I expect it should work:
```
:) select CAST(CAST('{"x" : 1}', 'Object(\'json\')'), 'Object(Nullable(\'json\'))')

SELECT CAST(CAST('{"x" : 1}', 'Object(\'json\')'), 'Object(Nullable(\'json\'))')

Query id: fb057b45-b72a-4d22-9b38-a3136cdf2465


0 rows in set. Elapsed: 0.002 sec.

Received exception from server (version 22.5.1):
Code: 53. DB::Exception: Received from localhost:9000. DB::Exception: Cast to Object can be performed only from flatten named Tuple, Map or String. Got: Object('json'): While processing CAST(CAST('{"x" : 1}', 'Object(\'json\')'), 'Object(Nullable(\'json\'))'). (TYPE_MISMATCH)
```

@CurtizJ what do you think?
: DateTimeTransformImpl<DataTypeFloat64, DataTypeDateTime64, ToDateTime64TransformFloat<DataTypeFloat64, Float64>> {};
/** Conversion of DateTime64 to Date or DateTime: discards fractional part.
 */
template <typename Transform>
struct FromDateTime64Transform
{
    static constexpr auto name = Transform::name;
    const DateTime64::NativeType scale_multiplier = 1;
    FromDateTime64Transform(UInt32 scale) 
        : scale_multiplier(DecimalUtils::scaleMultiplier<DateTime64::NativeType>(scale))
    {}
    inline auto execute(DateTime64::NativeType dt, const DateLUTImpl & time_zone) const
    {
        const auto c = DecimalUtils::splitWithScaleMultiplier(DateTime64(dt), scale_multiplier);
        return Transform::execute(static_cast<UInt32>(c.whole), time_zone);
    }
};
/** Conversion of DateTime64 to Date or DateTime: discards fractional part.
 */
template <typename Name> struct ConvertImpl<DataTypeDateTime64, DataTypeDate, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeDateTime64, DataTypeDate, TransformDateTime64<ToDateImpl>> {};
template <typename Name> struct ConvertImpl<DataTypeDateTime64, DataTypeDateTime, Name, ConvertDefaultBehaviorTag>
    : DateTimeTransformImpl<DataTypeDateTime64, DataTypeDateTime, TransformDateTime64<ToDateTimeImpl>> {};
struct ToDateTime64Transform
{
    static constexpr auto name = "toDateTime64";
    const DateTime64::NativeType scale_multiplier = 1;
    ToDateTime64Transform(UInt32 scale = 0) 
        : scale_multiplier(DecimalUtils::scaleMultiplier<DateTime64::NativeType>(scale))
    {}