`@errSetCast` refuses to accept an error union operand

[andrewrk]

Zig Version

0.12.0-dev.695+303181901

Steps to Reproduce and Observed Behavior

const std = @import("std");

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

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

$ stage4/bin/zig test test.zig 
test.zig:4:33: error: expected error set type, found 'i32'
    const bar: error{Bad}!i32 = @errSetCast(foo());
                                ^~~~~~~~~~~~~~~~~~

Expected Behavior

Test passed.

[andrewrk]

Follow-up issues:

• @errorCast crashes on inferred error set #17353
• returning @errorCast from an inline inferred error set function crashes the compiler #17354
• @errorCast on error unions should allow disjoint sets #17355