std.fmt on comptime_int fails to handle some decimal values

[luca-i]

comptimePrint doesn't handle correctly some decimal values (such as 300.0, but other are fine, such as 300.5).

How to replicate the issue:

const std = @import("std");


pub fn main() void {
    const value = 30.0;
    const value_to_str = std.fmt.comptimePrint("{d}", .{value});
    std.debug.print("value = {s}\n", .{value_to_str});
}

Expected output:

value = 30.0

Retrieved output:

An error occurred:
/usr/local/bin/lib/std/fmt.zig:1367:47: error: overflow of integer type 'u1' with value '2'
/usr/local/bin/lib/std/fmt.zig:802:27: note: called from here
/usr/local/bin/lib/std/fmt.zig:732:58: note: called from here
/usr/local/bin/lib/std/fmt.zig:487:31: note: called from here
/usr/local/bin/lib/std/fmt.zig:184:23: note: called from here
/usr/local/bin/lib/std/fmt.zig:2008:11: note: called from here
/usr/local/bin/lib/std/fmt.zig:2052:83: note: called from here

Code that works:

const std = @import("std");


pub fn main() void {
    const value = 30.1;
    const value_to_str = std.fmt.comptimePrint("{d}", .{value});
    std.debug.print("value = {s}\n", .{value_to_str});
}

Output:

value = 30.1

[luca-i]

I think the offending line could be 1366 in lib.std.fmt.zig

    var i = num_digits_whole_no_pad;

I'm not an expert of Zig by no means, but it seems to me that i takes size 1 bit (u1).
Could somebody confirm this?

Changing the line to

    var i: u8 = num_digits_whole_no_pad;

compiles.

If I'm right, could somebody spend some time to mentor me (I'm absolutely a newbie) how to prove my solution has no side effects / regression and how to submit a pull request?

Thank you, Luca-

[Vexu]

That's correct but it needs to be a usize to also work for runtime known values.

how to prove my solution has no side effects / regression

Add a new test to fmt.zig using expectFmt and as long as the new test and all old ones pass you're good.

how to submit a pull request?

That depends on your setup, GitHub probably has tutorials for all the common ones.

[karlseguin]

I believe this has regressed (or maybe it was never fully fixed). Using the latest 0.12.0-dev.3428+d8bb139da the original code works, but changing value to 0.05 gives an overflow:

const std = @import("std");
pub fn main() void {
    const value = 0.05;
    const value_to_str = std.fmt.comptimePrint("{d}", .{value});
    std.debug.print("value = {s}\n", .{value_to_str});
}

[Vexu]

Probably regressed with #19229