Issue #1547 'use strict' duplicate formal parameter are prohibited

updated error message (thanks @davidchambers)

code style fixes

src/nodes.coffee

@@ -1162,8 +1162,10 @@ exports.Code = class Code extends Base
     o.scope.shared  = del(o, 'sharedScope')
     o.indent        += TAB
     delete o.bare
-    vars   = []
+    params     = []
     exprs      = []
+    for name in @paramNames() # this step must be performed before the others
+      unless o.scope.check name then o.scope.parameter name
     for param in @params when param.splat
       o.scope.add p.name.value, 'var', yes for p in @params when p.name.value
       splats = new Assign new Value(new Arr(p.asReference o for p in @params)),
@@ -1180,11 +1182,15 @@ exports.Code = class Code extends Base
           lit = new Literal ref.name.value + ' == null'
           val = new Assign new Value(param.name), param.value, '='
           exprs.push new If lit, val
-      vars.push ref unless splats
+      params.push ref unless splats
     wasEmpty = @body.isEmpty()
     exprs.unshift splats if splats
     @body.expressions.unshift exprs... if exprs.length
-    o.scope.parameter vars[i] = v.compile o for v, i in vars unless splats
+    o.scope.parameter params[i] = p.compile o for p, i in params
+    uniqs = []
+    for name in @paramNames()
+      throw SyntaxError "multiple parameters named '#{name}'" if name in uniqs
+      uniqs.push name
     @body.makeReturn() unless wasEmpty or @noReturn
     if @bound
       if o.scope.parent.method?.bound
@@ -1194,12 +1200,18 @@ exports.Code = class Code extends Base
     idt   = o.indent
     code  = 'function'
     code  += ' ' + @name if @ctor
-    code  += '(' + vars.join(', ') + ') {'
+    code  += '(' + params.join(', ') + ') {'
     code  += "\n#{ @body.compileWithDeclarations o }\n#{@tab}" unless @body.isEmpty()
     code  += '}'
     return @tab + code if @ctor
     if @front or (o.level >= LEVEL_ACCESS) then "(#{code})" else code
 
+  # A list of parameter names, excluding those generated by the compiler.
+  paramNames: ->
+    names = []
+    names.push param.names()... for param in @params
+    names
+
   # Short-circuit `traverseChildren` method to prevent it from crossing scope boundaries
   # unless `crossScope` is `true`.
   traverseChildren: (crossScope, func) ->
@@ -1223,7 +1235,8 @@ exports.Param = class Param extends Base
     node = @name
     if node.this
       node = node.properties[0].name
-      node = new Literal '_' + node.value if node.value.reserved
+      if node.value.reserved
+        node = new Literal o.scope.freeVariable node.value
     else if node.isComplex()
       node = new Literal o.scope.freeVariable 'arg'
     node = new Value node
@@ -1233,6 +1246,36 @@ exports.Param = class Param extends Base
   isComplex: ->
     @name.isComplex()
 
+  # Finds the name or names of a `Param`; useful for detecting duplicates.
+  # In a sense, a destructured parameter represents multiple JS parameters,
+  # thus this method returns an `Array` of names. 
+  # Reserved words used as param names, as well as the Object and Array 
+  # literals used for destructured params, get a compiler generated name 
+  # during the `Code` compilation step, so this is necessarily an incomplete
+  # list of a parameter's names.
+  names: (name = @name)->
+    atParam = (obj) ->
+      {value} = obj.properties[0].name
+      return if value.reserved then [] else [value]
+    # * simple literals `foo`
+    return [name.value] if name instanceof Literal
+    # * at-params `@foo`
+    return atParam(name) if name instanceof Value
+    names = []
+    for obj in name.objects
+      # * assignments within destructured parameters `{foo:bar}`
+      if obj instanceof Assign
+        names.push obj.variable.base.value
+      # * destructured parameters within destructured parameters `[{a}]`
+      else if obj.isArray() or obj.isObject()
+        names.push @names(obj.base)...
+      # * at-params within destructured parameters `{@foo}`
+      else if obj.this
+        names.push atParam(obj)...
+      # * simple destructured parameters {foo}
+      else names.push obj.base.value 
+    names
+
 #### Splat
 
 # A splat, either as a parameter to a function, an argument to a call,

test/functions.coffee

@@ -91,9 +91,9 @@ test "self-referencing functions", ->
 
 test "splats", ->
   arrayEq [0, 1, 2], (((splat...) -> splat) 0, 1, 2)
-  arrayEq [2, 3], (((_, _, splat...) -> splat) 0, 1, 2, 3)
+  arrayEq [2, 3], (((_, _1, splat...) -> splat) 0, 1, 2, 3)
-  arrayEq [0, 1], (((splat..., _, _) -> splat) 0, 1, 2, 3)
+  arrayEq [0, 1], (((splat..., _, _1) -> splat) 0, 1, 2, 3)
-  arrayEq [2], (((_, _, splat..., _) -> splat) 0, 1, 2, 3)
+  arrayEq [2], (((_, _1, splat..., _2) -> splat) 0, 1, 2, 3)
 
 test "@-parameters: automatically assign an argument's value to a property of the context", ->
   nonce = {}
@@ -131,7 +131,7 @@ test "destructuring in function definition", ->
 test "default values", ->
   nonceA = {}
   nonceB = {}
-  a = (_,_,arg=nonceA) -> arg
+  a = (_,_1,arg=nonceA) -> arg
   eq nonceA, a()
   eq nonceA, a(0)
   eq nonceB, a(0,0,nonceB)
@@ -139,7 +139,7 @@ test "default values", ->
   eq nonceA, a(0,0,null)
   eq false , a(0,0,false)
   eq nonceB, a(undefined,undefined,nonceB,undefined)
-  b = (_,arg=nonceA,_,_) -> arg
+  b = (_,arg=nonceA,_1,_2) -> arg
   eq nonceA, b()
   eq nonceA, b(0)
   eq nonceB, b(0,nonceB)
@@ -147,7 +147,7 @@ test "default values", ->
   eq nonceA, b(0,null)
   eq false , b(0,false)
   eq nonceB, b(undefined,nonceB,undefined)
-  c = (arg=nonceA,_,_) -> arg
+  c = (arg=nonceA,_,_1) -> arg
   eq nonceA, c()
   eq      0, c(0)
   eq nonceB, c(nonceB)