[Bug c++/91759] [8/9/10 Regression] g++ accepts ill-formed deduction guides in wrong scope since r7-6608-ga56c0ac08242269b

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91759

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 48045
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48045&action=edit
gcc10-pr91759.patch

So, shall we restore the old diagnostics for the case where set_decl_namespace
isn't called like this?  Seems to work on check-c++-all
RUNTESTFLAGS=dg.exp=*deduct* so far.