[Bug c++/80290] [6/7 Regression] g++ uses unreasonable amount of memory compiling nested string maps

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80290

--- Comment #14 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #13)
> typedef std::pair<unsigned, std::pair<const char *, std::pair<const char *,
> std::pair<const char *, const char *> > > > FruMap;
> extern const FruMap frus[] = {
>     { 1, { "", { "", {"",""}, }, }, },
> };
> 
> is twice as fast as
> 
> typedef std::pair<unsigned, std::pair<const char *, std::pair<const char *,
> std::pair<const char *, std::pair<const char *, const char *> > > > > FruMap;
> 
> extern const FruMap frus[] = {
>     { 1, { "", { "", { "", {"",""}, }, } }, },
> };
> 
> the latter with 70713 implicit_conversion calls while the former with "only"
> 14331 implicit_conversion calls.  Removing another std::pair<> level reduces
> it to 4389 calls (still 10689 tsubst_copy_and_build calls(!)).

Similar amount of cxx_eval_call_expression so it boils down to constexpr
handling...