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std.meta.FieldEnum for type with one field does not work in std.EnumArray #22506

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Sandalmoth opened this issue Jan 16, 2025 · 1 comment · May be fixed by #23701
Open

std.meta.FieldEnum for type with one field does not work in std.EnumArray #22506

Sandalmoth opened this issue Jan 16, 2025 · 1 comment · May be fixed by #23701
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bug Observed behavior contradicts documented or intended behavior standard library This issue involves writing Zig code for the standard library.
Milestone
0.15.0

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@Sandalmoth
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Zig Version

0.14.0-dev.2647+5322459a0

Steps to Reproduce and Observed Behavior

Using an enum generated by std.meta.FieldEnum from a type with exactly one field in a std.EnumArray fails to compile if iterator.next() is called.

Example

const std = @import("std");

const A = struct {
    x: u32,
    // y: u32, // it works if A has a second field, or if A has no fields
};

const AEnum = std.meta.FieldEnum(A);

const MyEnum = enum { x };

pub fn main() void {
    // this doesn't compile
    var a = std.EnumArray(AEnum, u32).initFill(123);
    var it = a.iterator();
    while (it.next()) |n| _ = n;

    // this compiles
    var b = std.EnumArray(MyEnum, u32).initFill(123);
    var it2 = b.iterator();
    while (it2.next()) |n| _ = n;
}

fails to compile with the error message

/home/j/Programs/zig-linux-x86_64-0.14.0-dev.2647+5322459a0/lib/std/enums.zig:1158:39: error: value stored in comptime field does not match the default value of the field
                        .key = Indexer.keyForIndex(index),
                        ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
/home/j/Programs/zig-linux-x86_64-0.14.0-dev.2647+5322459a0/lib/std/enums.zig:1141:13: note: default value set here
            key: Key,

Expected Behavior

I would expect it to compile and behave like an enum implemented by hand.
@Sandalmoth Sandalmoth added the bug Observed behavior contradicts documented or intended behavior label Jan 16, 2025
@andrewrk andrewrk added the standard library This issue involves writing Zig code for the standard library. label Jan 25, 2025
@andrewrk andrewrk added this to the 0.15.0 milestone Jan 25, 2025
@poypoyan
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poypoyan commented Mar 14, 2025 •
edited
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This looks like a variant of #19985 that doesn't compile. An example:

const std = @import("std");

// these work fine
// const E = enum { foo };
// const E = enum(u0) { foo };
const E = @Type(.{ .@"enum" = .{
    .tag_type = u0,
    .is_exhaustive = true,
    .fields = &.{.{ .name = "foo", .value = 0 }},
    .decls = &.{},
} });

const Entry = struct {
    k: E,
};

pub fn main() void {
    const s = Entry{ .k = E.foo };
    _ = s;
}

When setting .tag_type to u1, the above compiles.
I'll investigate this further.

@poypoyan poypoyan linked a pull request Apr 27, 2025 that will close this issue
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bug Observed behavior contradicts documented or intended behavior standard library This issue involves writing Zig code for the standard library.
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std.EnumArray: edit return of iterator.next() poypoyan/zig
3 participants
@andrewrk @Sandalmoth @poypoyan