ES-conformant symbol typing

[Michael FIG (michaelfig)]

AI-generated from my test cases that showed discrepancies between TypeScript typing and ES standard behaviour.

Summary

• Add a RegisteredSymbol<Key> intrinsic and preserve literal registry keys in the return type of Symbol.for.
• Represent unresolved registry keys with a deferred symbol type, so separate generic calls do not receive one unsound unique-symbol identity.
• Preserve narrow unique symbol types in const-like declarations and allow const assertions on them.
• Emit and resolve symbol-keyed object properties using their registered and unique symbol types.
• Add compiler cases and accepted baselines for symbol typing, property access, and deferred registry keys.

Tests

• go -C ./tsc test -run='TestLocal/(registeredSymbolDeferredKey|registeredSymbolIntrinsic|registeredSymbolPropertyAccess|uniqueSymbolConstLikeWidening)\.ts$' ./internal/testrunner -count=1
• go -C ./tsc test ./internal/checker ./internal/api

Refs: #35562, #37469, #54778, #60052

Closes: #35909
Closes: #27524
Closes: #55901

Co-authored-by: Codex codex@openai.com

[typescript-automation]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.

[Copilot]

Copilot review overview

🟡 Changes recommended

Non-literal and union registry keys currently receive unsound single unique-symbol identities.

Get a fresh assessment by requesting another Copilot review.

Review effort: Balanced
Findings: None

What changed in this PR

Adds registered-symbol typing and preserves symbol identities through inference, property access, and declaration emit.

Changes:

• Introduces RegisteredSymbol<Key> and generic Symbol.for.
• Preserves unique-symbol types in const-like contexts.
• Adds compiler tests and baselines for symbol-keyed properties.

File	Description
tsc/​internal/​checker/​checker.go	Implements registered-symbol types and unique-symbol preservation.
tsc/​internal/​checker/​nodebuilderimpl.go	Emits registered symbol types and computed property names.
tsc/​internal/​bundled/​libs/​lib.es2015.symbol.d.ts	Declares the new intrinsic and Symbol.for signature.
tsc/​testdata/​tests/​cases/​compiler/​registeredSymbolIntrinsic.ts	Tests intrinsic inference.
tsc/​testdata/​tests/​cases/​compiler/​registeredSymbolPropertyAccess.ts	Tests symbol-keyed property access and emit.
tsc/​testdata/​tests/​cases/​compiler/​uniqueSymbolConstLikeWidening.ts	Tests const-like unique-symbol widening.
tsc/​testdata/​baselines/​reference/​compiler/​registeredSymbolIntrinsic.types	Records inferred types.
tsc/​testdata/​baselines/​reference/​compiler/​registeredSymbolIntrinsic.symbols	Records resolved symbols.
tsc/​testdata/​baselines/​reference/​compiler/​registeredSymbolPropertyAccess.types	Records property-access types.
tsc/​testdata/​baselines/​reference/​compiler/​registeredSymbolPropertyAccess.symbols	Records property symbols.
tsc/​testdata/​baselines/​reference/​compiler/​registeredSymbolPropertyAccess.js	Records declaration emit.
tsc/​testdata/​baselines/​reference/​compiler/​uniqueSymbolConstLikeWidening.types	Records widening types.
tsc/​testdata/​baselines/​reference/​compiler/​uniqueSymbolConstLikeWidening.symbols	Records widening symbols.

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[typescript-automation]

The TypeScript team hasn't accepted the linked issue #27524. If you can get it accepted, this PR will have a better chance of being reviewed.

[Daniel Rosenwasser (DanielRosenwasser)]

How does this keep the type parameter deferred? Doesn't this just return symbol?

I think you not only need a new intrinsic type, you need a new kind of type (e.g. see newStringMappingType). Similar to how a type with TypeFlagsStringMapping doesn't have TypeFlagsString, it wouldn't have TypeFlagsSymbol - maybe because it could be instantiated to never? I'd have to consult with others.

[Daniel Rosenwasser (DanielRosenwasser)]

I guess we just handle that in assignability separately and keeping the bitflags disjoint on this avoids other issues.

[Michael FIG (michaelfig)]

I agree that a separate type flag is a better way to represent a registered symbol whose key is still a type parameter. I’ve updated the PR to use that representation and added a regression test where two Symbol.for calls with generic keys previously shared an unsound unique-symbol identity.

My main hesitation was the limited remaining space in the TypeFlags uint32. It seems that all the bits up to bit 31 are already in use, so this uses the former Reserved3 bit at 31 and keeps TypeFlags as uint32. That consumes the remaining reserved bit, but seems justified for a distinct type kind.

I’d welcome your assessment of that tradeoff.

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