Fix event sorting

A warning and an inline option boundary event can share location
when a function declaration is unreachable code (a push event is generated
for each closure start). Previously event comparator assumed that only two
warnings or two inline option events can share location, resulting in an
error when this assumption is violated.

The fix is to compare events of different types by set priority before
falling back to code comparison for warning. Also remove a hack
that partially worked around this issue by adding `code` field
to some inline option events.

Ref #74.

spec/check_spec.lua

@@ -617,6 +617,17 @@ end
       ]])
    end)
 
+   it("detects unreachable functions", function()
+      assert.same({
+         {code = "231", name = "f", line = 1, column = 7, end_column = 7},
+         {code = "511", line = 3, column = 1, end_column = 8}
+      }, check[[
+local f = nil
+do return end
+function f() end
+      ]])
+   end)
+
    it("detects unreachable code in nested function", function()
       assert.same({
          {code = "511", line = 4, column = 7, end_column = 12}

src/luacheck/core_utils.lua

@@ -58,16 +58,33 @@ function core_utils.is_definition(opts, warning)
    return opts.allow_defined or (opts.allow_defined_top and warning.top)
 end
 
-local function location_comparator(event1, event2)
-   -- If two events share location, neither can be an invalid comment event.
-   -- However, they can be equal by identity due to the way table.sort is implemented.
-   return event1.line < event2.line or
-      event1.line == event2.line and (event1.column < event2.column or
-      event1.column == event2.column and event1.code and event1.code < event2.code)
+local function event_priority(event)
+   -- Inline option boundaries have priority over inline option declarations
+   -- so that `-- luacheck: push ignore foo` is interpreted correctly (push first).
+   if event.push or event.pop then
+      return -2
+   elseif event.options then
+      return -1
+   else
+      return tonumber(event.code)
+   end
+end
+
+local function event_comparator(event1, event2)
+   if event1.line ~= event2.line then
+      return event1.line < event2.line
+   elseif event1.column ~= event2.column then
+      return event1.column < event2.column
+   else
+      return event_priority(event1) < event_priority(event2)
+   end
 end
 
+-- Sorts an array of warnings, inline options (tables with `options` field)
+-- or inline option boundaries (tables with `push` or `pop` field) by location
+-- information as provided in `line` and `column` fields.
 function core_utils.sort_by_location(array)
-   table.sort(array, location_comparator)
+   table.sort(array, event_comparator)
 end
 
 return core_utils

src/luacheck/inline_options.lua

@@ -98,7 +98,7 @@ local function add_inline_option(events, per_line_opts, body, location, end_colu
    end
 
    if body == "push" or body == "pop" then
-      table.insert(events, {code = 1, [body] = true, line = location.line, column = location.column, end_column = end_column})
+      table.insert(events, {[body] = true, line = location.line, column = location.column, end_column = end_column})
 
       if after_push then
          body = after_push
@@ -120,7 +120,7 @@ local function add_inline_option(events, per_line_opts, body, location, end_colu
 
       table.insert(per_line_opts[location.line], opts)
    else
-      table.insert(events, {code = 2, options = opts, line = location.line, column = location.column, end_column = end_column})
+      table.insert(events, {options = opts, line = location.line, column = location.column, end_column = end_column})
    end
 
    return true
@@ -216,7 +216,7 @@ local function handle_events(events, per_line_opts)
 
    -- Go through all events.
    for _, event in ipairs(events) do
-      if type(event.code) == "string" then
+      if event.code then
          -- It's a warning, put it into list of not handled warnings.
          table.insert(unfiltered_warnings, event)
       elseif event.options then