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?
{
    static constexpr auto name = "toDateTime64";
    const DateTime64::NativeType scale_multiplier = 1;
    ToDateTime64TransformUnsigned(UInt32 scale = 0) 
        : scale_multiplier(DecimalUtils::scaleMultiplier<DateTime64::NativeType>(scale))
    {}
    inline NO_SANITIZE_UNDEFINED DateTime64::NativeType execute(FromType from, const DateLUTImpl &) const
    {
        from = std::min<time_t>(from, LUT_MAX_TIME);
        return DecimalUtils::decimalFromComponentsWithMultiplier<DateTime64>(from, 0, scale_multiplier);
    }
};
template <typename FromType>
struct ToDateTime64TransformSigned
{
    static constexpr auto name = "toDateTime64";
    const DateTime64::NativeType scale_multiplier = 1;
    ToDateTime64TransformSigned(UInt32 scale = 0) 
        : scale_multiplier(DecimalUtils::scaleMultiplier<DateTime64::NativeType>(scale))
    {}
    inline NO_SANITIZE_UNDEFINED DateTime64::NativeType execute(FromType from, const DateLUTImpl &) const
    {
        from = std::max<time_t>(from, LUT_MIN_TIME);
        from = std::min<time_t>(from, LUT_MAX_TIME);
        return DecimalUtils::decimalFromComponentsWithMultiplier<DateTime64>(from, 0, scale_multiplier);
    }