Created attachment 55262 [details]
Testcase for slow -fimplicit-constexpr

The attached test case behaves as follows:

% time g++ -std=c++20 -o /dev/null -c u.ii
g++ -std=c++20 -o /dev/null -c u.ii  0.07s user 0.01s system 99% cpu 0.078 total

% time g++ -std=c++20 -o /dev/null -fimplicit-constexpr -c u.ii
g++ -std=c++20 -o /dev/null -fimplicit-constexpr -c u.ii  1637.98s user 41.63s system 99% cpu 27:59.91 total

The second run also consumes a large amount of memory in the region of 100GB.

I'm a bit stuck on how to reduce this further.