https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120237
--- Comment #2 from Tobias Burnus <burnus at gcc dot gnu.org> --- Iain pointed out (on IRC) that ISL > 0.24 will cause an in-tree build fail with GCC-10.5 due to its use of C++17 (and seemingly not properly adding -std=c++17, cf. PR 115077). [BTW: GCC requires C++14; GCC 9.5 declared C++17 as stable, but only GCC 11 enabled C++17 by default.] Thus, sticking to ISL 0.24 and only updating all others (or at least MPFR) seems to be decent.
