deptypes1.zig

const std = @import("std");

pub fn BoolOrNat(comptime b : bool) type {
    if (b) {
        return bool;
    } else {
        return usize;
    }
}

pub fn trueOrOne(b : bool) BoolOrNat(b) {
    if (b) {
        return true;
    } else {
        return 1;
    }
}

pub fn main() !void {
    const b = trueOrOne(true);
    const i = trueOrOne(false);
    std.debug.warn("{} {}", .{b, i});
}

deptypes2.zig

const std = @import("std");

pub fn BoolOrNat(comptime b : bool) type {
    if (b) {
        return bool;
    } else {
        return usize;
    }
}

pub fn trueOrOne(comptime b : bool) BoolOrNat(b) {
    if (b) {
        return true;
    } else {
        return 1;
    }
}

pub fn main() !void {
    const b = trueOrOne(true);
    const i = trueOrOne(false);
    std.debug.warn("{} {}", .{b, i});
}

error1.txt

./deptypes.zig:12:38: error: use of undeclared identifier 'b'
pub fn trueOrOne(b : bool) BoolOrNat(b) {
                                     ^
./deptypes.zig:32:15: note: referenced here
    const b = trueOrOne(true);
              ^
/usr/lib/zig/std/start.zig:253:40: note: referenced here
            const result = root.main() catch |err| {
                                       ^