Allow omission of parentheses in one line pattern matching [Feature #16182]

doc/syntax/pattern_matching.rdoc

@@ -140,7 +140,7 @@ Both array and hash patterns support "rest" specification:
   end
   #=> "matched"
 
-In +case+ (but not in <code>=></code> and +in+) expressions, parentheses around both kinds of patterns could be omitted:
+Parentheses around both kinds of patterns could be omitted:
 
   case [1, 2]
   in Integer, Integer
@@ -158,6 +158,12 @@ In +case+ (but not in <code>=></code> and +in+) expressions, parentheses around
   end
   #=> "matched"
 
+ [1, 2] => a, b
+ [1, 2] in a, b
+
+ {a: 1, b: 2, c: 3} => a:
+ {a: 1, b: 2, c: 3} in a:
+
 Find pattern is similar to array pattern but it can be used to check if the given object has any elements that match the pattern:
 
   case ["a", 1, "b", "c", 2]

parse.y

@@ -1732,7 +1732,7 @@ expr		: command_call
 			p->ctxt.in_kwarg = 1;
 		    }
 		    {$<tbl>$ = push_pvtbl(p);}
-		  p_expr
+		  p_top_expr_body
 		    {pop_pvtbl(p, $<tbl>4);}
 		    {
 			p->ctxt.in_kwarg = $<ctxt>3.in_kwarg;
@@ -1750,7 +1750,7 @@ expr		: command_call
 			p->ctxt.in_kwarg = 1;
 		    }
 		    {$<tbl>$ = push_pvtbl(p);}
-		  p_expr
+		  p_top_expr_body
 		    {pop_pvtbl(p, $<tbl>4);}
 		    {
 			p->ctxt.in_kwarg = $<ctxt>3.in_kwarg;

spec/ruby/language/pattern_matching_spec.rb

@@ -1135,5 +1135,16 @@ ruby_version_is "2.7" do
         result.should == true
       end
     end
+
+    ruby_version_is "3.1" do
+      it "can omit parentheses in one line pattern matching" do
+        [1, 2] => a, b
+        a.should == 1
+        b.should == 2
+
+        {a: 1} => a:
+        a.should == 1
+      end
+    end
   end
 end

test/ruby/test_pattern_matching.rb

@@ -1519,13 +1519,13 @@ END
     assert_raise(NoMatchingPatternError) do
       {a: 1} => {a: 0}
     end
-    assert_syntax_error("if {} => {a:}; end", /void value expression/)
-    assert_syntax_error(%q{
-      1 => a, b
-    }, /unexpected/, '[ruby-core:95098]')
-    assert_syntax_error(%q{
-      1 => a:
-    }, /unexpected/, '[ruby-core:95098]')
+
+    [1, 2] => a, b
+    assert_equal 1, a
+    assert_equal 2, b
+
+    {a: 1} => a:
+    assert_equal 1, a
 
     assert_equal true, (1 in 1)
     assert_equal false, (1 in 2)