I don't think the issue has anything to do with GCC 5. I realized that I had 2 projects in my workspace with the same namespace name in them. This is the same workspace layout that I had in v20, so something is different. When I removed one of the projects everything in that namespace works fine.
However, I can still reproduce issues in my single project where I have something like this:
namespace Foo {
}
namespace Bar {
namespace Foo {
}
}
When I type "Foo::", autocomplete and indexing are only giving me the things in Bar::Foo, vs. the things in Foo.
I'm having trouble coming up with a self contained repro. I will keep working on it. But maybe this will help if there have been any changes in this area in v21.