Parse hex colors with alpha channels

See sass/sass#2179

doc-src/SASS_CHANGELOG.md

@@ -8,6 +8,11 @@
 * Add support for importing an `_index.scss` or `_index.sass` file when
   importing a directory.
 
+### Backwards Incompatibilities -- Must Read!
+
+* Tokens such as `#abcd` that are ambiguous between ID strings and hex colors
+  with an alpha channel are now parsed as colors.
+
 ## 3.5.6 (22 March 2018)
 
 * Allow `!` in custom property values.

lib/sass/script/lexer.rb

@@ -368,24 +368,20 @@ def id
         # IDs in properties are used in the Basic User Interface Module
         # (http://www.w3.org/TR/css3-ui/).
         return unless scan(REGULAR_EXPRESSIONS[:id])
-        if @scanner[0] =~ /^\#[0-9a-fA-F]+$/
-          if @scanner[0].length == 4 || @scanner[0].length == 7
-            return [:color, Script::Value::Color.from_hex(@scanner[0])]
-          elsif @scanner[0].length == 5 || @scanner[0].length == 9
-            filename = @options[:filename]
-            Sass::Util.sass_warn <<MESSAGE
-DEPRECATION WARNING on line #{line}, column #{offset}#{" of #{filename}" if filename}:
-The value "#{@scanner[0]}" is currently parsed as a string, but it will be parsed as a color in
-future versions of Sass. Use "unquote('#{@scanner[0]}')" to continue parsing it as a string.
-MESSAGE
-          end
+        if @scanner[0] =~ /^\#[0-9a-fA-F]+$/ &&
+          (@scanner[0].length == 4 || @scanner[0].length == 5 ||
+           @scanner[0].length == 7 || @scanner[0].length == 9)
+          return [:color, Script::Value::Color.from_hex(@scanner[0])]
         end
         [:ident, @scanner[0]]
       end
 
       def color
         return unless @scanner.match?(REGULAR_EXPRESSIONS[:color])
-        return unless @scanner[0].length == 4 || @scanner[0].length == 7
+        unless @scanner[0].length == 4 || @scanner[0].length == 5 ||
+               @scanner[0].length == 7 || @scanner[0].length == 9
+          return
+        end
         script_color = Script::Value::Color.from_hex(scan(REGULAR_EXPRESSIONS[:color]))
         [:color, script_color]
       end

lib/sass/script/value/color.rb

@@ -277,13 +277,14 @@ def initialize(attrs, representation = nil, allow_both_rgb_and_hsl = false)
     #
     # @return [Color]
     def self.from_hex(hex_string, alpha = nil)
-      unless hex_string =~ /^#?([0-9a-f]{2})([0-9a-f]{2})([0-9a-f]{2})$/i ||
-             hex_string =~ /^#?([0-9a-f])([0-9a-f])([0-9a-f])$/i
+      unless hex_string =~ /^#?([0-9a-f]{2})([0-9a-f]{2})([0-9a-f]{2})([0-9a-f]{2})?$/i ||
+             hex_string =~ /^#?([0-9a-f])([0-9a-f])([0-9a-f])([0-9a-f])?$/i
         raise ArgumentError.new("#{hex_string.inspect} is not a valid hex color.")
       end
       red   = $1.ljust(2, $1).to_i(16)
       green = $2.ljust(2, $2).to_i(16)
       blue  = $3.ljust(2, $3).to_i(16)
+      alpha = $4.ljust(2, $4).to_i(16).to_f/0xff if $4
 
       hex_string = "##{hex_string}" unless hex_string[0] == ?#
       attrs = {:red => red, :green => green, :blue => blue, :representation => hex_string}