automatic coercion from unitless numbers

lib/sass/constant/number.rb

@@ -104,7 +104,11 @@ module Sass::Constant
     end
 
     def operate(other, operation)
+      if unitless?
+        self.coerce!(operation, other.numerator_units, other.denominator_units)
+      else
         other.coerce!(operation, numerator_units, denominator_units)
+      end
       v = self.value.send(operation, other.value)
       Number.from_value(v, *compute_units(other, operation))
     rescue ArgumentError
@@ -113,8 +117,10 @@ module Sass::Constant
 
     def coerce!(operation, num_units, den_units)
       if [:+, :-].include?(operation)
+        unless unitless?
           @value *= coercion_factor(numerator_units, num_units)
           @value /= coercion_factor(denominator_units, den_units)
+        end
         @numerator_units = num_units.dup
         @denominator_units = den_units.dup
       end

test/sass/engine_test.rb

@@ -38,7 +38,6 @@ class SassEngineTest < Test::Unit::TestCase
     "!a = 1b + 2c" => "Incompatible units: b and c.",
     "a\n  :b= 1b * 2c" => "Incompatible units: b and c.",
     "a\n  :b= 1b % 2c" => "Cannot modulo by a number with units: 2c.",
-    "!a = 1b + 2" => "Incompatible units: b and unitless.",
     "!a = 2px + #ccc" => "Cannot add a number with units (2px) to a color (#cccccc).",
     "!a = #ccc + 2px" => "Cannot add a number with units (2px) to a color (#cccccc).",
     "& a\n  :b c" => ["Base-level rules cannot contain the parent-selector-referencing character '&'.", 1],

test/sass/templates/constants.sass

@@ -1,4 +1,4 @@
-!width = 10em + 20em
+!width = 10em + 20
 !color = #00ff98
 !main_text = #ffa
 !num = 10
@@ -39,7 +39,7 @@
   :num
     :num= 5+2
     :num-un= 10em + 15em
-    :num-un2= 10em + 13em
+    :num-un2= 10 + 13em
     :num-neg= 10 + -.13
     :str= 100 + px
     :col= 13 + #aaa