Only allow `yield return` as a statement

Fixes #4097. Also happens to fix #4096. I also took the liberty to simplify the
error message for invalid use of `yield`.

lib/coffee-script/grammar.js

@@ -41,7 +41,7 @@
         return $1.push($3);
       }), o('Body TERMINATOR')
     ],
-    Line: [o('Expression'), o('Statement')],
+    Line: [o('Expression'), o('Statement'), o('YieldReturn')],
     Statement: [
       o('Return'), o('Comment'), o('STATEMENT', function() {
         return new Literal($1);
@@ -51,8 +51,6 @@
     Yield: [
       o('YIELD', function() {
         return new Op($1, new Value(new Literal('')));
-      }), o('YIELD Statement', function() {
-        return new Op($1, $2);
       }), o('YIELD Expression', function() {
         return new Op($1, $2);
       }), o('YIELD FROM Expression', function() {
@@ -142,6 +140,13 @@
         return new Return;
       })
     ],
+    YieldReturn: [
+      o('YIELD RETURN Expression', function() {
+        return new YieldReturn($3);
+      }), o('YIELD RETURN', function() {
+        return new YieldReturn;
+      })
+    ],
     Comment: [
       o('HERECOMMENT', function() {
         return new Comment($1);

lib/coffee-script/nodes.js

@@ -1,6 +1,6 @@
 // Generated by CoffeeScript 1.10.0
 (function() {
-  var Access, Arr, Assign, Base, Block, Call, Class, Code, CodeFragment, Comment, Existence, Expansion, Extends, For, HEXNUM, IDENTIFIER, IS_REGEX, IS_STRING, If, In, Index, LEVEL_ACCESS, LEVEL_COND, LEVEL_LIST, LEVEL_OP, LEVEL_PAREN, LEVEL_TOP, Literal, NEGATE, NO, NUMBER, Obj, Op, Param, Parens, RESERVED, Range, Return, SIMPLENUM, STRICT_PROSCRIBED, Scope, Slice, Splat, Switch, TAB, THIS, Throw, Try, UTILITIES, Value, While, YES, addLocationDataFn, compact, del, ends, extend, flatten, fragmentsToText, isComplexOrAssignable, isLiteralArguments, isLiteralThis, locationDataToString, merge, multident, parseNum, ref1, ref2, some, starts, throwSyntaxError, unfoldSoak, utility,
+  var Access, Arr, Assign, Base, Block, Call, Class, Code, CodeFragment, Comment, Existence, Expansion, Extends, For, HEXNUM, IDENTIFIER, IS_REGEX, IS_STRING, If, In, Index, LEVEL_ACCESS, LEVEL_COND, LEVEL_LIST, LEVEL_OP, LEVEL_PAREN, LEVEL_TOP, Literal, NEGATE, NO, NUMBER, Obj, Op, Param, Parens, RESERVED, Range, Return, SIMPLENUM, STRICT_PROSCRIBED, Scope, Slice, Splat, Switch, TAB, THIS, Throw, Try, UTILITIES, Value, While, YES, YieldReturn, addLocationDataFn, compact, del, ends, extend, flatten, fragmentsToText, isComplexOrAssignable, isLiteralArguments, isLiteralThis, locationDataToString, merge, multident, parseNum, ref1, ref2, some, starts, throwSyntaxError, unfoldSoak, utility,
     extend1 = function(child, parent) { for (var key in parent) { if (hasProp.call(parent, key)) child[key] = parent[key]; } function ctor() { this.constructor = child; } ctor.prototype = parent.prototype; child.prototype = new ctor(); child.__super__ = parent.prototype; return child; },
     hasProp = {}.hasOwnProperty,
     indexOf = [].indexOf || function(item) { for (var i = 0, l = this.length; i < l; i++) { if (i in this && this[i] === item) return i; } return -1; },
@@ -652,18 +652,13 @@
     };
 
