Don’t allow mixing different types of whitespace for indentation, per line.

src/lexer.coffee

@@ -349,6 +349,10 @@ exports.Lexer = class Lexer
     noNewlines = @unfinished()
 
     newIndentLiteral = if size > 0 then indent[-size..] else ''
+    unless /^(.?)\1*$/.exec newIndentLiteral
+      @error 'mixed indentation', offset: indent.length
+      return indent.length
+
     minLiteralLength = Math.min newIndentLiteral.length, @indentLiteral.length
     if newIndentLiteral[...minLiteralLength] isnt @indentLiteral[...minLiteralLength]
       @error 'indentation mismatch', offset: indent.length

test/error_messages.coffee

@@ -41,18 +41,6 @@ test "compiler error formatting", ->
                  ^^^^
   '''
 
-test "compiler error formatting with mixed tab and space", ->
-  assertErrorFormat """
-    \t  if a
-    \t  test
-  """,
-  '''
-    [stdin]:1:4: error: unexpected if
-    \t  if a
-    \t  ^^
-  '''
-
-
 if require?
   fs   = require 'fs'
   path = require 'path'

test/formatting.coffee

@@ -266,21 +266,24 @@ test "don’t allow mixing of spaces and tabs for indentation", ->
   catch e
     eq 'indentation mismatch', e.message
 
-test "indentation can be a mix of spaces and tabs, if each line is the same", ->
+test "each code block that starts at indentation 0 can use a different style", ->
   doesNotThrow ->
+    CoffeeScript.compile '''
+      new Layer
+       x: 0
+       y: 1
+      new Layer
+      	x: 0
+      	y: 1
+    '''
+
+test "tabs and spaces cannot be mixed for indentation", ->
+  try
     CoffeeScript.compile '''
       new Layer
       	 x: 0
       	 y: 1
     '''
-
-test "each code block that starts at indentation 0 can use a different style", ->
-    doesNotThrow ->
-      CoffeeScript.compile '''
-        new Layer
-         x: 0
-         y: 1
-        new Layer
-        	x: 0
-        	y: 1
-      '''
+    ok no
+  catch e
+    eq 'mixed indentation', e.message

test/scope.coffee

@@ -59,21 +59,21 @@ test "#1183: super + fat arrows", ->
   dolater = (cb) -> cb()
 
   class A
-  	constructor: ->
-  		@_i = 0
-  	foo : (cb) ->
-  		dolater =>
-  			@_i += 1
-  			cb()
+    constructor: ->
+      @_i = 0
+    foo : (cb) ->
+      dolater =>
+        @_i += 1
+        cb()
 
   class B extends A
-  	constructor : ->
-  		super
-  	foo : (cb) ->
-  		dolater =>
-  			dolater =>
-  				@_i += 2
-  				super cb
+    constructor : ->
+      super
+    foo : (cb) ->
+      dolater =>
+        dolater =>
+          @_i += 2
+          super cb
 
   b = new B
   b.foo => eq b._i, 3