JMESPath - Merge function can't merge when receiving a json_const_ref

[BartelNieuwenhuyse]

JMESPath expressions in which the merge function is used may give the following error if the first argument of the merge function is of storage kind json_const_ref. This because the insert_or_assign method is not implemented for storage kind json_const_ref:

Attempting to access a member of a value that is not an object

Various JMESPath functions produce a json_const_ref, so if one of these is used as input to a merge function, the JMESPath expression doesn't work:

• map
• projections (object, list, flatten, slice, filter)
• multi select list and hash

Enumerate the steps to reproduce the bug
given:

{ "foz": "baz" }

expression with, for example, a multi select hash:
{ object: @ }.merge(object, { foo: 'bar' })

expected:

{ "foo": "bar", "foz": "baz" }

actual:
error Attempting to access a member of a value that is not an object: 'foo'

What jsoncons library version?

• Latest release 1.7.0
• Other release ______
• master

[danielaparker]

json_const_ref is an optimization that allows json values to hold references to already existing json values and so minimize expensive copies while evaluating a chain of expressions. At the end of the chain, before returning a value to the user, a deep copy must be made to make a self contained value.

In the merge function, the issue is probably that

    auto result = context.create_json(arg0);

needs to be

    auto result = context.create_json(deep_copy(arg0));

I believe the jmespath extension is the only part of this library that uses json_const_ref.

[BartelNieuwenhuyse]

Thank you for the clarification. I think a shallow copy will be enough for the merge function and retain some of the benefits of using refs instead of copies during the jmes evaluation.

I'll submit a pull request once I have it ready.