[CS2] Throw an error for ambiguous get or set keywords or function calls (#4484)

* Throw an error for ambiguous `get` or `set` function calls or ES5 getter/setter keywords, to warn the user to use parentheses if they intend a function call (or to inform them that `get` or `set` cannot be used as a keyword)

* Code golf

* Catch get or set keyword before static method

* DRY up getting the previous token

* Throw an error if get or set are used as keywords before what looks like a function or method with an interpolated/dynamic name

* Allow `get` or `set` parentheses-less function calls when first argument is a string without a colon (so a plain string, not a property accessor)

* Revert "Allow `get` or `set` parentheses-less function calls when first argument is a string without a colon (so a plain string, not a property accessor)"

This reverts commit 2d1addf5a4.

* Optimization

* No longer throw an error on `get` or `set` function calls to objects with dynamic property names (introduces a way to circumvent our check for trying to avoid the `get` or `set` keywords, but not worth the complications for this tiny edge case)

src/lexer.coffee

@@ -132,7 +132,7 @@ exports.Lexer = class Lexer
       @token 'DEFAULT', id
       return id.length
 
-    [..., prev] = @tokens
+    prev = @prev()
 
     tag =
       if colon or prev? and
@@ -170,6 +170,16 @@ exports.Lexer = class Lexer
        isForFrom(prev)
       tag = 'FORFROM'
       @seenFor = no
+    # Throw an error on attempts to use `get` or `set` as keywords, or
+    # what CoffeeScript would normally interpret as calls to functions named
+    # `get` or `set`, i.e. `get({foo: function () {}})`
+    else if tag is 'PROPERTY' and prev
+      if prev.spaced and prev[0] in CALLABLE and /^[gs]et$/.test(prev[1])
+        @error "'#{prev[1]}' cannot be used as a keyword, or as a function call without parentheses", prev[2]
+      else
+        prevprev = @tokens[@tokens.length - 2]
+        if prev[0] in ['@', 'THIS'] and prevprev and prevprev.spaced and /^[gs]et$/.test(prevprev[1])
+          @error "'#{prevprev[1]}' cannot be used as a keyword, or as a function call without parentheses", prevprev[2]
 
     if tag is 'IDENTIFIER' and id in RESERVED
       @error "reserved word '#{id}'", length: id.length
@@ -237,8 +247,9 @@ exports.Lexer = class Lexer
 
     # If the preceding token is `from` and this is an import or export statement,
     # properly tag the `from`.
-    if @tokens.length and @value() is 'from' and (@seenImport or @seenExport)
-      @tokens[@tokens.length - 1][0] = 'FROM'
+    prev = @prev()
+    if prev and @value() is 'from' and (@seenImport or @seenExport)
+      prev[0] = 'FROM'
 
     regex = switch quote
       when "'"   then STRING_SINGLE
@@ -318,7 +329,7 @@ exports.Lexer = class Lexer
         [regex, body, closed] = match
         @validateEscapes body, isRegex: yes, offsetInChunk: 1
         index = regex.length
-        [..., prev] = @tokens
+        prev = @prev()
         if prev
           if prev.spaced and prev[0] in CALLABLE
             return 0 if not closed or POSSIBLY_DIVISION.test regex
@@ -440,7 +451,7 @@ exports.Lexer = class Lexer
   whitespaceToken: ->
     return 0 unless (match = WHITESPACE.exec @chunk) or
                     (nline = @chunk.charAt(0) is '\n')
-    [..., prev] = @tokens
+    prev = @prev()
     prev[if match then 'spaced' else 'newLine'] = true if prev
     if match then match[0].length else 0
 
@@ -468,7 +479,7 @@ exports.Lexer = class Lexer
     else
       value = @chunk.charAt 0
     tag  = value
-    [..., prev] = @tokens
+    prev = @prev()
 
     if prev and value in ['=', COMPOUND_ASSIGN...]
       skipToken = false
@@ -758,6 +769,10 @@ exports.Lexer = class Lexer
     [..., token] = @tokens
     token?[1]
 
+  # Get the previous token in the token stream.
+  prev: ->
+    @tokens[@tokens.length - 1]
+
   # Are we in the midst of an unfinished expression?
   unfinished: ->
     LINE_CONTINUER.test(@chunk) or

src/nodes.coffee

@@ -775,7 +775,7 @@ exports.Call = class Call extends Base
   constructor: (@variable, @args = [], @soak) ->
     super()
 
-    @isNew    = false
+    @isNew = no
     if @variable instanceof Value and @variable.isNotCallable()
       @variable.error "literal is not a function"