Previously Zig allowed you to write something like,
```zig
switch (x) {
    .y => |_| {
```

This seems a bit strange because in other cases, such as when
capturing the tag in a switch case,
```zig
switch (x) {
    .y => |_, _| {
```
this produces an error.

The only usecase I can think of for the previous behaviour is
if you wanted to assert that all union payloads are able
to coerce,
```zig
const X = union(enum) { y: u8, z: f32 };

switch (x) {
    .y, .z => |_| {
```

This will compile-error with the `|_|` and pass without it.

I don't believe this usecase is strong enough to keep the current
behaviour; it was never used in the Zig codebase and I cannot
find a single usage of this behaviour in the real world, searching
through Sourcegraph.