Fix TaggedUnion constraint to allow documented generic usage

[ATOM00blue]

The documented usage of TaggedUnion,

type Tagged<Fields extends Record<string, unknown>> = TaggedUnion<'type', Fields>;

failed to compile with: Type 'Fields' does not satisfy the constraint 'Record<string, Record<string, unknown>>'.

The UnionMembers parameter required Record<string, Record<string, unknown>> — a redundant extra level of Record. A wrapper constrained as the natural Record<string, unknown> cannot satisfy the nested constraint, so the documented pattern (and any generic passthrough) was rejected.

This loosens the constraint to Record<string, unknown>. Per-member field types are still fully enforced by the & UnionMembers[Name] intersection in the mapped type, so the existing negative tests continue to pass. Added a test mirroring the documented pattern.

Closes #1211

[Copilot]

Pull request overview

Note

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Updates TaggedUnion to support being wrapped by generics constrained as Record<string, unknown> (as documented), and adds type-level tests to prevent regressions.

Changes:

• Widened TaggedUnion’s UnionMembers constraint from Record<string, Record<string, unknown>> to Record<string, unknown>.
• Updated the documentation example to match the intended generic-wrapper constraint.
• Added a test-d case to validate the wrapper scenario and correct narrowing behavior.

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

File	Description
test-d/tagged-union.ts	Adds a regression test ensuring TaggedUnion works through a generic wrapper constrained as Record<string, unknown>.
source/tagged-union.d.ts	Widens the TaggedUnion type constraint and updates the docs example accordingly.

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