public static ErrorType asError(TypeMirror maybeErrorType) {
    return maybeErrorType.accept(ErrorTypeVisitor.INSTANCE, null);
  }
Returns a {@link ExecutableType} if the {@link TypeMirror} represents an executable type such
as may result from missing code, or bad compiles or throws an {@link IllegalArgumentException}.
common/src/main/java/com/google/auto/common/MoreTypes.java