code: RelDataType deriveTypeImpl(
  		SqlValidatorScope scope,
  		SqlNode operand) {
  		DeriveTypeVisitor v = new DeriveTypeVisitor(scope);
  		final RelDataType type = operand.accept(v);
  		return Objects.requireNonNull(scope.nullifyType(operand, type));
  	}
docs: Derives the type of a node, never null.