Zero size tuple field is implicitly comptime which leads to confusing error message

[tomboehmer]

Zig Version

0.14.0-dev.2563+af5e73172

Steps to Reproduce and Observed Behavior

const std = @import("std");

test "broken" {
    const I = struct {
        // Adding a field or making `self` a non-pointer fixes the error
        fn next(_: *@This()) i32 {
            return 2;
        }
    };

    var iters: struct { I, I } = .{ I{}, I{} };
    //@compileError(@typeName(@TypeOf(iters)));

    var result: struct { i32, i32 } = undefined;

    result[0] = iters[0].next(); // error: runtime value contains reference to comptime var
    result[1] = iters[1].next();

    try std.testing.expectEqual(result, .{ 2, 2 });
}

I get a compilation error. I find this very confusing as it refers to a "comptime var" when I have not used "comptime" anywhere in the code.

src/main.zig:38:25: error: runtime value contains reference to comptime var
    result[0] = iters[0].next(); // error: runtime value contains reference to comptime var
                ~~~~~~~~^~~~~
src/main.zig:38:25: note: comptime var pointers are not available at runtime

Printing the type of iters reveals that iters apparently has two comptime fields.

src/main.zig:30:5: error: struct { comptime main.test.broken.I = .{}, comptime main.test.broken.I = .{} }
    @compileError(@typeName(@TypeOf(iters)));

Expected Behavior

There are two behaviors i would think sensible.

1. The code should compile successfully and the test should pass
2. If it is invalid to have a runtime pointer to a zero bit type (is it?) this should be the content of the compilation error

Talking about a "comptime var" when there is no "comptime" in the code is not helpful.

Actually, I think option 1 should be correct, because the following also compiles:

const std = @import("std");

test "works" {
    const I = struct {
        fn next(_: *@This()) i32 {
            return 2;
        }
    };

    var iter0: I = I{};
    var iter1: I = I{};

    var result: struct { i32, i32 } = undefined;

    result[0] = iter0.next();
    result[1] = iter1.next();

    try std.testing.expectEqual(result, .{ 2, 2 });
}