If you are lucky enough to have a unit test that catches an error, or if you can add a new test that specifically checks for an error, it offers a good and objective measure of resolution.
If you perform continuous assembly with regression testing, then as long as the corresponding test passes through your primary branch, the error can be resolved. The advantage of this is that it makes it easy to examine the error resolved on one branch but not resolved on the other, which will lead to an attempt to integrate at the beginning and measure success.
Depending on your culture, you may want the error to be marked as truly resolved if it passes automatic assemblies in all branches.
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