Deprecate #RRGGBBAA and #RGBA id parsing. (#2226)

See #2179

doc-src/SASS_CHANGELOG.md

@@ -39,6 +39,16 @@
   this is likely to result in a situation where the framework cannot
   resolve the function in 4.0.
 
+* Values that can be interpreted as
+  [hex colors with alpha channels][hex alpha spec] and also as
+  [ID values][directional focus spec], such as `#abcd`, now emit deprecation
+  warnings in preparation for being parsed differently Sass 3.6. They were
+  previously parsed as strings, and in 3.6 they will be parsed as colors
+  instead.
+
+[hex alpha spec]: https://drafts.csswg.org/css-color/#hex-notation
+[directional focus spec]: https://www.w3.org/TR/css-ui-3/#nav-dir
+
 ### Backwards Incompatibilities -- Must Read!
 
 * The way [CSS variables][] are handled has changed to better correspond to the

lib/sass/script/lexer.rb

@@ -368,8 +368,17 @@ 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]+$/ && (@scanner[0].length == 4 || @scanner[0].length == 7)
-          return [:color, Script::Value::Color.from_hex(@scanner[0])]
+        if @scanner[0] =~ /^\#[0-9a-fA-F]+$/
+          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
         end
         [:ident, @scanner[0]]
       end