bug: Comptime variable assignment in branch with runtime condition

[felipepiovezan]

What

An assignment to a comptime variable is predicated by a run-time value, but the compiler doesn't emit an error.
Instead, it uses "program" order and performs assignments on all branches. In particular, flipping the branch
condition and inverting the branch blocks should produce an equivalent program, but it doesn't.

(This could be me misinterpreting the comptime documentation)

Code that reproduces the issue

Godbolt link

export fn foo(arg:i32) i32 {
   comptime var x = 0;
   x = 10000;
   if (arg > 0) { x = 1;}
   else {x = 2;}
   return x;
}

const std = @import("std");

pub fn main() !void {
    std.debug.print("foo(-1) = {}!\n", .{foo(-1)});
    std.debug.print("foo(1) = {}!\n", .{foo(1)});
    std.debug.print("foo(-1) = {}!\n", .{comptime foo(-1)});
    std.debug.print("foo(1) = {}!\n", .{comptime foo(1)});
}

Produced output

foo(-1) = 2!
foo(1) = 2!
foo(-1) = 2!
foo(1) = 1!

Expected output

// first print => error: writing to comptime variable on a path that depends on a runtime value
/// second print => error: writing to comptime variable on a path that depends on a runtime value
foo(-1) = 2!
foo(1) = 1!

Tags searched before filing this issue

comptime variable branch

Tested with Zig versions:

• trunk (version available at the time of writing on Godbolt)
• .7
• .6

[SpexGuy]

Thanks for the report! We have a plan to fix this, see #1470.

[felipepiovezan]

Neat, thanks for looking into it! I'll close this as a duplicate!