     Return.prototype.compileNode = function(o) {
-      var answer, exprIsYieldReturn, ref3;
+      var answer;
       answer = [];
-      exprIsYieldReturn = (ref3 = this.expression) != null ? typeof ref3.isYieldReturn === "function" ? ref3.isYieldReturn() : void 0 : void 0;
-      if (!exprIsYieldReturn) {
-        answer.push(this.makeCode(this.tab + ("return" + (this.expression ? " " : ""))));
-      }
+      answer.push(this.makeCode(this.tab + ("return" + (this.expression ? " " : ""))));
       if (this.expression) {
         answer = answer.concat(this.expression.compileToFragments(o, LEVEL_PAREN));
       }
-      if (!exprIsYieldReturn) {
-        answer.push(this.makeCode(";"));
-      }
+      answer.push(this.makeCode(";"));
       return answer;
     };
 
@@ -671,6 +666,24 @@
 
   })(Base);
 
+  exports.YieldReturn = YieldReturn = (function(superClass1) {
+    extend1(YieldReturn, superClass1);
+
+    function YieldReturn() {
+      return YieldReturn.__super__.constructor.apply(this, arguments);
+    }
+
+    YieldReturn.prototype.compileNode = function(o) {
+      if (o.scope.parent == null) {
+        this.error('yield can only occur inside functions');
+      }
+      return YieldReturn.__super__.compileNode.apply(this, arguments);
+    };
+
+    return YieldReturn;
+
+  })(Return);
+
   exports.Value = Value = (function(superClass1) {
     extend1(Value, superClass1);
 
@@ -1921,8 +1934,7 @@
       this.body = body || new Block;
       this.bound = tag === 'boundfunc';
       this.isGenerator = !!this.body.contains(function(node) {
-        var ref3;
-        return node instanceof Op && ((ref3 = node.operator) === 'yield' || ref3 === 'yield*');
+        return (node instanceof Op && node.isYield()) || node instanceof YieldReturn;
       });
     }
 
@@ -2400,10 +2412,6 @@
       return (ref3 = this.operator) === 'yield' || ref3 === 'yield*';
     };
 
-    Op.prototype.isYieldReturn = function() {
-      return this.isYield() && this.first instanceof Return;
-    };
-
     Op.prototype.isUnary = function() {
       return !this.second;
     };
@@ -2571,19 +2579,17 @@
       parts = [];
       op = this.operator;
       if (o.scope.parent == null) {
-        this.error('yield statements must occur within a function generator.');
+        this.error('yield can only occur inside functions');
       }
       if (indexOf.call(Object.keys(this.first), 'expression') >= 0 && !(this.first instanceof Throw)) {
-        if (this.isYieldReturn()) {
-          parts.push(this.first.compileToFragments(o, LEVEL_TOP));
-        } else if (this.first.expression != null) {
+        if (this.first.expression != null) {
           parts.push(this.first.expression.compileToFragments(o, LEVEL_OP));
         }
       } else {
         if (o.level >= LEVEL_PAREN) {
           parts.push([this.makeCode("(")]);
         }
-        parts.push([this.makeCode("" + op)]);
+        parts.push([this.makeCode(op)]);
         if (((ref3 = this.first.base) != null ? ref3.value : void 0) !== '') {
           parts.push([this.makeCode(" ")]);
         }

src/grammar.coffee

@@ -83,10 +83,13 @@ grammar =
     o 'Body TERMINATOR'
   ]
 
-  # Block and statements, which make up a line in a body.
+  # Block and statements, which make up a line in a body. YieldReturn is a
+  # statement, but not included in Statement because that results in an ambigous
+  # grammar.
   Line: [
     o 'Expression'
     o 'Statement'
+    o 'YieldReturn'
   ]
 
   # Pure statements which cannot be expressions.
@@ -118,7 +121,6 @@ grammar =
 
   Yield: [
     o 'YIELD',                                  -> new Op $1, new Value new Literal ''
-    o 'YIELD Statement',                        -> new Op $1, $2
     o 'YIELD Expression',                       -> new Op $1, $2
     o 'YIELD FROM Expression',                  -> new Op $1.concat($2), $3
   ]
@@ -205,6 +207,11 @@ grammar =
     o 'RETURN',                                 -> new Return
   ]
 
