Fixes #2155 -- existential assignment to a closure

lib/coffee-script/nodes.js

@@ -1566,6 +1566,7 @@
       o.scope.shared = del(o, 'sharedScope');
       o.indent += TAB;
       delete o.bare;
+      delete o.isExistentialEquals;
       params = [];
       exprs = [];
       _ref3 = this.paramNames();

src/nodes.coffee

@@ -1171,6 +1171,7 @@ exports.Code = class Code extends Base
     o.scope.shared  = del(o, 'sharedScope')
     o.indent        += TAB
     delete o.bare
+    delete o.isExistentialEquals
     params = []
     exprs  = []
     for name in @paramNames() # this step must be performed before the others

test/operators.coffee

@@ -269,3 +269,9 @@ test "Regression with implicit calls against an indented assignment", ->
     1
 
   eq a, 1
+
+test "#2155 ... conditional assignment to a closure", ->
+  x = null
+  func = -> x ?= (-> if true then 'hi')
+  func()
+  eq x(), 'hi'