Fix #4852, @get @set implicit object (#4867)

* Test cases for #4852, get/set oddities

* Fix soak before accessor call to `get` or `set`

* Fixes #4852: More get/set cases; cleanup style

* Check for tokens' existence before referencing them

Author: GeoffreyBooth

lib/coffeescript/lexer.js

@@ -192,13 +192,17 @@ exports.Lexer = class Lexer
     # 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$/.test(prev[1]) and @tokens[@tokens.length - 2][0] isnt '.'
-        @error "'#{prev[1]}' cannot be used as a keyword, or as a function call without parentheses", prev[2]
-      else
+      if prev.spaced and prev[0] in CALLABLE and /^[gs]et$/.test(prev[1]) and
+         @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 @tokens.length > 2
         prevprev = @tokens[@tokens.length - 2]
-        if prev[0] in ['@', 'THIS'] and prevprev and prevprev.spaced and /^[gs]et$/.test(prevprev[1]) and
-           @tokens[@tokens.length - 3][0] not in ['.', '@']
-          @error "'#{prevprev[1]}' cannot be used as a keyword, or as a function call without parentheses", prevprev[2]
+        if prev[0] in ['@', 'THIS'] and prevprev and prevprev.spaced and
+           /^[gs]et$/.test(prevprev[1]) and
+           @tokens[@tokens.length - 3][0] not in ['.', '?.', '@']
+          @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

src/lexer.coffee

@@ -823,6 +823,8 @@ test "#4524: functions named get or set can be used without parentheses when att
 
   eq 12, obj.get 10
   eq 13, obj.set 10
+  eq 12, obj?.get 10
+  eq 13, obj?.set 10
 
   eq 14, a.get 10
   eq 15, a.set 10
@@ -853,13 +855,32 @@ test "#4836: functions named get or set can be used without parentheses when att
 
   eq 12, this.get 10
   eq 13, this.set 10
+  eq 12, this?.get 10
+  eq 13, this?.set 10
   eq 6, this.get @a
   eq 7, this.set @a
+  eq 6, this?.get @a
+  eq 7, this?.set @a
 
   eq 12, @get 10
   eq 13, @set 10
+  eq 12, @?.get 10
+  eq 13, @?.set 10
   eq 6, @get @a
   eq 7, @set @a
+  eq 6, @?.get @a
+  eq 7, @?.set @a
+
+test "#4852: functions named get or set can be used without parentheses when attached to this or @, with an argument of an implicit object", ->
+  @get = ({ x }) -> x + 2
+  @set = ({ x }) -> x + 3
+
+  eq 12, @get x: 10
+  eq 13, @set x: 10
+  eq 12, @?.get x: 10
+  eq 13, @?.set x: 10
+  eq 12, this?.get x: 10
+  eq 13, this?.set x: 10
 
 test "#4473: variable scope in chained calls", ->
   obj =