Fix crash when checking Meta values for mutations

Author: tarleb

src/resources/filters/ast/runemulation.lua

@@ -72,7 +72,7 @@ local function copy_table (tbl, depth)
 end
 
 --- Checks if two tables are equal
-function equals(o1, o2, seen)
+function equals(o1, o2)
   if o1 == o2 then
     return true
   end
@@ -83,22 +83,16 @@ function equals(o1, o2, seen)
   end
 
   local keys = {}
-  seen = seen or {}
-  if seen[o1] then
-    return true
-  end
-
-  seen[o1] = true
 
   for key1, value1 in pairs(o1) do
     local value2 = o2[key1]
-    if value2 == nil or equals(value1, value2, seen) == false then
+    if value2 == nil or equals(value1, value2) == false then
       return false
     end
     keys[key1] = true
   end
 
-  for key2, _ in pairs(o2) do
+  for key2 in pairs(o2) do
     if not keys[key2] then return false end
   end
   return true
@@ -117,13 +111,15 @@ local function check_nondestructive_property (namedfilter)
   for name, fn in pairs(namedfilter.filter) do
     if type(fn) == 'function' then
       local copy = function (x)
-        return type(x) == 'table' and copy_table(x) or x:clone()
+        local tp = type(x)
+        return tp ~= 'table' and x:clone() or
+          (pandoc.utils.type(x) == 'Meta' and pandoc.Meta(x) or copy_table(x))
       end
       namedfilter.filter[name] = function (obj, context)
         local orig = copy(obj)
         local result, descend = fn(obj, context)
         if result == nil then
-          if not equals(obj, orig) then
+          if type(obj) ~= 'table' and not equals(obj, orig) then
             warn(
               "\nFunction '" .. name .. "' in filter '" .. namedfilter.name ..
               "' returned `nil`, but modified the input."