"ValueError: Invalid Choice" with neither property name nor value

[lhupfeldt]

Describe the problem

During copier update I get ValueError: Invalid Choice but there is no mention of the property name or the invalid value.
This is caused an earlier update in copier.yaml making difficult to debug (i.e. I don't know which change is causing this.)
Issue may be related to multi select choice, I will update if I figure it out.

Template

Sorry, cannot provide the template.

To Reproduce

No response

Logs

Expected behavior

The exception message includes property name, valid choices and bad choice causing the error.

Screenshots/screencasts/logs

No response

Operating system

Linux

Operating system distribution and version

Fedora 40

Copier version

9.7.1

Python version

3.12.9

Installation method

uvx+pypi

Additional context

Install is uv venv ...; uv pip ...

[lhupfeldt]

The issue was caused by a wrong handling of a merge conflict storing a value for a multi select property which was invalid for the template version registered in copier-answers.
But is it really necessary to throw an exception in this case? Why not simply prompt for the new value or accept valid data argument?