Match.tag crashes on union type including null/undefined

[Hurtak]

What version of Effect is running?

3.19.15

What steps can reproduce the bug?

When using Match.tag on union type that includes undefined/null, it will produce runtime error when the Match.tag is before Match.when(undefined, ...). The code type checks correctly.

Reproduciton steps

import { Match } from "effect"

const value = undefined as ({_tag: "A"} | undefined)

const res = Match.value(value).pipe(
  Match.tag('A', () => 'mached A'), // <- crashes with `Cannot read properties of undefined (reading '_tag')`
  Match.when(undefined, () => 'matched undefined'),
  Match.exhaustive
);
console.log(res);

Or in playground: https://effect.website/play/#b2df3d05e29a

What is the expected behavior?

The code does not crash

What do you see instead?

No response

Additional information

No response

[codewithkenzo]

Hi! I'd like to work on this.

The issue is in packages/effect/src/internal/matcher.ts - the discriminator function (used by Match.tag) doesn't check for null/undefined before accessing the discriminator field:

// Lines 353-355
const pred = values.length === 1
  ? (_: any) => _[field] === values[0]      // crashes if _ is null/undefined
  : (_: any) => values.includes(_[field])

Interestingly, discriminators (used by Match.tags) already has this guard:

// Line 413
(arg: any) => arg != null && arg[field] in fields

The fix would be to add the same _ != null && check to discriminator and discriminatorStartsWith.

Happy to submit a PR if this approach looks good!