Fixes #2490 -- adding guard-style if blocks

lib/coffee-script/grammar.js

@@ -507,7 +507,10 @@
       })
     ],
     IfBlock: [
-      o('IF Expression Block', function() {
+      o('IF INDENT GuardBlock OUTDENT', function() {
+        $3.type = $1;
+        return $3;
+      }), o('IF Expression Block', function() {
         return new If($2, $3, {
           type: $1
         });
@@ -517,6 +520,15 @@
         }));
       })
     ],
+    GuardBlock: [
+      o('Expression Block', function() {
+        return new If($1, $2);
+      }), o('GuardBlock TERMINATOR Expression Block', function() {
+        return $1.addElse(new If($3, $4));
+      }), o('GuardBlock ELSE Block', function() {
+        return $1.addElse($3);
+      })
+    ],
     If: [
       o('IfBlock'), o('IfBlock ELSE Block', function() {
         return $1.addElse($3);

src/grammar.coffee

@@ -510,10 +510,19 @@ grammar =
   # if-related rules are broken up along these lines in order to avoid
   # ambiguity.
   IfBlock: [
+    o 'IF INDENT GuardBlock OUTDENT',           -> $3.type = $1; $3
     o 'IF Expression Block',                    -> new If $2, $3, type: $1
     o 'IfBlock ELSE IF Expression Block',       -> $1.addElse new If $4, $5, type: $3
   ]
 
+  # The guard-style form of *if*, where each expression serves as the truthy
+  # conditional.
+  GuardBlock: [
+    o 'Expression Block',                       -> new If $1, $2
+    o 'GuardBlock TERMINATOR Expression Block', -> $1.addElse new If $3, $4
+    o 'GuardBlock ELSE Block',                  -> $1.addElse $3
+  ]
+
   # The full complement of *if* expressions, including postfix one-liner
   # *if* and *unless*.
   If: [

test/control_flow.coffee

@@ -427,3 +427,23 @@ test "Throw should be usable as an expression.", ->
     throw new Error 'failed'
   catch e
     ok e is 'up'
+
+
+test "#2490. Block-style, guard-style if/else chain.", ->
+  result = if
+    1 - 1
+      false
+    process.someUndefinedThing
+      false
+    1 + 1
+      'correct'
+
+  eq result, 'correct'
+
+  func = -> if
+    null
+      false
+    else
+      'correct'
+
+  eq func(), 'correct'