Produce compiler error when required return is missing

[kant2002]

int foo(void)
{
}

should produce compiler error

[kant2002]

Aah. That should be warning from compiler by default

https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4715?view=msvc-170

or -Werror=return-type

[ForNeVeR]

I'd say we should add this by default if we can detect that a function may terminate but doesn't return.

[kant2002]

We already have it it by default. But we don’t have warnings yet.

[kant2002]

Sorry. To clarify. By default we emit return, without warnings.

I think we should produce warning first, and second we should support switch which make this error

[ForNeVeR]

I'd vote for error by default and switch to make it a warning. But we might need to re-read the corresponding section of the standard.

[evgTSV]

I'd like to tackle this task

[ForNeVeR]

Reference https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3483.pdf — a proposal for a future standard (not approved) to make this code unconditionally erroneous.

For now this seems to be UB, so we can do whatever we wish. If you think there's that much bad code in the wild, and we wanna allow to compile it by default — I can't approve, but I'd agree.

Meaning — let's make it opt-in. I don't like it but meh. We can always improve later.