[Bug c++/64826] New: Better diagnostics for when ADL/overloading doesn't apply as desired

[b.r.longbons at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64826

            Bug ID: 64826
           Summary: Better diagnostics for when ADL/overloading doesn't
                    apply as desired
           Product: gcc
           Version: 4.9.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: b.r.longbons at gmail dot com

Created attachment 34596
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34596&action=edit
test set 1, regressions on namespace splitting

Recently I refactored a fair bit of code between namespaces, and found a number
of situations that could use better error messages.

Additionally, it would be nice if at least some attempt was made to provide a
warning when even *before* the namespace.

I mostly understand *why*, but in real world code bases it is often harder than
in these reduced testcases.

Attached are 3 files, each with testcases of similar code inside and outside
namespaces. One version of each test fails, the other works (except for
works-both-ways tests, which are type sensitive).