[Bug #21669] Implement void value expression check for case/when

If the all `when` and `else` branches are void, the `case` is also
void.

parse.y

@@ -13864,6 +13864,18 @@ value_expr_check(struct parser_params *p, NODE *node)
           case NODE_RETRY:
             goto found;
 
+          case NODE_CASE:
+          case NODE_CASE2:
+            for (node = RNODE_CASE(node)->nd_body;
+                 node && nd_type_p(node, NODE_WHEN);
+                 node = RNODE_WHEN(node)->nd_next) {
+                if (!(vn = value_expr_check(p, RNODE_WHEN(node)->nd_body))) {
+                    return NULL;
+                }
+                if (!void_node) void_node = vn;
+            }
+            break;
+
           case NODE_CASE3:
             if (!RNODE_CASE3(node)->nd_body || !nd_type_p(RNODE_CASE3(node)->nd_body, NODE_IN)) {
                 compile_error(p, "unexpected node");

test/ruby/test_syntax.rb

@@ -2053,6 +2053,30 @@ eom
     };
   end
 
+  def test_value_expr_in_case
+    assert_syntax_error("#{<<~"{#"}\n#{<<~'};'}", /void value expression/, nil, "#{BUG_21669} 1.3")
+    {#
+      x =
+        case a
+        when 1; return
+        when 2; return
+        else return
+        end
+    };
+  end
+
+  def test_value_expr_in_case2
+    assert_syntax_error("#{<<~"{#"}\n#{<<~'};'}", /void value expression/, nil, "#{BUG_21669} 1.3")
+    {#
+      x =
+        case
+        when 1; return
+        when 2; return
+        else return
+        end
+    };
+  end
+
   def test_tautological_condition
     assert_valid_syntax("def f() return if false and invalid; nil end")
     assert_valid_syntax("def f() return unless true or invalid; nil end")