Merge pull request #821 from ruby/fix/empty-rule-alternatives

Reject %empty misuse in later rule alternatives

Author: ydah

lib/lrama/parser.rb

@@ -446,6 +446,12 @@ rule
       IDENT_COLON named_ref? ":" rhs_list
         {
           result = val[3].map do |rhs|
+            if rhs.reject {|sym| sym.is_a?(Grammar::Node::RuleRhs::Action) || sym.is_a?(Grammar::Node::RuleRhs::Prec) }.count > 1
+              empties = rhs.select { |sym| sym.is_a?(Grammar::Node::RuleRhs::Empty) }
+              empties.each do |empty|
+                on_action_error("%empty on non-empty rule", empty)
+              end
+            end
             Grammar::Node::Rule.new(
               id: val[0],
               alias_name: val[1],
@@ -458,12 +464,6 @@ rule
   rhs_list:
       rhs
         {
-          if val[0].reject {|sym| sym.is_a?(Grammar::Node::RuleRhs::Action) || sym.is_a?(Grammar::Node::RuleRhs::Prec) }.count > 1
-            empties = val[0].select { |sym| sym.is_a?(Grammar::Node::RuleRhs::Empty) }
-            empties.each do |empty|
-              on_action_error("%empty on non-empty rule", empty)
-            end
-          end
           result = [val[0]]
         }
     | rhs_list "|" rhs

parser.y

@@ -4601,6 +4601,31 @@ class : keyword_class tSTRING keyword_end { code 1 }
           ERROR
         end
       end
+
+      context "when %empty is misused in a later alternative" do
+        [
+          ["A | B %empty", 13],
+          ["A | %empty B", 11],
+          ["A | %empty %empty", 11],
+          ["A | A | B %empty", 17],
+        ].each do |rhs, column|
+          it "rejects #{rhs}" do
+            y = <<~INPUT
+              %token A B
+              %%
+              start: #{rhs};
+            INPUT
+
+            parser = Lrama::Parser.new(y, "error_messages/parse.y")
+
+            expect { parser.parse }.to raise_error(ParseError, <<~ERROR)
+              error_messages/parse.y:3:#{column}: %empty on non-empty rule
+                 3 | start: #{rhs};
+                   | #{' ' * column}^~~~~~
+            ERROR
+          end
+        end
+      end
     end
   end