Fix #4487: Outdentation bug (#4488)

Author: Julian Rosse

lib/coffeescript/lexer.js

@@ -414,12 +414,9 @@ exports.Lexer = class Lexer
       lastIndent = @indents[@indents.length - 1]
       if not lastIndent
         moveOut = 0
-      else if lastIndent is @outdebt
-        moveOut -= @outdebt
-        @outdebt = 0
-      else if lastIndent < @outdebt
-        @outdebt -= lastIndent
-        moveOut  -= lastIndent
+      else if @outdebt and moveOut <= @outdebt
+        @outdebt -= moveOut
+        moveOut   = 0
       else
         dent = @indents.pop() + @outdebt
         if outdentLength and @chunk[outdentLength] in INDENTABLE_CLOSERS

src/lexer.coffee

@@ -287,3 +287,20 @@ test "tabs and spaces cannot be mixed for indentation", ->
     ok no
   catch e
     eq 'mixed indentation', e.message
+
+test "#4487: Handle unusual outdentation", ->
+  a =
+    switch 1
+      when 2
+          no
+         when 3 then no
+      when 1 then yes
+  eq yes, a
+
+  b = do ->
+    if no
+      if no
+            1
+       2
+      3
+  eq b, undefined