Cannot use C++ on APPLICATION level initialization

[r2r0]

Describe the bug
Cannot use C++ in functions called at APPLICATION initialization level.
Because setup of C++ subsystem is performed after APPLICATION level initialization then C++ usage is limited to main & following threads.

To Reproduce
Steps to reproduce the behavior:

1. Define static instance of class with non trivial constructor
2. Use SYS_INIT macro to define function called at APPLICATION level
3. Try to use C++ object in function defined in point 2
4. Object is not properly constructed

Expected behavior
C++ sybsystem should be avialable at APPLICATION initialization level.
It would be great if the POST_KERNEL level will be also usable from C++ but maybe I am going too far.
If it is not possible (i.e. because of Zephyr design philosophy) then this limitation should be clearly mentioned in the documentation.

Impact
Annoyance.

Screenshots or console output

Environment (please complete the following information):

• OS: Linux
• Toolchain: Zephyr SDK
• Commit: 6789ecb

Additional context

[pabigot]

I agree; I personally would like this to occur even earlier, but there was strong pushback at the suggestion that C++ might be used in drivers.

The documentation says it's supported for applications. It is ambiguous whether that includes initialization routines run at the application init level, but I think it's worth trying.