Fix PlaceholderInExtend to report comma sequences

Fixes sds#747

CHANGELOG.md

@@ -14,6 +14,8 @@
 * Fix `Shorthand` to not report shorthanded values with `!important` as having
   a shorthand length not specified in `allowed_shorthands`
 * Fix `UnnecessaryMantissa` to ignore decimal values in URL literals
+* Fix `PlaceholderInExtend` to report comma sequences starting with a
+  placeholder
 
 ## 0.49.0
 

lib/scss_lint/linter/placeholder_in_extend.rb

@@ -12,11 +12,13 @@ def visit_extend(node)
       # every word boundary (so %placeholder becomes ['%', 'placeholder']).
       selector = node.selector.join
 
-      # Ignore if this is a placeholder
-      return if selector.start_with?('%')
-
-      add_lint(node, 'Prefer using placeholder selectors (e.g. ' \
-                     '%some-placeholder) with @extend')
+      if selector.include?(',')
+        add_lint(node, 'Avoid comma sequences in `@extend` directives; ' \
+                       'prefer single placeholder selectors (e.g. `%some-placeholder`)')
+      elsif !selector.start_with?('%')
+        add_lint(node, 'Prefer using placeholder selectors (e.g. ' \
+                       '%some-placeholder) with @extend')
+      end
     end
   end
 end

spec/scss_lint/linter/placeholder_in_extend_spec.rb

@@ -31,6 +31,16 @@
     it { should report_lint line: 2 }
   end
 
+  context 'when extending with a comma sequence starting with a placeholder' do
+    let(:scss) { <<-SCSS }
+      p {
+        @extend %placeholder, .item;
+      }
+    SCSS
+
+    it { should report_lint line: 2 }
+  end
+
   context 'when extending with a placeholder' do
     let(:scss) { <<-SCSS }
       p {