errdefer |err| uses the local errorset instead of the wrapping errorset

[fengb]

This fails to compile. Seems like the compiler does not transfer the correct errorset into the errdefer handler:

fn foo(val: bool) !void {
    // error: expected type 'error{Truth}', found 'error{Justice}'
    errdefer |err| switch (err) {
        error.Truth => {},
        error.Justice => {},
    };

    if (val) {
        return error.Truth;
    } else {
        return error.Justice;
    }
}

Even more bizarrely, sometimes the errdefer receives an error union:

fn foo(val: bool) error{Truth,Justice}!void {
    // error: invalid switch target type 'error:1:19!void'
    errdefer |err| switch (err) {};
    return if (val) error.Truth else error.Justice;
}

[mnemnion]

I thought it was possible that this was a consequence of comptime branch elimination, but that appears unlikely.

test "expect Justice" {
    var err_type: bool = true;
    err_type = false;
    // error: expected type 'error{Truth}', found 'error{Justice}'
    try expectError(error.Justice, foo(err_type));
}

Adding the annotation error{Truth,Justice}!void doesn't fix the bug, but it no longer changes the error message, I assume that was fixed by #20371.

My last experiment was changing the return type to bool, that makes no difference:

// Same error here
fn foo(val: bool) error{ Truth, Justice }!bool {
    // error: expected type 'error{Truth}', found 'error{Justice}'
    errdefer |err| switch (err) {
        error.Truth => {},
        error.Justice => {},
    };

    if (val) {
        return error.Truth;
    } else if (val == false) {
        return error.Justice;
    } else {
        return true;
    }
}