned size() const { return length(m_list); }
     typedef list<context_entry>::iterator iterator;
     iterator begin() const { return m_list.begin(); }
diff --git a/src/kernel/environment.h b/src/kernel/environment.h
index 7ea90b9b9..9a5c1a6e6 100644
--- a/src/kernel/environment.h
+++ b/src/kernel/environment.h
@@ -148,7 +148,7 @@ public:
     object const & find_object(name const & n) const;
 
     /** \brief Return true iff the environment has an object with the given name */
-    bool has_object(name const & n) const { return find_object(n); }
+    bool has_object(name const & n) const { return static_cast<bool>(find_object(n)); }
 
     /**
        \brief Return the type of \c e in the given context and this environment.
diff --git a/src/kernel/expr.h b/src/kernel/expr.h
index d147efd4a..4fd952b55 100644
--- a/src/kernel/expr.h
+++ b/src/kernel/expr.h
@@ -123,7 +123,7 @@ public:
 
     expr_cell * raw() const { return m_ptr; }
 
-    operator bool() const { return m_ptr != nullptr; }
+    explicit operator bool() const { return m_ptr != nullptr; }
 
     friend expr mk_var(unsigned idx);
     friend expr mk_constant(name const & n, expr const & t);
diff --git a/src/kernel/justification.h b/src/kernel/justification.h
index a8434a9bf..3bd8b9764 100644
--- a/src/kernel/justification.h
+++ b/src/kernel/justification.h
@@ -61,7 +61,7 @@ public:
     justification(justification && s):m_ptr(s.m_ptr) { s.m_ptr = nullptr; }
     ~justification() { if (m_ptr) m_ptr->dec_ref(); }
 
-    operator bool() const { return m_ptr != nullptr; }
+    explicit operator bool() const { return m_ptr != nullptr; }
 
     justification_cell * raw() const { return m_ptr; }
 
diff --git a/src/kernel/metavar.cpp b/src/kernel/metavar.cpp
index 6c23de869..01d5c37f9 100644
--- a/src/kernel/metavar.cpp
+++ b/src/kernel/metavar.cpp
@@ -126,7 +126,7 @@ expr metavar_env::get_type(name const & m) {
 bool metavar_env::has_type(name const & m) const {
     auto it = const_cast<metavar_env*>(this)->m_metavar_data.splay_find(m);
     lean_assert(it);
-    return it->m_type;
+    return static_cast<bool>(it->m_type);
 }
 
 bool metavar_env::has_type(expr const & m) const {
@@ -343,7 +343,7 @@ expr add_inst(expr const & m, unsigned s, expr const & v) {
 }
 
 bool has_local_context(expr const & m) {
-    return metavar_lctx(m);
+    return static_cast<bool>(metavar_lctx(m));
 }
 
 expr pop_meta_context(expr const & m) {
@@ -361,12 +361,12 @@ bool has_metavar(expr const & e, expr const & m, metavar_env const & menv) {
     if (has_metavar(e)) {
         lean_assert(is_metavar(m));
         lean_assert(!menv.is_assigned(m));
-        return find(e, [&](expr const & m2) {
-                return
-                    is_metavar(m2) &&
-                    ((metavar_name(m) == metavar_name(m2)) ||
-                     (menv.is_assigned(m2) && has_metavar(menv.get_subst(m2), m, menv)));
-            });
+        return static_cast<bool>(find(e, [&](expr const & m2) {
+                    return
+                        is_metavar(m2) &&
+                        ((metavar_name(m) == metavar_name(m2)) ||
+                         (menv.is_assigned(m2) && has_metavar(menv.get_subst(m2), m, menv)));
+                }));
     } else {
         return false;
     }
diff --git a/src/kernel/object.h b/src/kernel/object.h
index 51df40ea3..832c9c06c 100644
--- a/src/kernel/object.h
+++ b/src/kernel/object.h
@@ -98,7 +98,7 @@ public:
     object & operator=(object const & s) { LEAN_COPY_REF(object, s); }
     object & operator=(object && s) { LEAN_MOVE_REF(object, s); }
 
-    operator bool() const { return m_ptr != nullptr; }
+    explicit operator bool() const { return m_ptr != nullptr; }
 
     object_kind kind() const { return m_ptr->kind(); }
 
diff --git a/src/kernel/occurs.cpp b/src/kernel/occurs.cpp
index 4b6e65ede..1396c6f45 100644
--- a/src/kernel/occurs.cpp
+++ b/src/kernel/occurs.cpp
@@ -9,11 +9,11 @@ Author: Leonardo de Moura
 
 namespace lean {
 bool occurs(name const & n, context const * c, unsigned sz, expr const * es) {
-    return find(c, sz, es, [&](expr const & e) { return is_constant(e) && const_name(e) == n; });
+    return static_cast<bool>(find(c, sz, es, [&](expr const & e) { return is_constant(e) && const_name(e) == n; }));
 }
 
 bool occurs(expr const & n, context const * c, unsigned sz, expr const * es) {
-    return find(c, sz, es, [&](expr const & e) { return e == n; });
+    return static_cast<bool>(find(c, sz, es, [&](expr const & e) { return e == n; }));
 }
 bool occurs(expr const & n, expr const & m) { return occurs(n, nullptr, 1, &m); }
 bool occurs(name const & n, expr const & m) { return occurs(n, nullptr, 1, &m); }
diff --git a/src/kernel/unification_constraint.h b/src/kernel/unification_constraint.h
index d635f541d..de2c279e4 100644
--- a/src/kernel/unification_constraint.h
+++ b/src/kernel/unification_constraint.h
@@ -69,7 +69,7 @@ public:
     unification_constraint_kind kind() const { return m_ptr->kind(); }
     unification_constraint_cell * raw() const { return m_ptr; }
 
