TypeScript Port-Divergence Notes

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Companion to TS_DIAGNOSTIC_CODE_STATUS.md and TS_PARITY_PLAN.md. Records diagnostic codes where Home's faithful-parity reference (the Microsoft Go port, _submodules/typescript-go) and the original TypeScript compiler (_submodules/typescript-go/_submodules/TypeScript/src) disagree, and how Home resolves the disagreement.

Policy

Home targets parity with TypeScript's observable diagnostic behavior. The Go port is the primary porting reference because its data structures map cleanly onto Home's, but the port is incomplete: some subsystems (notably the isolatedDeclarations / JS declaration-emit transformer) are not yet ported, so a handful of codes the original TypeScript compiler genuinely emits have no emission site in the Go port today.

Decision tree when a catalog-only code has no emission site in internal/{checker,parser,binder,transformers} of the Go port:

Original TS also never emits it (the message exists only in diagnosticMessages.json / the generated table, with no Diagnostics.<Name> use in _submodules/TypeScript/src/compiler). → Skip. There is no reference behavior to match; implementing it would invent semantics. (Example historically flagged: TS2553.)
Original TS emits it, the Go port just hasn't ported that subsystem yet. → Keep in scope. Implement faithfully against the original TypeScript source and record the divergence here. Do not skip it merely because the Go port dropped it — it is real, shipping TypeScript behavior that "makes sense having."

Verification for case (2) uses the original TS source + that compiler's own test baselines (_submodules/TypeScript/tests/baselines/reference/…) since the Go port has no baseline for an unported feature.

Codes kept against original TypeScript (Go port has not ported them)

Code	Message (abridged)	Original-TS emission site	Subsystem the Go port lacks	Home status
TS9005	Declaration emit … requires using private name '{0}'. An explicit type annotation may unblock declaration emit.	`compiler/transformers/declarations.ts` `transformDeclarationsForJS` (≈L451)	JS (`allowJs`+`declaration`) decl-emit privacy fallback	planned — privacy-family JS variant
TS9006	Declaration emit … requires using private name '{0}' from module '{1}'. …	same site (the `errorModuleName` branch)	same	planned
TS9026	Declaration emit … requires preserving this import for augmentations. Not supported with --isolatedDeclarations.	`compiler/transformers/declarations.ts` (augmentation-preservation path)	isolatedDeclarations augmentation handling	deferred — needs the .d.ts transformer

Notes:

TS9005/TS9006 are the JavaScript-file analogues of the TS40xx declaration-emit privacy family (TS4023/4024/4025/4030/4031/…). Upstream installs a getSymbolAccessibilityDiagnostic override in transformDeclarationsForJS so that, when emitting .d.ts for a .js/checkJs file, a symbol whose type would leak a private name reports the generic "An explicit type annotation may unblock declaration emit" message (TS9005, or TS9006 when the name comes from another module) instead of the position-specific TS40xx error. They reuse the same module-local-private and cross-module export detection Home already has for the TS40xx family — only the message selection and the JS-file gate differ. Tracked under the privacy-walk work in the parity plan.
TS9026 needs Home to actually attempt isolatedDeclarations .d.ts emission and detect an import that must be preserved for an augmentation; that depends on the declaration-emit transformer, which Home has not built. Left deferred (not skipped).