Enum value wrong in nested switch

[kuon]

Zig Version

0.12.0-dev.1270+6c9d34bce

Steps to Reproduce and Observed Behavior

I have code similar to this: (I was unable to reproduce the bug with a minimal example)

const std = @import("std");

fn BuildId(comptime name: []const u8) type {
    return @Type(.{ .Struct = .{
        .layout = .Auto,
        .fields = &.{
            .{
                .name = name,
                .type = i64,
                .default_value = null,
                .is_comptime = false,
                .alignment = 0,
            },
        },
        .decls = &.{},
        .is_tuple = false,
    } });
}

// Actual code has 20 id types
pub const IdentityId = BuildId("identity_id");
pub const AccountId = BuildId("account_id");

// Actual code has 20 members
pub const Id = union(enum) {
    identity_id: IdentityId,
    account_id: AccountId,
};

// Actual code has about a hundred members
pub const Msg = union(enum) {
    ok,
    id: Id,
};

pub fn main() !void {
    var msg: Msg = .{ .id = .{ .account_id = .{ .account_id = 10 } } };

    std.debug.print("msg {any}\n", .{msg});
    std.debug.print("tag A {}\n", .{@intFromEnum(msg.id)});
    switch (msg) {
        .id => |id| {
            std.debug.print("ID {any}\n", .{id});
            std.debug.print("tag B {}\n", .{@intFromEnum(id)});
            switch (id) {
                .account_id => std.debug.print("yes\n", .{}),
                else => std.debug.print("no\n", .{}),
            }
        },
        else => std.debug.print("no\n", .{}),
    }
}

The tag should be the same both time, it should print tag A 1 and tag B 1.

But in my code, the second one is 0.

Here is the actual code and it's output:

        var r1 = try self.send_recv(Msg{ .register_endpoint = params });
        defer r1.deinit();

        std.debug.print("msg {any}\n", .{r1.msg});
        std.debug.print("tag A {}\n", .{@intFromEnum(r1.msg.id)});
        switch (r1.msg) {
            .id => |id| {
                std.debug.print("ID {any}\n", .{id});
                std.debug.print("tag B {}\n", .{@intFromEnum(id)});
                switch (id) {
                    .endpoint_id => |endpoint_id| return endpoint_id,
                    else => return error.BadId,
                }
            },
            else => return error.BadMsg,
        }

msg msg.Msg{ .id = client.Id{ .endpoint_id = client.BuildId("endpoint_id"){ .endpoint_id = 1 } } }
tag A 12
ID client.Id{ .identity_id = client.BuildId("identity_id"){ .identity_id = 1 } }
tag B 0

If I do .id => |*id| and switch (id.*) the bug does not occurs.

Actual code is built for wasm.

Expected Behavior

Enum tag should be the same within the switch.

[kuon]

I did a few more tests:

The code does exhibit the bug only when run in the browser in wasm.

I suspected some weird memory corruption, so I replaced the line with send_recv (which does some allocation) with var msg: Msg = .{ .id = .{ .endpoint_id = .{ .endpoint_id = 23 } } }; and replaced r1.msg with msg and it does trigger the bug, but in this case, even the first debug prints tag A 0

[kuon]

Ok, I discovered something.

I noticed that the enum is actually correct up to 4 and then it starts back to 0. My endpoint_id was actually 12 in the enum, if I move it to 13 I get 1, and if I move it to 11 I get 4.

It made me realize that the enum underlying type must be incorrectly inferred under some circumstances.

If I explicitly specify u8 as enum type, so

// from 
pub const Id = union(enum) {
// to 
pub const Id = union(enum(u8)) {

The bug does not happen.

[Vexu]

Cannot reproduce on x86-64_linux:

msg a.Msg{ .id = a.Id{ .account_id = a.BuildId("account_id"){ .account_id = 10 } } }
tag A 1
ID a.Id{ .account_id = a.BuildId("account_id"){ .account_id = 10 } }
tag B 1
yes

[kuon]

Yes I cannot reproduce it outside of the browser with my own code base.

[Vexu]

Ah, I didn't notice the line about wasm. Either way I can't reproduce it using wasmtime either, can you share your js code for it?

[kuon]

Yeah I cannot reproduce it with command line wasm runtime either. I cannot share the JS code as it is part of a proprietary plugin loading system. I'm trying to extract the most important bits of it to get a reproduction.

[kuon]

I spent the day trying to reproduce it, without success. I tried to compare the .wasm but it was very hard to get anything out of it. Changing pub const Id = union(enum) { to pub const Id = union(enum(u8)) { generates much different output with about 10kB size difference (on a 2.4MB file).