Dedupe identical object literal types before subtype reduction

[Camille Barneaud (gadcam)]

Fixes #48364: this fixes the checker blow-up on large JSON arrays. Editor responsiveness on very large JSON files may still warrant .d.json.ts.

removeSubtypes relates every pair of union constituents. Array literals of records, typically from resolveJsonModule imports, produce one object literal type per element, so their element type costs O(n²) structural checks, and the relation cache grows with them.

This removes object literal types that are identical to an earlier constituent before the pairwise loop. Identical types are mutual subtypes, so the loop would keep only the first anyway and the reduced type is the same. One visible difference: array literals with many records of one shape followed by another shape (e.g. 1,000 × { a: "" } + 100 × { a: 0 }) no longer hit the subtype reduction limit and report a false TS2590. Two types count as identical when they have the same hash of their object flags and properties (names, flags, check flags and types), like the other CacheHashKey-keyed caches in the checker. Members and property types are only read when they are already resolved (valueSymbolLinks.resolvedType, no accessors, methods, or deferred/instantiated/mapped properties), so the check never triggers type resolution and can't change circularity behavior (see #46981).

Results

colornerd.json from #48364 (29,177 objects, 3 shapes):

	before	after
check	did not finish (>12 min, >4.5 GB RSS)	0.06s

The blow-up starts between 23K and 24K elements. Below that, the same file checks in ~0.1s.

Interleaved A/B runs (Mann-Whitney U on wall time):

project	wall	p
vscode @ 9cf0128	+0.37%	0.85
typescript-benchmarking Compiler	0%	—
typescript-benchmarking Compiler-Unions	−3.45% (check −0.42%)	0.029
app with 5 JSON imports (807-object file with 113 shapes)	−12.3% (check −16.5%)	<0.001

Type and symbol counts are identical in every run. Declaration emit for the JSON imports is identical.

Tests

arrayLiteralIdenticalObjectLiteralsSubtypeReduction.ts covers JSON records with reordered keys, mixed property types and null, plus the cases the shape key must handle or skip: reordered keys, optional properties, spreads, methods, accessors and as const. Its baselines are byte-identical with and without the checker change, and a temporary trace confirmed the new path runs for most of these cases.

arrayLiteralManyObjectLiteralsNotTooComplex.ts reports TS2590 without this change and no error with it.

AI disclosure

This change was written with Claude Code (investigation, patch, test, and benchmarks). I have reviewed it, understand it, and will handle review feedback myself.

[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.

[Camille Barneaud (gadcam)]

TypeScript Bot (@typescript-bot) It fixes issue #48364

[Copilot]

🟡 Changes recommended

Shape-key computation can trigger member resolution and the test does not detect regression of the optimization.

2 open findings

• Avoid resolving members before checking instantiation status · New
• Add a regression test that exercises the deduplication optimization · New

What changed in this PR

Optimizes subtype reduction by deduplicating structurally identical object literal types before pairwise comparison.

Changes:

• Adds object-literal shape hashing and deduplication.
• Adds compiler coverage and generated baselines for JSON and object-literal cases.

File	Description
tsc/​internal/​checker/​checker.go	Implements shape-based deduplication.
tsc/​testdata/​tests/​cases/​compiler/​arrayLiteralIdenticalObjectLiteralsSubtypeReduction.ts	Adds compiler scenarios.
tsc/​testdata/​baselines/​reference/​compiler/​arrayLiteralIdenticalObjectLiteralsSubtypeReduction.types	Records inferred types.
tsc/​testdata/​baselines/​reference/​compiler/​arrayLiteralIdenticalObjectLiteralsSubtypeReduction.symbols	Records resolved symbols.
tsc/​testdata/​baselines/​reference/​compiler/​arrayLiteralIdenticalObjectLiteralsSubtypeReduction.js	Records emit and declarations.

🧠 Review effort: Balanced

[Copilot]

🔵 Needs a closer look

The optimization changes core subtype-reduction semantics and warrants final human validation despite comprehensive regression coverage.

0 open findings

2 resolved since last review

• Avoid resolving members before checking instantiation status
• Add a regression test that exercises the deduplication optimization

🧠 Review effort: Balanced