@errorCast on error unions should allow disjoint sets

[andrewrk]

Zig Version

0.12.0-dev.705+af40bce08

Steps to Reproduce and Observed Behavior

const std = @import("std");

test {
    const bar: error{}!i32 = @errorCast(foo());
    std.testing.expect((try bar) == 1234);
}

fn foo() anyerror!i32 {
    return 1234;
}

$ stage4/bin/zig test test3.zig 
test3.zig:4:30: error: error sets 'anyerror' and 'error{}' have no common errors
    const bar: error{}!i32 = @errorCast(foo());
                             ^~~~~~~~~~~~~~~~~

Expected Behavior

Test passed. All the logic for disjoint sets should be skipped when casting error unions.

However, there should be a safety check which would be triggered in the above example if foo is changed to return an error. In order to close this issue, an additional runtime safety test is needed to cover this.