[BUG] inout parameter without non-const use of x works

[filipsajdak]

In section 1.3.2 inout parameters of https://github.com/hsutter/708/blob/main/708.pdf you specify that

If the function is nonvirtual, at least one path must contain a non-const use of x (else the parameter should be in).

In the current implementation of cppfront the following code compiles:

f2: (inout x) -> _ = {
    return x * 2;
}

main: () -> int = {
    x := 21;
    std::cout << f2(x) << std::endl;
    x = 0; // workaround for another bug
}

And generate output:

42

Expectations

cppfront should end with an error informing to use in argument passing instead of inout.

[hsutter]

Right, I'm not able to detect that right now so it's not possible to issue that diagnostic.

I could probably do this if I required a mutating argument to be qualified with inout, which I've thought of and @jbatez suggested in #198.

I see there's a related issue in #231 which might also be solved with an inout argument (call-site) requirement...

[filipsajdak]

I found that issue while playing with for..do loops.

I assume that there is no call-site there.