Fix #4878: Compile error when using destructuring with a splat or expansion in an array (#4879)

* fix #4878

* improvements

* test

* refactor

Author: zdenko

lib/coffeescript/nodes.js

@@ -2408,8 +2408,8 @@ exports.Assign = class Assign extends Base
       # Sort 'splatsAndExpans' so we can show error at first disallowed token.
       objects[splatsAndExpans.sort()[1]].error "multiple splats/expansions are disallowed in an assignment"
 
-    isSplat = splats.length
-    isExpans = expans.length
+    isSplat = splats?.length > 0
+    isExpans = expans?.length > 0
     isObject = @variable.isObject()
     isArray = @variable.isArray()
 
@@ -2458,7 +2458,7 @@ exports.Assign = class Assign extends Base
 
     # "Complex" `objects` are processed in a loop.
     # Examples: [a, b, {c, r...}, d], [a, ..., {b, r...}, c, d]
-    loopObjects = (objs, vvarTxt) =>
+    loopObjects = (objs, vvar, vvarTxt) =>
       objSpreads = hasObjSpreads objs
       for obj, i in objs
         # `Elision` can be skipped.
@@ -2488,16 +2488,16 @@ exports.Assign = class Assign extends Base
         assigns.push new Assign(vvar, vval, null, param: @param, subpattern: yes).compileToFragments o, LEVEL_LIST
 
     # "Simple" `objects` can be split and compiled to arrays, [a, b, c] = arr, [a, b, c...] = arr
-    assignObjects = (objs, vvarTxt) =>
+    assignObjects = (objs, vvar, vvarTxt) =>
       vvar = new Value new Arr(objs, yes)
       vval = if vvarTxt instanceof Value then vvarTxt else new Value new Literal(vvarTxt)
       assigns.push new Assign(vvar, vval, null, param: @param, subpattern: yes).compileToFragments o, LEVEL_LIST
 
-    processObjects = (objs, vvarTxt) ->
+    processObjects = (objs, vvar, vvarTxt) ->
       if complexObjects objs
-        loopObjects objs, vvarTxt
+        loopObjects objs, vvar, vvarTxt
       else
-        assignObjects objs, vvarTxt
+        assignObjects objs, vvar, vvarTxt
 
     # In case there is `Splat` or `Expansion` in `objects`,
     # we can split array in two simple subarrays.
@@ -2514,7 +2514,7 @@ exports.Assign = class Assign extends Base
       expIdx = splatsAndExpans[0]
       leftObjs = objects.slice 0, expIdx + (if isSplat then 1 else 0)
       rightObjs = objects.slice expIdx + 1
-      processObjects leftObjs, vvarText if leftObjs.length isnt 0
+      processObjects leftObjs, vvar, vvarText if leftObjs.length isnt 0
       if rightObjs.length isnt 0
         # Slice or splice `objects`.
         refExp = switch
@@ -2524,10 +2524,10 @@ exports.Assign = class Assign extends Base
           restVar = refExp
           refExp = o.scope.freeVariable 'ref'
           assigns.push [@makeCode(refExp + ' = '), restVar.compileToFragments(o, LEVEL_LIST)...]
-        processObjects rightObjs, refExp
+        processObjects rightObjs, vvar, refExp
     else
       # There is no `Splat` or `Expansion` in `objects`.
-      processObjects objects, vvarText
+      processObjects objects, vvar, vvarText
     assigns.push vvar unless top or @subpattern
     fragments = @joinFragmentArrays assigns, ', '
     if o.level < LEVEL_LIST then fragments else @wrapInParentheses fragments

src/nodes.coffee

@@ -945,3 +945,43 @@ test "#4673: complex destructured object spread variables", ->
 
   {{g}...} = g: 1
   eq g, 1
+
+test "#4878: Compile error when using destructuring with a splat or expansion in an array", ->
+  arr = ['a', 'b', 'c', 'd']
+
+  f1 = (list) ->
+    [first, ..., last] = list
+
+  f2 = (list) ->
+    [first..., last] = list
+
+  f3 = (list) ->
+    ([first, ...] = list); first
+
+  f4 = (list) ->
+    ([first, ...rest] = list); rest
+
+  arrayEq f1(arr), arr
+  arrayEq f2(arr), arr
+  arrayEq f3(arr), 'a'
+  arrayEq f4(arr), ['b', 'c', 'd']
+
+  foo = (list) ->
+    ret =
+      if list?.length > 0
+        [first, ..., last] = list
+        [first, last]
+      else
+        []
+
+  arrayEq foo(arr), ['a', 'd']
+
+  bar = (list) ->
+    ret =
+      if list?.length > 0
+        [first, ...rest] = list
+        [first, rest]
+      else
+        []
+
+  arrayEq bar(arr), ['a', ['b', 'c', 'd']]