ADL deserializer not picked up for non default-constructible type

[SylvainCorlay]

Bug Report

• What is the issue you have?

I am seeing a situation where

v = j;

picks up the default deserializer, which yields in my case

Standard Exception: [json.exception.type_error.302] type must be number, but is array

while an unqualified call to from_Json like

from_json(j, v);

results in the right overload being picked up through ADL.

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?

This came up in the xtensor PR xtensor-stack/xtensor#830 in which I joined an example notebook:

https://github.com/SylvainCorlay/xtensor/blob/d4a67ac488fc10a99b7238d9314a9970f5562c34/notebooks/json.ipynb

• Which compiler and operating system are you using? Is it a supported compiler?

Clang (through cling) for the example, but I see the same thing in gcc.

• Did you use a released version of the library or the version from the develop branch?

no

• If you experience a compilation error: can you compile and run the unit tests?

no compilation error

[SylvainCorlay]

Note that the same thing works for other xt:: types that are default-constructible (like containers) while it fails for xview, which are not default-constructible.

[SylvainCorlay]

Hum, I guess that this would probably require xt::xview to have an operator= taking the JSON object as an rhs.