function promotedPolicy( dtypes ) {
	var dt = promoteDataTypes( dtypes );
	if ( dt === null ) {
		throw new Error( format( 'invalid argument. Unable to apply type promotion rules when resolving a data type to which the input data types can be safely cast. Data types: [%s].', join( dtypes, ', ' ) ) );
	}
	return dt;
}
Applies the "promoted" policy by applying type promotion to the list of provided data types.

@private
@param {StringArray} dtypes - input ndarray data types
@throws {Error} must provide data types which can be safely cast to a common data type
@returns {string} output ndarray data type