Fix duplicated warnings in nested functions

Unreachable variable access and unreachable code checks could be
executed several times in nested functions.

spec/check_spec.lua

@@ -439,6 +439,19 @@ end
       ]])
    end)
 
+   it("detects unreachable code in nested function", function()
+      assert.same({
+         {code = "511", line = 4, column = 7, end_column = 12}
+      }, check[[
+return function()
+   return function()
+      do return end
+      return
+   end
+end
+      ]])
+   end)
+
    it("detects accessing uninitialized variables", function()
       assert.same({
          {code = "113", name = "get", line = 6, column = 8, end_column = 10},
@@ -456,6 +469,25 @@ return a
       ]])
    end)
 
+   it("detects accessing uninitialized variables in nested functions", function()
+      assert.same({
+         {code = "113", name = "get", line = 7, column = 8, end_column = 10},
+         {code = "321", name = "a", line = 7, column = 12, end_column = 12}
+      }, check[[
+return function() return function(...)
+local a
+
+if ... then
+   a = 5
+else
+   a = get(a)
+end
+
+return a
+end end
+      ]])
+   end)
+
    it("does not detect accessing unitialized variables incorrectly in loops", function()
       assert.same({
          {code = "113", name = "get", line = 4, column = 8, end_column = 10}

src/luacheck/reachability.lua

@@ -4,14 +4,14 @@ local reachability
 
 local function noop_callback() end
 
-local function reachability_callback(_, _, item, chstate)
+local function reachability_callback(_, _, item, chstate, nested)
    if not item then
       return true
    end
 
-   if item.lines then
+   if not nested and item.lines then
       for _, subline in ipairs(item.lines) do
-         reachability(chstate, subline)
+         reachability(chstate, subline, true)
       end
    end
 
@@ -29,9 +29,9 @@ local function reachability_callback(_, _, item, chstate)
 end
 
 -- Emits warnings: unreachable code, uninitialized access.
-function reachability(chstate, line)
+function reachability(chstate, line, nested)
    local reachable_indexes = {}
-   core_utils.walk_line_once(line, reachable_indexes, 1, reachability_callback, chstate)
+   core_utils.walk_line_once(line, reachable_indexes, 1, reachability_callback, chstate, nested)
 
    for i, item in ipairs(line.items) do
       if not reachable_indexes[i] then