Recognize mutation with a long key chain

Previously `x.y = 1` was considered a mutation but `x.y.z = 2` was
a simple access. Recursively handle indexes in lhs of assignments
so that such long key chains are recognized as mutations, too.

spec/cli_spec.lua

@@ -234,7 +234,7 @@ Total: 4 warnings / 0 errors in 1 file
       assert.equal([[
 Checking spec/samples/bad_code.lua                6 warnings
 
-    spec/samples/bad_code.lua:1:1: accessing undefined variable 'package'
+    spec/samples/bad_code.lua:1:1: mutating non-standard global variable 'package'
     spec/samples/bad_code.lua:3:16: unused function 'helper'
     spec/samples/bad_code.lua:3:23: unused variable length argument
     spec/samples/bad_code.lua:7:10: setting non-standard global variable 'embrace'
@@ -750,7 +750,7 @@ spec/samples/good_code.lua
 return {}
 spec/samples/bad_code.lua
 (%d+)
-local A="113";return {{A,"package",1,1,7},{"211","helper",3,16,21,%[10%]=true},{"212","...",3,23,25},{"111","embrace",7,10,16,%[12%]=true},{"412","opt",8,10,12,7,18},{A,"hepler",9,11,16}}
+return {{"112","package",1,1,7},{"211","helper",3,16,21,%[10%]=true},{"212","...",3,23,25},{"111","embrace",7,10,16,%[12%]=true},{"412","opt",8,10,12,7,18},{"113","hepler",9,11,16}}
 spec/samples/python_code.lua
 (%d+)
 return {{"011",%[3%]=1,%[4%]=6,%[5%]=15,%[23%]="expected '=' near '__future__'"}}

src/luacheck/cache.lua

@@ -9,7 +9,7 @@ local cache = {}
 -- third is check result in lua table format.
 -- String fields are compressed into array indexes.
 
-cache.format_version = 5
+cache.format_version = 6
 
 local fields = {
    "code", "name", "line", "column", "end_column", "prev_line", "prev_column", "secondary",

src/luacheck/linearize.lua

@@ -456,15 +456,7 @@ function LinState:emit_stmt_Set(node)
          end
       else
          assert(expr.tag == "Index")
-
-         if expr[1].tag == "Id" and not self:resolve_var(expr[1][1]) then
-            -- Warn about mutated global.
-            self:check_var(expr[1], "mutate")
-         else
-            self:scan_expr(item, expr[1])
-         end
-
-         self:scan_expr(item, expr[2])
+         self:scan_lhs_index(item, expr)
       end
    end
 
@@ -529,6 +521,19 @@ function LinState:scan_expr_Dots(item, node)
    self:mark_access(item, node)
 end
 
+function LinState:scan_lhs_index(item, node)
+   if node[1].tag == "Id" and not self:resolve_var(node[1][1]) then
+      -- Warn about mutated global.
+      self:check_var(node[1], "mutate")
+   elseif node[1].tag == "Index" then
+      self:scan_lhs_index(item, node[1])
+   else
+      self:scan_expr(item, node[1])
+   end
+
+   self:scan_expr(item, node[2])
+end
+
 LinState.scan_expr_Index = LinState.scan_exprs
 LinState.scan_expr_Call = LinState.scan_exprs
 LinState.scan_expr_Invoke = LinState.scan_exprs