use of undefined only compiler error if const, not var

[mitchellh]

Zig Version

0.10.0-dev.500+66cf011aa

Steps to Reproduce

1. Save the following code as bug.zig
2. zig run bug.zig -- passes
3. Change var to const
4. zig run bug.zig -- compiler error

const std = @import("std");

pub fn main() !void {
    var x: bool = undefined;
    if (!x) {
        std.log.info("YES", .{});
    }
}

Expected Behavior

Compiler error regardless of var vs const usage, since either way you're comptime-provably using an undefined value.

Actual Behavior

Compiler error only if const.

[andrewrk]

This is actually working as designed because Zig is not able to prove that x is undefined in the var case.

This case looks easy to prove, and indeed it is, but we could construct an arbitrarily difficult problem for which solving it would be the halting problem. Currently Zig's analysis stops at var. So, on the line with the if expression, the !x is an undefined value, and then if (undefined) is unchecked undefined behavior due to branching on an undefined value.

The issue for adding a safety check for this kind of undefined behavior is #63.

I don't think we have a proposal issue open for advanced control flow analysis which would attempt to prove that an undefined value gets branched on, such as in this case. If you'd like to transform this issue into such a proposal, feel free to do so. Just edit your issue and title and I'll add the appropriate labels.

[mitchellh]

Ah #63 was what I was looking for. Yeah sorry I figured the comptime angle wasn’t exhaustively solvable but was unsure about the simple case. Runtime made a lot more sense. Thanks!