Thanks for the PR, but why not free the buffer with HeapFree (GetProcessHeap(), allocatedMessage) as suggested in https://msdn.microsoft.com/en-us/library/windows/desktop/ms679351(v=vs.85).aspx instead of increasing the buffer size in a loop?

The documentation for the define indirectly calls out that it wasn't available for "Windows Store apps" for Windows 8 and 8.1 and the latest Windows SDK only defines it for "desktop family" applications. #280 also pointed out that FORMAT_MESSAGE_ALLOCATE_BUFFER was not defined. It not being defined for non-desktop family applications if Windows 10 is being targeted is probably a Windows header file bug, but cppformat would still have to deal with old SDKs even if that gets fixed.

So, basically, just to maximize compatibility with as many different Windows "versions" as possible.

I need to also amend this PR with a test to make sure that TBS_E_PROVISIONING_NOT_ALLOWED works consistently with directly calling FormatMessageW(ALLOCATE) since that error code forces the retry, I just wasn't aware of that case when I started.

Fair enough. I'm not quite sure what TBS_E_PROVISIONING_NOT_ALLOWED is about but I'll wait for you to update the PR.

The message text for TBS_E_PROVISIONING_NOT_ALLOWED is more than 500 characters long, so it'll just be to verify that the buffer is too small case is handled correctly.

Merged, thanks!