Allow @ values as indices in for expressions

This loosens the compilation of `for` expressions to allow the index
variable to be an `@` value, e.g.

    do @visit for @node, @Index in nodes

Within `@visit`, the index of the current node (`@node`) would be
available as `@index`.

Fixes #4411.

src/nodes.coffee

@@ -2312,7 +2312,7 @@ exports.For = class For extends While
     @index.error 'cannot use index with for-from' if @from and @index
     source.ownTag.error "cannot use own with for-#{if @from then 'from' else 'in'}" if @own and not @object
     [@name, @index] = [@index, @name] if @object
-    @index.error 'index cannot be a pattern matching expression' if @index instanceof Value
+    @index.error 'index cannot be a pattern matching expression' if @index instanceof Value and not @index.isAssignable()
     @range   = @source instanceof Value and @source.base instanceof Range and not @source.properties.length and not @from
     @pattern = @name instanceof Value
     @index.error 'indexes do not apply to range loops' if @range and @index
@@ -2334,7 +2334,7 @@ exports.For = class For extends While
     name        = @name  and (@name.compile o, LEVEL_LIST) if not @pattern
     index       = @index and (@index.compile o, LEVEL_LIST)
     scope.find(name)  if name and not @pattern
-    scope.find(index) if index
+    scope.find(index) if index and @index not instanceof Value
     rvar        = scope.freeVariable 'results' if @returns
     if @from
       ivar = scope.freeVariable 'x', single: true if @pattern

test/comprehensions.coffee

@@ -515,6 +515,16 @@ test "#2274: Allow @values as loop variables", ->
   obj.method()
   eq obj.item, 3
 
+test "#4411: Allow @values as loop indices", ->
+  obj =
+    index: null
+    get: -> @index
+    method: ->
+      @get() for _, @index in [1, 2, 3]
+  eq obj.index, null
+  arrayEq obj.method(), [0, 1, 2]
+  eq obj.index, 3
+
 test "#2525, #1187, #1208, #1758, looping over an array forwards", ->
   list = [0, 1, 2, 3, 4]
 

test/error_messages.coffee

@@ -1179,3 +1179,10 @@ test "tagged template literals must be called by an identifier", ->
     1"""#{b}"""
     ^
   '''
+
+test "can't use pattern matches for loop indices", ->
+  assertErrorFormat 'a for b, {c} in d', '''
+    [stdin]:1:10: error: index cannot be a pattern matching expression
+    a for b, {c} in d
+             ^^^
+  '''