chore(*): use 'explicit operator bool' everywhere.

operator bool() may produce unwanted conversions.
For example, we had the following bug in the code base.

...
   object const & obj = find_object(const_name(n));
   if (obj && obj.is_builtin() && obj.get_name() == n)
...

obj.get_name() has type lean::name
n              has type lean::expr

Both have 'operator bool()', then the compiler uses the operator to
convert them to Boolean, and then compare the result.
Of course, this is not our intention.

After this commit, the compiler correctly signs the error.
The correct code is

...
   object const & obj = find_object(const_name(n));
   if (obj && obj.is_builtin() && obj.get_name() == const_name(n))
...

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

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;

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);

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 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) {

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(); }

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.

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);

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; }
 

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 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;
     }

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(); }
 

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); }

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);

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;
 };

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);

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); }

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.

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; }

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);

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);