Fix #4877: Exponentiation operators (#4881)

* Passthrough exponentiation operator; remove tests that are invalid JavaScript

* Treat **= as a passthrough assignment

* Get tests passing in Node 6

* Improve scoping

* Move exponentiation tests into their own file, now that it's filtered out by Cakefile

* Restore original test

Author: GeoffreyBooth

lib/coffeescript/nodes.js

@@ -2222,9 +2222,9 @@ exports.Assign = class Assign extends Base
 
       return @compileSplice       o if @variable.isSplice()
       return @compileConditional  o if @context in ['||=', '&&=', '?=']
-      return @compileSpecialMath  o if @context in ['**=', '//=', '%%=']
+      return @compileSpecialMath  o if @context in ['//=', '%%=']
 
-    unless @context
+    if not @context or @context is '**='
       varBase = @variable.unwrapAll()
       unless varBase.isAssignable()
         @variable.error "'#{@variable.compile o}' can't be assigned"
@@ -2548,7 +2548,7 @@ exports.Assign = class Assign extends Base
       fragments = new Op(@context[...-1], left, new Assign(right, @value, '=')).compileToFragments o
       if o.level <= LEVEL_LIST then fragments else @wrapInParentheses fragments
 
-  # Convert special math assignment operators like `a **= b` to the equivalent
+  # Convert special math assignment operators like `a //= b` to the equivalent
   # extended form `a = a ** b` and then compiles that.
   compileSpecialMath: (o) ->
     [left, right] = @variable.cacheReference o
@@ -3253,7 +3253,6 @@ exports.Op = class Op extends Base
     return @compileChain        o if isChain
     switch @operator
       when '?'  then @compileExistence o, @second.isDefaultValue
-      when '**' then @compilePower o
       when '//' then @compileFloorDivision o
       when '%%' then @compileModulo o
       else
@@ -3320,11 +3319,6 @@ exports.Op = class Op extends Base
       parts.push [@makeCode ")"] if o.level >= LEVEL_PAREN
     @joinFragmentArrays parts, ''
 
-  compilePower: (o) ->
-    # Make a Math.pow call
-    pow = new Value new IdentifierLiteral('Math'), [new Access new PropertyName 'pow']
-    new Call(pow, [@first, @second]).compileToFragments o
-
   compileFloorDivision: (o) ->
     floor = new Value new IdentifierLiteral('Math'), [new Access new PropertyName 'floor']
     second = if @second.shouldCache() then new Parens @second else @second

src/nodes.coffee

@@ -0,0 +1,19 @@
+# The `**` and `**=` operators are only supported in Node 7.5+, so the tests
+# for these exponentiation operators are split out into their own file to be
+# loaded only by supported runtimes.
+
+test "exponentiation operator", ->
+  eq 27, 3 ** 3
+
+test "exponentiation operator has higher precedence than other maths operators", ->
+  eq 55, 1 + 3 ** 3 * 2
+  eq -4, -2 ** 2
+  eq 0, (!2) ** 2
+
+test "exponentiation operator is right associative", ->
+  eq 2, 2 ** 1 ** 3
+
+test "exponentiation operator compound assignment", ->
+  a = 2
+  a **= 3
+  eq 8, a

test/exponentiation.coffee

@@ -168,7 +168,7 @@ test "`throw` can be yielded", ->
   throws -> x.next()
 
 test "symbolic operators has precedence over the `yield`", ->
-  symbolic   = '+ - * / << >> & | || && ** ^ // or and'.split ' '
+  symbolic   = '+ - * / << >> & | || && ^ // or and'.split ' '
   compound   = ("#{op}=" for op in symbolic)
   relations  = '< > == != <= >= is isnt'.split ' '
 

test/generators.coffee

@@ -139,7 +139,7 @@ eq 'multiline nested "interpolations" work', """multiline #{
 
 eq 'function(){}', "#{->}".replace /\s/g, ''
 ok /^a[\s\S]+b$/.test "a#{=>}b"
-ok /^a[\s\S]+b$/.test "a#{ (x) -> x ** 2 }b"
+ok /^a[\s\S]+b$/.test "a#{ (x) -> x %% 2 }b"
 
 # Regular Expression Interpolation
 

test/interpolation.coffee

@@ -305,24 +305,6 @@ test "#2508: Existential access of the prototype", ->
   eq NonExistent?::nothing, undefined
   ok Object?::toString
 
-test "power operator", ->
-  eq 27, 3 ** 3
-
-test "power operator has higher precedence than other maths operators", ->
-  eq 55, 1 + 3 ** 3 * 2
-  eq -4, -2 ** 2
-  eq false, !2 ** 2
-  eq 0, (!2) ** 2
-  eq -2, ~1 ** 5
-
-test "power operator is right associative", ->
-  eq 2, 2 ** 1 ** 3
-
-test "power operator compound assignment", ->
-  a = 2
-  a **= 3
-  eq 8, a
-
 test "floor division operator", ->
   eq 2, 7 // 3
   eq -3, -7 // 3