A DESCRIPTION OF THE PROBLEM :
Compilation fails if yield statement is followed by underscore, even if it is legal syntax, like yield _ -> {}. This is issue that has been present in jdk since introduction of unnamed variables and patterns

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile following code:

public class SwitchYieldAndUnderscore {
  public static void main(String[] args) {
    Consumer<String> value = switch (args[0]) {
      case "foo" -> {
        yield _ -> {};
      }
      default -> throw new AssertionError();
    };
  }
}

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Compilation successful
ACTUAL -
Compilation failes with 2 errors:
";" exprected
"=" exprected

---------- BEGIN SOURCE ----------
public class SwitchYieldAndUnderscore {
  public static void main(String[] args) {
    Consumer<String> value = switch (args[0]) {
      case "foo" -> {
        yield _ -> {};
      }
      default -> throw new AssertionError();
    };
  }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Enclose _ in paretheses like so: yield (_) -> {}

FREQUENCY : always