fix(frontends/lean/parser): segfault when ending scope without opening, fixes #244

src/frontends/lean/local_decls.h

@@ -46,6 +46,7 @@ public:
         std::tie(m_map, m_counter, m_entries) = head(m_scopes);
         m_scopes  = tail(m_scopes);
     }
+    bool has_scopes() const { return !is_nil(m_scopes); }
     bool empty() const { return m_map.empty(); }
     struct mk_scope {
         local_decls & m_d;

src/frontends/lean/parser.cpp

@@ -417,6 +417,9 @@ void parser::push_local_scope(bool save_options) {
 }
 
 void parser::pop_local_scope() {
+    if (!m_local_level_decls.has_scopes()) {
+        throw parser_error("invalid 'end', there is no open namespace/section/context", pos());
+    }
     m_local_level_decls.pop();
     m_local_decls.pop();
     lean_assert(!is_nil(m_parser_scope_stack));

tests/lean/bad_end.lean

@@ -0,0 +1 @@
+end foo

tests/lean/bad_end.lean.expected.out

@@ -0,0 +1 @@
+bad_end.lean:1:4: error: invalid 'end', there is no open namespace/section/context