fix(Match): handle null/undefined in Match.tag and Match.tagStartsWith

[codewithkenzo]

Summary

Fixes #6017

Match.tag and Match.tagStartsWith now correctly handle nullable union types instead of crashing with "Cannot read properties of undefined (reading '_tag')".

Problem

When using Match.tag on a union type that includes null or undefined, the predicate would crash because it accessed _[field] without first checking if _ is nullish:

const value = undefined as ({_tag: "A"} | undefined)
const res = Match.value(value).pipe(
  Match.tag('A', () => 'matched A'),  // crashes!
  Match.when(undefined, () => 'matched undefined'),
  Match.exhaustive
)

Solution

Add _ != null && guard to the predicates in discriminator and discriminatorStartsWith, following the same pattern already used in discriminators (which powers Match.tags).

Changes

• packages/effect/src/internal/matcher.ts: Add null checks to predicates
• packages/effect/test/Match.test.ts: Add test cases for nullable unions

Testing

Added 3 test cases:

• Match.tag with undefined union
• Match.tag with null union
• Match.tagStartsWith with undefined union

All 62 Match tests pass.

[changeset-bot]

🦋 Changeset detected

Latest commit: 971727c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 36 packages

Name	Type
effect	Patch
@effect/cli	Patch
@effect/cluster	Patch
@effect/experimental	Patch
@effect/opentelemetry	Patch
@effect/platform-browser	Patch
@effect/platform-bun	Patch
@effect/platform-node-shared	Patch
@effect/platform-node	Patch
@effect/platform	Patch
@effect/printer-ansi	Patch
@effect/printer	Patch
@effect/rpc	Patch
@effect/sql-clickhouse	Patch
@effect/sql-d1	Patch
@effect/sql-drizzle	Patch
@effect/sql-kysely	Patch
@effect/sql-libsql	Patch
@effect/sql-mssql	Patch
@effect/sql-mysql2	Patch
@effect/sql-pg	Patch
@effect/sql-sqlite-bun	Patch
@effect/sql-sqlite-do	Patch
@effect/sql-sqlite-node	Patch
@effect/sql-sqlite-react-native	Patch
@effect/sql-sqlite-wasm	Patch
@effect/sql	Patch
@effect/typeclass	Patch
@effect/vitest	Patch
@effect/workflow	Patch
@effect/ai	Patch
@effect/ai-amazon-bedrock	Patch
@effect/ai-anthropic	Patch
@effect/ai-google	Patch
@effect/ai-openai	Patch
@effect/ai-openrouter	Patch

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[codewithkenzo]

This follows my comment on #6017. The fix adds the same _ != null && guard that discriminators (used by Match.tags) already has.

Minimal change - just 3 lines modified in the predicates.