I’m running into an odd comptime issue when I was trying some sample code using evented + Grand Central Dispatch, and I’m not quite understanding it. I suspect this is just a standard library bug but I’m still not positive that it isn’t user error. The issue happens for me on both master (0.17.0-dev.1275+59a628c6d) and 0.16.0.

A minimal failing example (adapted from here, which also fails if copied verbatim):

const std = @import("std");

pub fn main() !void {
    var debug_allocator: std.heap.DebugAllocator(.{}) = .init;
    const gpa = debug_allocator.allocator();

    var evented: std.Io.Evented = undefined;
    try evented.init(gpa, .{
        .backing_allocator_needs_mutex = false,
    });
    defer evented.deinit();
}

Which gives this comptime error:

Compile output 
❯ zig build
install
└─ install zidk
   └─ compile exe zidk Debug native 1 errors
/opt/homebrew/Cellar/zig/0.16.0_1/lib/zig/std/debug.zig:420:14: error: reached unreachable code
    if (!ok) unreachable; // assertion failure
             ^~~~~~~~~~~
/opt/homebrew/Cellar/zig/0.16.0_1/lib/zig/std/mem/Allocator.zig:445:20: note: called at comptime here
    comptime assert(slice_info.size == .slice);
             ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
referenced by:
    deinit: /opt/homebrew/Cellar/zig/0.16.0_1/lib/zig/std/Io/Dispatch.zig:584:30
    main: src/main.zig:11:25
    4 reference(s) hidden; use '-freference-trace=6' to see all references
error: 1 compilation errors
failed command: /opt/homebrew/Cellar/zig/0.16.0_1/bin/zig build-exe -ODebug --dep zidk -Mroot=/Users/perry/git/zidk/src/main.zig -Mzidk=/Users/perry/git/zidk/src/root.zig --cache-dir .zig-cache --global-cache-dir /Users/perry/.cache/zig --name zidk --zig-lib-dir /opt/homebrew/Cellar/zig/0.16.0_1/lib/zig/ --listen=-

Build Summary: 0/3 steps succeeded (1 failed)
install transitive failure
└─ install zidk transitive failure
   └─ compile exe zidk Debug native 1 errors

error: the following build command failed with exit code 1:
.zig-cache/o/0cd16de5dbb9d338e110e2d0aa3a08b7/build /opt/homebrew/Cellar/zig/0.16.0_1/bin/zig /opt/homebrew/Cellar/zig/0.16.0_1/lib/zig /Users/perry/git/zidk .zig-cache /Users/perry/.cache/zig --seed 0xc1fa9371 -Za691d53d472b4d28

Convenience link for Evented.deinit’s definition.

vulpesx July 9, 2026, 3:15am 2

Only Threaded is guaranteed to work, other implementations are WIP

There are also 3rd party implementations such as zio

alanza July 9, 2026, 12:06pm 3

I think the fix for this was merged recently?

https://codeberg.org/ziglang/zig/pulls/35222#issuecomment-17886197

In particular the assertion that you’re tripping should not exist on master anymore; could you try updating Zig and seeing if your minimal program still fails to compile?

Ah, good catch—I somehow didn’t notice that building with master resulting in a different (albeit similar) error message. Here’s what the compile log looks like on master (0.17.0-dev.1282+c0f9b51d8):

❯ zig build
install
└─ install newzidk
   └─ compile exe newzidk Debug native 1 errors
/Users/perry/.cache/zig/p/N-V-__8AAMlw-BajASJzv2qM-Wcf7ijzIYrCy_OFSd6KO2-Z/lib/std/debug.zig:435:14: error: reached unreachable code
    if (!ok) unreachable; // assertion failure
             ^~~~~~~~~~~
/Users/perry/.cache/zig/p/N-V-__8AAMlw-BajASJzv2qM-Wcf7ijzIYrCy_OFSd6KO2-Z/lib/std/mem.zig:4711:11: note: called at comptime here
    assert(info.size == .slice);
    ~~~~~~^~~~~~~~~~~~~~~~~~~~~
/Users/perry/.cache/zig/p/N-V-__8AAMlw-BajASJzv2qM-Wcf7ijzIYrCy_OFSd6KO2-Z/lib/std/mem.zig:4718:63: note: called at comptime here
pub fn absorbSentinel(slice: anytype) AbsorbSentinelReturnType(@TypeOf(slice)) {
                                      ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
/Users/perry/.cache/zig/p/N-V-__8AAMlw-BajASJzv2qM-Wcf7ijzIYrCy_OFSd6KO2-Z/lib/std/mem/Allocator.zig:452:63: note: generic function instantiated here
    const bytes: []u8 = @ptrCast(@constCast(mem.absorbSentinel(memory)));
                                            ~~~~~~~~~~~~~~~~~~^~~~~~~~
referenced by:
    deinit: /Users/perry/.cache/zig/p/N-V-__8AAMlw-BajASJzv2qM-Wcf7ijzIYrCy_OFSd6KO2-Z/lib/std/Io/Dispatch.zig:583:30
    main: src/main.zig:11:25
    4 reference(s) hidden; use '-freference-trace=6' to see all references
error: 1 compilation errors
failed command: /Users/perry/.cache/zig/p/N-V-__8AAMlw-BajASJzv2qM-Wcf7ijzIYrCy_OFSd6KO2-Z/zig build-exe -ODebug --dep newzidk -Mroot=src/main.zig -Mnewzidk=src/root.zig --cache-dir .zig-cache --global-cache-dir /Users/perry/.cache/zig --name newzidk --zig-lib-dir /Users/perry/.cache/zig/p/N-V-__8AAMlw-BajASJzv2qM-Wcf7ijzIYrCy_OFSd6KO2-Z/lib/ --listen=-

Build Summary: 0/3 steps succeeded (1 failed)
install transitive failure
└─ install newzidk transitive failure
   └─ compile exe newzidk Debug native 1 errors

alanza July 9, 2026, 6:21pm 5

Ope, that makes total sense as an error to me. I put up a PR:

https://codeberg.org/ziglang/zig/pulls/36113

Amazing; thank you kindly!!