+  YieldReturn: [
+    o 'YIELD RETURN Expression',                -> new YieldReturn $3
+    o 'YIELD RETURN',                           -> new YieldReturn
+  ]
+
   # A block comment.
   Comment: [
     o 'HERECOMMENT',                            -> new Comment $1

src/nodes.coffee

@@ -453,15 +453,21 @@ exports.Return = class Return extends Base
 
   compileNode: (o) ->
     answer = []
-    exprIsYieldReturn = @expression?.isYieldReturn?()
     # TODO: If we call expression.compile() here twice, we'll sometimes get back different results!
-    unless exprIsYieldReturn
-      answer.push @makeCode @tab + "return#{if @expression then " " else ""}"
+    answer.push @makeCode @tab + "return#{if @expression then " " else ""}"
     if @expression
       answer = answer.concat @expression.compileToFragments o, LEVEL_PAREN
-    answer.push @makeCode ";" unless exprIsYieldReturn
+    answer.push @makeCode ";"
     return answer
 
+# `yield return` works exactly like `return`, except that it turns the function
+# into a generator.
+exports.YieldReturn = class YieldReturn extends Return
+  compileNode: (o) ->
+    unless o.scope.parent?
+      @error 'yield can only occur inside functions'
+    super
+
 #### Value
 
 # A value, variable or literal or parenthesized, indexed or dotted into,
@@ -1384,7 +1390,7 @@ exports.Code = class Code extends Base
     @body        = body or new Block
     @bound       = tag is 'boundfunc'
     @isGenerator = !!@body.contains (node) ->
-      node instanceof Op and node.operator in ['yield', 'yield*']
+      (node instanceof Op and node.isYield()) or node instanceof YieldReturn
 
   children: ['params', 'body']
 
@@ -1701,9 +1707,6 @@ exports.Op = class Op extends Base
   isYield: ->
     @operator in ['yield', 'yield*']
 
-  isYieldReturn: ->
-    @isYield() and @first instanceof Return
-
   isUnary: ->
     not @second
 
@@ -1829,13 +1832,10 @@ exports.Op = class Op extends Base
   compileYield: (o) ->
     parts = []
     op = @operator
-    if not o.scope.parent?
-      @error 'yield statements must occur within a function generator.'
+    unless o.scope.parent?
+      @error 'yield can only occur inside functions'
     if 'expression' in Object.keys(@first) and not (@first instanceof Throw)
-      if @isYieldReturn()
-        parts.push @first.compileToFragments o, LEVEL_TOP
-      else if @first.expression?
-        parts.push @first.expression.compileToFragments o, LEVEL_OP
+      parts.push @first.expression.compileToFragments o, LEVEL_OP if @first.expression?
     else
       parts.push [@makeCode "("] if o.level >= LEVEL_PAREN
       parts.push [@makeCode op]

test/error_messages.coffee

@@ -824,3 +824,28 @@ test "#3926: implicit object in parameter list", ->
     (one, two, {three, four: five}, key: value) ->
                                        ^
   '''
+
+test "`yield` outside of a function", ->
+  assertErrorFormat '''
+    yield 1
+  ''', '''
+    [stdin]:1:1: error: yield can only occur inside functions
+    yield 1
+    ^^^^^^^
+  '''
+  assertErrorFormat '''
+    yield return
+  ''', '''
+    [stdin]:1:1: error: yield can only occur inside functions
+    yield return
+    ^^^^^^^^^^^^
+  '''
+
+test "#4097: `yield return` as an expression", ->
+  assertErrorFormat '''
+    -> (yield return)
+  ''', '''
+    [stdin]:1:5: error: cannot use a pure statement in an expression
+    -> (yield return)
+        ^^^^^^^^^^^^
+  '''

test/generators.coffee

@@ -70,9 +70,6 @@ test "bound generator", ->
   ok obj isnt obj.unbound().next().value
   eq obj, obj.nested().next().value.next().value.next().value
 
-test "error if `yield` occurs outside of a function", ->
-  throws -> CoffeeScript.compile 'yield 1'
-
 test "`yield from` support", ->
   x = do ->
     yield from do ->