The 5.7 kernel is out

> vector<bool>, unordered_set perf pessimizations

This should be fixed by disabling standard library.

> the language itself (operator overloading, ADL, etc.) that is unlikely to be allowed beyond RAII and some better typing behaviors.

This is more than enough. Just check out the horror show called latest incarnation of min()/max() macros.

Kernel was reinventing C++ for quite some time already:
TMP in kfifo.h, macroified bsearch and interval trees (include/linux/interval_tree_generic.h),
function overloading -- grep for __builtin_choose_expr()),
destructors and unique_ptr<>-- devm_alloc_*().

The real losses are compilation slowdown, mangled names and a _lot_ of casts from void * (at least initially).