REGRESSION: ~760 class test262 tests broke — 'm descriptor should be configurable'

[proggeramlug]

Severity: HIGH (class method property descriptors)

A re-measure of the honest test262 sweep caught a regression: overall parity dropped 91.8% → 90.4% despite annexB (+10.6) and intl402 (+3.7) gains — because language fell 94.5% → 89.3%.

What regressed

770 newly-failing tests (passed on main @ dbedba916, fail on main @ 73d1fede4), of which 760 are class tests:

• 384 language/statements/class
• 376 language/expressions/class

752 share one reason: Uncaught exception: m descriptor should be configurable — i.e. a class method's property descriptor is now wrong (a method that should be configurable: true no longer is, per verifyProperty in propertyHelper.js).

Trigger is a specific shape

Isolated repros are FINE — Object.getOwnPropertyDescriptor(C.prototype,'m') on both a plain class C { m(){} } and class C { #x=1; m(v){...} } correctly yields {writable:true,enumerable:false,configurable:true} on the current binary. So the break is a narrower class-method shape exercised by the flags: [generated] grammar-permutation class tests (generator/computed/private-adjacent method forms). Repro: run scripts/test262_subset.py --dir language/statements/class (node v26), grab a m descriptor should be configurable failure, and inspect its class body.

Regression window + prime suspects

Between main dbedba916 (2026-06-17) and 73d1fede4 (2026-06-19). PRs touching class method/descriptor/reflection in that window:

• fix(class): super-in-static resolution + accessor .name reflection (#5345) #5395 fix(class): super-in-static resolution + accessor .name reflection ← prime suspect (touched class method/accessor property definition)
• fix(runtime): throw TypeError for Object.setPrototypeOf on non-extensible target (#5347) #5396 Object.setPrototypeOf TypeError, codegen: derived-ctor this-TDZ on standalone constructor-symbol path (toward #5345) #5362 derived-ctor this-TDZ, fix(hir,codegen): route class-extends userland stream-shim ctor through its real constructor (winston) #5357 class-extends stream-shim ctor, and the Object-descriptor PRs.

git bisect over that range against one failing case will pin it fast.

Fix

Restore the correct class-method descriptor (configurable:true, enumerable:false, writable:true) for whatever method shape #5395 (or the culprit) altered, without losing that PR's super-in-static / accessor-.name gain.

Found via the honest re-measure (#5300 follow-up). Class-tail tracker: #5345.