Semantics of Expr(:new) underspecified

[Keno]

The semantics of Expr(:new) with missing parameters for immutables are underspecified.
In particular, we need to decide whether the value is undefined or merely possibly uninitialized.
The primary difference between the two is how we represent this to LLVM. In particular undefined values may change on every access. As an illustrative example, consider:

struct Foo
    x::UInt8
    Foo() = new()
    Foo(x::UInt8) = new(x)
end

function bar(i...)
    f = Foo(i...)
    y = (f.x + UInt8(128))
    if f.x >= UInt8(128)
        println("World")
    end
    if y >= UInt8(128)
        println("Hello")
    end
    return nothing
end

What happens when you call bar() depends on LLVM versions and other considerations, but on my local copy, it prints nothing. However, any actual UInt8 value would print either "Hello" or "World". Unfortunately, there isn't really a way to say to llvm "I don't care what this value is, but it always has to be the same". If we decide we do not like the above behavior, we'll have to initialize all (stack? - will have to check what assumptions LLVM makes) allocations visible to LLVM.

[JeffBezanson]

We should move towards initializing to zero, unless the cost is prohibitive. I suspect in these cases it's not.

[StefanKarpinski]

Or just disallow incomplete construction of immutable structs.

[KristofferC]

At least for the isbits fields, see #24943. I have hit that twice when I have forgotten updating my new call when adding new isbits field and silently gotten uninitialized data in that field.

[JeffBezanson]

It would be very difficult to define a type like Nullable if uninitialized fields are disallowed.

[KristofferC]

Opt in to it by enforcing putting undef in that argument in new . Safe by default.

[vtjnash]

If only we had thought to add optimizations to make it more performant to explicitly mark undef by describing the field as having a Union of multiple possible types, rather than to define a type like Nullable with possibly-undef fields. And then had deprecated Nullable in preparation for v0.7 when that work was largely done.

[JeffBezanson]

explicitly mark undef by describing the field as having a Union of multiple possible types

Not the same as undef, since the value can propagate.

[StefanKarpinski]

It is exactly the same behavior as the undef that is passed to array constructors provides for arrays: #undef for pointer types and some value for bits types.

[JeffBezanson]

We might be talking about different things. I'm talking about undefined references that throw errors on access, as opposed to Union{T,Nothing}, which is how I interpreted Jameson's comment.

It would be ok to have some syntax inside new to indicate that a field is undefined (in the current sense). But we couldn't use the first class value undef for that, since that means using that value itself as the value of the field. Unlike the Array constructor, where the first argument can mean whatever we want.

[StefanKarpinski]

I was talking about what @KristofferC proposed. But I actually don't like that either since if new(undef) means what new() means now, then how do you write what new(undef) means currently?

[KristofferC]

then how do you write what new(undef) means currently?

You don't since that is a useless construct :). But yes, undef would have to be "lifted" to be something special and not just a normal singleton.
But there could of course also be some other special syntax, only valid inside new?

[JeffBezanson]

We could use @undef, expanding to a special expression that can only appear inside new.

[StefanKarpinski]

That would be ok, but I'd be rather worried that we have #undef, undef and @undef.

[StefanKarpinski]

Not to mention Nothing, nothing, Cvoid, C_NULL, Union{}, Missing and missing.

[vtjnash]

Related to #9147 xref