Built-in shapes default-on via base class (opt-out); shape Temporal.Now

[lahma]

Summary

Make built-in shape storage the default for any [JsObject] host that derives from BuiltinShapeObject — deriving from the base is now the opt-in, so a built-in adopts shapes with a one-line base change and no attribute flag. UseShape = false is the explicit opt-out; hosts that don't derive from BuiltinShapeObject keep the dictionary path regardless.

This was suggested on #2555: rather than an explicit [JsObject(UseShape = true)], shapes are on by default and you opt out.

What changed

• Auto-detect: the generator walks the base-type chain; a host deriving from BuiltinShapeObject uses the shape path by default. The UseShape attribute property now defaults to true and only means "opt this derived host back out" when set false.
• New diagnostic JINT022: a BuiltinShapeObject-derived host that declares an unsupported member (accessor / intrinsic reference / thrower / instance-or-mutable property) fails the build instead of silently dropping it.
• Math, JSON, Reflect, Atomics drop the now-redundant UseShape = true (auto-detected from the base).
• Temporal.Now (9 functions + Symbol.toStringTag, no properties) adopts shapes by simply deriving from BuiltinShapeObject — demonstrating the one-line adoption and adding a win.

Benchmarks (BuiltinShapeBenchmark, default job, vs main)

EngineOnly control is identical (13.69 KB) on both.

Benchmark	Allocation	Time
EngineInitMath / EngineInitBuiltins	15.51 / 20.20 KB == 15.51 / 20.20 KB (byte-identical — the default change is neutral for the existing four)	neutral
EngineInitTemporalNow	20.43 → 19.45 KB (−4.8%)	2.86 → 2.70 µs (−5.6%)

Conformance & tests

• Targeted Test262: Math 662, JSON 502, Reflect 306, Atomics 779, Temporal.Now 138 — all 0 failures.
• Full Test262: 0 new failures (the annexB/RegExp-*-escape-BMP slow-eval timeout tests and the Atomics.waitAsync no-spurious-wakeup test are pre-existing timing flakes under concurrent load — they flake identically on main and pass in isolation).
• Jint.Tests 3138/3076; Jint.Tests.PublicInterface 79/79; Jint.Tests.SourceGenerators 30/30 (snapshots updated for the attribute default value).

🤖 Generated with Claude Code

https://claude.ai/code/session_01RDS23QeSSNRkaUB2KL74U9