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 FunctionPtr create() { return std::make_shared<FunctionConvertFromString>(); }
    String getName() const override
    {
        return name;
    }
    bool isVariadic() const override { return true; }
    bool isSuitableForShortCircuitArgumentsExecution(const DataTypesWithConstInfo & ) const override { return true; }
    size_t getNumberOfArguments() const override { return 0; }
    bool useDefaultImplementationForConstants() const override { return true; }
    bool canBeExecutedOnDefaultArguments() const override { return false; }
    ColumnNumbers getArgumentsThatAreAlwaysConstant() const override { return {1}; }
    DataTypePtr getReturnTypeImpl(const ColumnsWithTypeAndName & arguments) const override
    {
        DataTypePtr res;
        if (isDateTime64<Name, ToDataType>(arguments))
        {
            validateFunctionArgumentTypes(*this, arguments,
                FunctionArgumentDescriptors{{"string", &isStringOrFixedString<IDataType>, nullptr, "String or FixedString"}},
                
                FunctionArgumentDescriptors{
                    {"precision", &isUInt8<IDataType>, isColumnConst, "const UInt8"},
                    {"timezone", &isStringOrFixedString<IDataType>, isColumnConst, "const String or FixedString"},
                });
            UInt64 scale = to_datetime64 ? DataTypeDateTime64::default_scale : 0;
            if (arguments.size() > 1)
                scale = extractToDecimalScale(arguments[1]);
            const auto timezone = extractTimeZoneNameFromFunctionArguments(arguments, 2, 0);
            res = scale == 0 ? res = std::make_shared<DataTypeDateTime>(timezone) : std::make_shared<DataTypeDateTime64>(scale, timezone);
        }