parse.y: else without rescue

* parse.y (bodystmt): [EXPERIMENTAL] make `else` without `rescue`
  a syntax error.  [DevelopersMeeting20180315Japan]

  https://bugs.ruby-lang.org/projects/ruby/wiki/DevelopersMeeting20180315Japan
  https://docs.google.com/document/d/1RT0ijSo8uJ4Awn3CEvuYkjH0TVeXSYgeAFNmVGYC3ak/edit#
  > * do-else-end
  >   https://twitter.com/joker1007/status/974173396006129664
  >   https://twitter.com/joker1007/status/974173641347756032
  >   https://twitter.com/joker1007/status/974176512554369027
  >   Will be SyntaxError in 2.6-preview2
  >   All of begin/do/def (experimental)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62901 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

NEWS

@@ -18,6 +18,8 @@ with all sufficient information, see the ChangeLog file or Redmine
 
 * refinements take place at block passing.  [Feature #14223]
 
+* `else` without `rescue` is now causes a syntax error.  [EXPERIMENTAL]
+
 === Core classes updates (outstanding ones only)
 
 * Array

parse.y

@@ -1000,7 +1000,7 @@ bodystmt	: compstmt
 			    $$ = NEW_RESCUE($1, $2, $3, &@$);
 			}
 			else if ($3) {
-			    rb_warn0("else without rescue is useless");
+			    compile_error(p, "else without rescue is useless");
 			    $$ = block_append(p, $$, $3);
 			}
 			if ($4) {

spec/ruby/language/rescue_spec.rb

@@ -195,18 +195,26 @@ describe "The rescue keyword" do
     ScratchPad.recorded.should == [:one, :else_ran, :ensure_ran, :outside_begin]
   end
 
-  it "will execute an else block even without rescue and ensure" do
-    lambda {
-      eval <<-ruby
+  else_without_rescue = lambda {
+    eval <<-ruby
         begin
           ScratchPad << :begin
         else
           ScratchPad << :else
         end
       ruby
-    }.should complain(/else without rescue is useless/)
+  }
 
-    ScratchPad.recorded.should == [:begin, :else]
+  ruby_version_is ""..."2.6" do
+    it "will execute an else block even without rescue and ensure" do
+      else_without_rescue.should complain(/else without rescue is useless/)
+
+      ScratchPad.recorded.should == [:begin, :else]
+    end
+  end
+
+  ruby_version_is "2.6" do
+    else_without_rescue.should raise_error(SyntaxError)
   end
 
   it "will not execute an else block if an exception was raised" do

test/ruby/test_parse.rb

@@ -14,13 +14,12 @@ class TestParse < Test::Unit::TestCase
   end
 
   def test_else_without_rescue
-    x = eval <<-END, nil, __FILE__, __LINE__+1
+    assert_syntax_error(<<-END, /else without rescue/)
       begin
       else
         42
       end
     END
-    assert_equal(42, x)
   end
 
   def test_alias_backref