feat(kernel/environment): make the environment throw an exception when weak-ref has expired

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

src/kernel/environment.cpp

@@ -435,6 +435,12 @@ environment::environment(std::shared_ptr<imp> const & ptr):
     m_ptr(ptr) {
 }
 
+environment::environment(weak_ref const & r) {
+    if (r.expired())
+        throw exception("weak reference to environment object has expired (i.e., the environment has been deleted)");
+    m_ptr = r.lock();
+}
+
 environment::~environment() {
 }
 

src/kernel/environment.h

@@ -282,6 +282,6 @@ public:
 public:
     typedef std::weak_ptr<imp> weak_ref;
     weak_ref to_weak_ref() const { return weak_ref(m_ptr); }
-    environment(weak_ref const & r):m_ptr(r) { lean_assert(!r.expired()); }
+    environment(weak_ref const & r);
 };
 }

src/tests/kernel/environment.cpp

@@ -270,6 +270,18 @@ static void tst12() {
     lean_assert(ext.get_parent<my_extension>() == nullptr);
 }
 
+static void tst13() {
+    environment::weak_ref wref;
+    {
+        environment env;
+        wref = env.to_weak_ref();
+    }
+    try {
+        environment env2(wref);
+        lean_unreachable();
+    } catch (exception &) {}
+}
+
 int main() {
     enable_trace("is_convertible");
     tst1();
@@ -284,5 +296,6 @@ int main() {
     tst10();
     tst11();
     tst12();
+    tst13();
     return has_violations() ? 1 : 0;
 }