-    operator bool() const { return m_ptr != nullptr; }
+    explicit operator bool() const { return m_ptr != nullptr; }
 
     format pp(formatter const & fmt, options const & opts, pos_info_provider const * p = nullptr, bool include_justification = false) const {
         lean_assert(m_ptr);
diff --git a/src/library/tactic/goal.h b/src/library/tactic/goal.h
index fedded9bd..48b47c3c7 100644
--- a/src/library/tactic/goal.h
+++ b/src/library/tactic/goal.h
@@ -26,7 +26,7 @@ public:
     goal(hypotheses const & hs, expr const & c);
     hypotheses const & get_hypotheses() const { return m_hypotheses; }
     expr const & get_conclusion() const { return m_conclusion; }
-    operator bool() const { return m_conclusion; }
+    explicit operator bool() const { return static_cast<bool>(m_conclusion); }
     format pp(formatter const & fmt, options const & opts) const;
     name mk_unique_hypothesis_name(name const & suggestion) const;
 };
diff --git a/src/library/tactic/tactic.h b/src/library/tactic/tactic.h
index acf1139d0..66606ab7d 100644
--- a/src/library/tactic/tactic.h
+++ b/src/library/tactic/tactic.h
@@ -74,7 +74,7 @@ public:
     tactic(tactic const & s):m_ptr(s.m_ptr) { if (m_ptr) m_ptr->inc_ref(); }
     tactic(tactic && s):m_ptr(s.m_ptr) { s.m_ptr = nullptr; }
     ~tactic() { if (m_ptr) m_ptr->dec_ref(); }
-    operator bool() const { return m_ptr != nullptr; }
+    explicit operator bool() const { return m_ptr != nullptr; }
     friend void swap(tactic & a, tactic & b) { std::swap(a.m_ptr, b.m_ptr); }
     tactic & operator=(tactic const & s);
     tactic & operator=(tactic && s);
diff --git a/src/util/list.h b/src/util/list.h
index 33321b132..21bdb0a6e 100644
--- a/src/util/list.h
+++ b/src/util/list.h
@@ -79,7 +79,7 @@ public:
     cell * raw() const { return m_ptr; }
 
     /** \brief Return true iff it is not the empty/nil list. */
-    operator bool() const { return m_ptr != nullptr; }
+    explicit operator bool() const { return m_ptr != nullptr; }
 
     friend bool is_nil(list const & l) { return l.m_ptr == nullptr; }
     friend bool empty(list const & l) { return is_nil(l); }
diff --git a/src/util/name.h b/src/util/name.h
index fc8ee0d76..2a8be053d 100644
--- a/src/util/name.h
+++ b/src/util/name.h
@@ -74,7 +74,7 @@ public:
     bool is_anonymous() const { return kind() == name_kind::ANONYMOUS; }
     bool is_string() const    { return kind() == name_kind::STRING; }
     bool is_numeral() const   { return kind() == name_kind::NUMERAL; }
-    operator bool() const     { return !is_anonymous(); }
+    explicit operator bool() const     { return !is_anonymous(); }
     unsigned get_numeral() const;
     /**
        \brief If the tail of the given hierarchical name is a string, then it returns this string.
diff --git a/src/util/optional.h b/src/util/optional.h
index 76db77b81..8761fc51d 100644
--- a/src/util/optional.h
+++ b/src/util/optional.h
@@ -40,7 +40,7 @@ public:
             m_value.~T();
     }
 
-    operator bool() const { return m_some; }
+    explicit operator bool() const { return m_some; }
     T const * operator->() const { lean_assert(m_some); return &m_value; }
     T * operator->() { lean_assert(m_some); return &m_value; }
     T const & operator*() const { lean_assert(m_some); return m_value; }
diff --git a/src/util/sexpr/format.h b/src/util/sexpr/format.h
index 75fbf8a04..798637ef2 100644
--- a/src/util/sexpr/format.h
+++ b/src/util/sexpr/format.h
@@ -186,7 +186,7 @@ public:
     }
     unsigned hash() const { return m_value.hash(); }
 
-    operator bool() const { return m_value; }
+    explicit operator bool() const { return static_cast<bool>(m_value); }
 
     friend format compose(format const & f1, format const & f2);
     friend format nest(int i, format const & f);
diff --git a/src/util/sexpr/sexpr.h b/src/util/sexpr/sexpr.h
index cfaa6c6ad..b8d0023ea 100644
--- a/src/util/sexpr/sexpr.h
+++ b/src/util/sexpr/sexpr.h
@@ -60,7 +60,7 @@ public:
 
     sexpr_kind kind() const;
 
-    operator bool() const { return m_ptr != nullptr; }
+    explicit operator bool() const { return m_ptr != nullptr; }
 
     friend bool is_nil(sexpr const & s) { return s.m_ptr == nullptr; }
     friend sexpr const & head(sexpr const & s);