Bug opened to track the Windows arm64 thing (only a full reproducer available at the moment). https://bugs.llvm.org/show_bug.cgi?id=50227

Just completed testing... everything seems to be working correctly. So long as the all the tests pass, let's commit! :D

And with those modifications, everything seems to work correctly. I'd be fine with the LLVM portion landing at this stage, but one more rev and another test cycle for this patch would be ideal.

Apologies, I meant to write "here's what Clang is trying to call" in the previous comment. It's clear from looking at MSVC's output that MSVC expects to return indirectly via X0, implying that _Smanip is not aggregate by MSVC's definition.

The current diff (196894) seems to have the same std::setw issue as the previous one. Here's what it's trying to compile:

Confirmed just now that the current diff (196774) does work, but it'd be good to correct user-provided constructors and trivial destructors before this lands.