checker: add fntype casting validations

[felipensp]

Fix #23852

[huly-for-github]

Connected to Huly®: V_0.6-22276

[spytheman]

none should be allowed for option values:

type Foo = fn (bar string)

fn test_main() {
    mut func := ?Foo(none)
    dump(func)
}

[spytheman]

On master that passes, but it fails here currently (which is the CI's problem):

(as an aside, the error is not shown on the CI, because of the -silent flag that was passed to reduce the scrolling for the normally OK lines in the test)

[felipensp]

On master that passes, but it fails here currently (which is the CI's problem):

(as an aside, the error is not shown on the CI, because of the -silent flag that was passed to reduce the scrolling for the normally OK lines in the test)

Right, I'll add the option test too. 👍🏻

[medvednikov]

Thanks!

[medvednikov]

casting number to function value should be done inside unsafe{} blocks

This should not be allowed at all. We recently forced requiring nil instead of 0 for the same reason.

[medvednikov]

We can do a warning for now (use nil instead of 0)

[spytheman]

This should not be allowed at all. We recently forced requiring nil instead of 0 for the same reason.

No, it should be allowed in unsafe {}.

[medvednikov]

Why?

[spytheman]

Because there are cases (for example with -freestanding, you can write code that can then be booted on bare metal), when you write code, where you do know the actual address of a function (because it is an external requirement), and do want to cast it to a function pointer.

Restricting what can be done in unsafe{} blocks in general kind of defeats the whole purpose of the unsafe expressions.

[medvednikov]

Then just use Func(voidptr(1234))

It's more clear this way too. Func(1234) looks like a function call.

V positions itself as a safe language. You can't cast numbers to functions in a safe language :)

[spytheman]

Then just use Func(voidptr(1234))

It's more clear this way too. Func(1234) looks like a function call.

V positions itself as a safe language. You can't cast numbers to functions in a safe language :)

We are talking then about:
unsafe { Func(voidptr(1234)) }

I think that is very verbose for no gain at all.

[spytheman]

V positions itself as a safe language.

V itself does not do anything on its own. It is a language, not a person.

You, however position it as a language, that I will not want to use for certain tasks, because of the verbosity and edge cases.

Unsafe blocks are in V, to allow people to write unsafe code in them, not to make them jump through hoops to avoid weird quirks in the compiler.

[felipensp]

I've added V UI PR fix vlang/ui#599

[spytheman]

Excellent work @felipensp, thank you.

However, I will not merge it, since @medvednikov expressed another opinion, and he is ultimately the arbiter of what should happen with his language.

I do hope, that he will reconsider, but that is of course up to him.