Fix #4609: support new.target (#5106)

* support new.target

* check token type

src/lexer.coffee

@@ -196,6 +196,8 @@ exports.Lexer = class Lexer
          @tokens.length > 1 and @tokens[@tokens.length - 2][0] not in ['.', '?.', '@']
         @error "'#{prev[1]}' cannot be used as a keyword, or as a function call
         without parentheses", prev[2]
+      else if prev[0] is '.' and @tokens.length > 1 and (prevprev = @tokens[@tokens.length - 2])[0] is 'UNARY' and prevprev[1] is 'new'
+        prevprev[0] = 'IDENTIFIER'
       else if @tokens.length > 2
         prevprev = @tokens[@tokens.length - 2]
         if prev[0] in ['@', 'THIS'] and prevprev and prevprev.spaced and

src/nodes.coffee

@@ -965,6 +965,7 @@ exports.Value = class Value extends Base
   # operators `?.` interspersed. Then we have to take care not to accidentally
   # evaluate anything twice when building the soak chain.
   compileNode: (o) ->
+    @checkNewTarget o
     @base.front = @front
     props = @properties
     if props.length and @base.cached?
@@ -984,6 +985,14 @@ exports.Value = class Value extends Base
 
     fragments
 
+  checkNewTarget: (o) ->
+    return unless @base instanceof IdentifierLiteral and @base.value is 'new' and @properties.length
+    if @properties[0] instanceof Access and @properties[0].name.value is 'target'
+      unless o.scope.parent?
+        @error "new.target can only occur inside functions"
+    else
+      @error "the only valid meta property for new is new.target"
+
   # Unfold a soak into an `If`: `a?.b` -> `a.b if a?`
   unfoldSoak: (o) ->
     @unfoldedSoak ?= do =>

test/classes.coffee

@@ -1899,4 +1899,26 @@ test "#4868: Incorrect ‘Can’t call super with @params’ error", ->
       super class then constructor: (@a) -> @a = 3
 
   d = new (new D).c
-  eq 3, d.a
+  eq 3, d.a
+
+test "#4609: Support new.target", ->
+  class A
+    constructor: ->
+      @calledAs = new.target.name
+
+  class B extends A
+
+  b = new B
+  eq b.calledAs, 'B'
+
+  newTarget = null
+  Foo = ->
+    newTarget = !!new.target
+
+  Foo()
+  eq newTarget, no
+
+  newTarget = null
+
+  new Foo()
+  eq newTarget, yes

test/error_messages.coffee

@@ -1907,3 +1907,21 @@ test "#3933: prevent implicit calls when cotrol flow is missing `THEN`", ->
       when a ->
              ^^
   '''
+
+test "`new.target` outside of a function", ->
+  assertErrorFormat '''
+    new.target
+  ''', '''
+    [stdin]:1:1: error: new.target can only occur inside functions
+    new.target
+    ^^^^^^^^^^
+  '''
+
+test "`new.target` is only allowed meta property", ->
+  assertErrorFormat '''
+    -> new.something
+  ''', '''
+    [stdin]:1:4: error: the only valid meta property for new is new.target
+    -> new.something
+       ^^^^^^^^^^^^^
+  '''