svn merge not possible without destroying branch? This is worrying ...

I suspect what you really want to do is use VCS -> Integrate Project.

Source 1 would be your trunk and Source 2 is the feature branch.  This should do what you want and preserve the svn:mergeinfo properties.

Thanks for your reply.

Unfortunately that does a different thing: it modifies my current trunk in a way that it is converted to what the branch looks like. All changes done to my trunk (after branching) are then lost/overwritten.

A real svn merge (e.g. done on the command line) conserves my changes in trunk and only applies to it what has been changed in my branch after branching. And this is what I would like to achieve from within PyCharm. Since this is such a common operation I would have assumed that PyCharm has some sort of support for it ...

I see what you mean. After testing it out, I think you can just use the Merge From command and ignore the warning. It appears to be spurious, or perhaps only applies to older versions of SVN. After doing 'Merge From...' I'm still given the option to review and commit, and it seems the normal svn:mergeinfo property is retained properly. I don't see any reason why you couldn't continue working on the branch and merge again later.

I should amend that to say that one oddity appears to be that the property change is not included in the new generated changeset, and so before you commit you must remember to move it over (or move the file changes to the default changeset).