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Arithmetic with comptime_int and runtime int throws "type cannot represent integer value" even when result is representable #35786

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opened 2026年06月15日 20:07:21 +02:00 by rileyophus · 2 comments

Zig Version

0.17.0-dev.857+2b2b85c5f

Steps to Reproduce, Observed Behavior, and Expected Behavior

The programs below work as expected if x is declared as a runtime int type, e.g. i16, but not when x is a comptime_int.
It seems like the compiler tries to coerce x to a type that's too small, fails, then never tries coercing to a type that actually fits. To work around this, I end up having to either assign problematic comptime values to temporary runtime variables or wrap them in @as.

Test programs

test1.zig:

pub fn main() void {
 const x = 200;
 const y: i8 = 100;
 const z: i8 = x - y;
 @import("std").debug.print("{}\n", .{z});
}

test2.zig:

pub fn main() void {
 const x = -200;
 const y: i8 = 100;
 const z: i8 = x + y;
 @import("std").debug.print("{}\n", .{z});
}

test3.zig:

pub fn main() void {
 const x = 200;
 const y: i8 = 2;
 const z: i8 = x / y;
 @import("std").debug.print("{}\n", .{z});
}

Observed behavior

> zig run test1.zig
test1.zig:4:19: error: type 'i8' cannot represent integer value '200'
 const z: i8 = x - y;
 ^
> zig run test2.zig
test2.zig:4:19: error: type 'i8' cannot represent integer value '-200'
 const z: i8 = x + y;
 ^
> zig run test3.zig
test3.zig:4:19: error: type 'i8' cannot represent integer value '200'
 const z: i8 = x / y;
 ^

Expected behavior

> zig run test1.zig
100
> zig run test2.zig
-100
> zig run test3.zig
100

Or, if this is the intended behavior for coercing/resolving comptime_int, that should be documented.

### Zig Version 0.17.0-dev.857+2b2b85c5f ### Steps to Reproduce, Observed Behavior, and Expected Behavior The programs below work as expected if `x` is declared as a runtime int type, e.g. `i16`, but not when `x` is a `comptime_int`. It seems like the compiler tries to coerce `x` to a type that's too small, fails, then never tries coercing to a type that actually fits. To work around this, I end up having to either assign problematic comptime values to temporary runtime variables or wrap them in `@as`. **Test programs** test1.zig: ``` pub fn main() void { const x = 200; const y: i8 = 100; const z: i8 = x - y; @import("std").debug.print("{}\n", .{z}); } ``` test2.zig: ``` pub fn main() void { const x = -200; const y: i8 = 100; const z: i8 = x + y; @import("std").debug.print("{}\n", .{z}); } ``` test3.zig: ``` pub fn main() void { const x = 200; const y: i8 = 2; const z: i8 = x / y; @import("std").debug.print("{}\n", .{z}); } ``` **Observed behavior** ``` > zig run test1.zig test1.zig:4:19: error: type 'i8' cannot represent integer value '200' const z: i8 = x - y; ^ ``` ``` > zig run test2.zig test2.zig:4:19: error: type 'i8' cannot represent integer value '-200' const z: i8 = x + y; ^ ``` ``` > zig run test3.zig test3.zig:4:19: error: type 'i8' cannot represent integer value '200' const z: i8 = x / y; ^ ``` **Expected behavior** ``` > zig run test1.zig 100 ``` ``` > zig run test2.zig -100 ``` ``` > zig run test3.zig 100 ``` Or, if this is the intended behavior for coercing/resolving `comptime_int`, that should be documented.

I don't know whether the current behavior was intended, but I believe the above code couldn't work no matter how implemented.
Whatever the comptime_int would coerce to in the peer type resolution (as not only i16 but actually i9 would be possible and maybe even make more sense), the compiler can't guarantee that the result will fit in an i8 as in the examples, as it depends on the runtime RHS, so you'd need an @intCast at any rate.
So indeed it should be more clear what one should expect, because the langref at least isn't clear about this specifically.

I don't know whether the current behavior was intended, but I believe the above code couldn't work no matter how implemented. Whatever the `comptime_int` would coerce to in the peer type resolution (as not only `i16` but actually `i9` would be possible and maybe even make more sense), the compiler can't guarantee that the result will fit in an `i8` as in the examples, as it depends on the runtime RHS, so you'd need an `@intCast` at any rate. So indeed it should be more clear what one should expect, because the langref at least isn't clear about this specifically.

@avis137 wrote in #35786 (comment):

I don't know whether the current behavior was intended, but I believe the above code couldn't work no matter how implemented. [...] you'd need an @intCast at any rate.

But you don't need an @intCast in these examples if the comptime_int is instead declared as an i16 or i9 or other runtime-compatible int. Probably because in all three examples y is also comptime-known. So, either @intCast should be required here even when the LHS is declared as a runtime-compatible type, or it shouldn't be required whether it's comptime_int or not, and neither of those are currently the case. The current behavior is confusingly inconsistent.

@avis137 wrote in https://codeberg.org/ziglang/zig/issues/35786#issuecomment-17785271: > I don't know whether the current behavior was intended, but I believe the above code couldn't work no matter how implemented. [...] you'd need an `@intCast` at any rate. But you _don't_ need an `@intCast` in these examples if the `comptime_int` is instead declared as an `i16` or `i9` or other runtime-compatible int. Probably because in all three examples `y` is also comptime-known. So, either `@intCast` should be required here even when the LHS is declared as a runtime-compatible type, or it shouldn't be required whether it's `comptime_int` or not, and neither of those are currently the case. The current behavior is confusingly inconsistent.
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