flat-js-no-es5-lowering

name	flat-js-no-es5-lowering
description	Use when adding or fixing ES2015+ language support in Flat JS, especially in `src/compiler/**`, `src/runtime/**`, opcode work, or syntax handlers. Enforces the project rule that newer JavaScript features must be implemented directly in compiler/runtime semantics rather than lowered into ES5-style helper code, synthetic AST rewrites, or userland compatibility shims.

Flat JS No ES5 Lowering

Use this skill when the task touches newer JavaScript semantics and there is any temptation to "just rewrite it" into older JavaScript before execution.

Rule

• Do not downlevel ES6+ user syntax into ES5-style source patterns inside codegen.
• Support the feature as first-class Flat JS behavior in bytecode, runtime metadata, opcodes, scope handling, iterator handling, or other VM semantics.
• Treat synthetic AST rewrites as a smell when they exist only to emulate a newer language feature with older JavaScript.

Reject These Patterns

• Do not rewrite rest parameters into [].slice.call(arguments, i).
• Do not rewrite spread call/new/super into .apply(...) helpers just to preserve syntax externally.
• Do not rewrite tagged templates into plain function calls with only cooked strings.
• Do not rewrite classes into prototype-assignment source patterns.
• Do not rewrite arrow functions into plain functions plus ad hoc this workarounds.
• Do not replace iterator-based semantics with array-only loops when the language feature is defined in terms of iterables.

Preferred Approach

1. Inspect the current syntax handler and runtime path first.
2. Identify whether the existing implementation lowers by creating replacement AST or helper-style source shapes.
3. Move the behavior into direct Flat JS semantics instead:
   • function-entry metadata
   • opcode additions
   • runtime helpers
   • iterator-aware execution
   • template-object caching
   • direct this / super / new.target handling
4. Preserve observable semantics, especially:
   • realm-sensitive errors
   • this
   • new.target
   • arguments
   • iterator consumption
   • template identity and raw
5. Add focused regressions near the changed feature and extend opcode coverage if a new opcode was introduced.

Files To Check First

• src/compiler/codegen/**
• src/compiler/shared.ts
• src/compiler/encoding.ts
• src/runtime/execution.ts
• src/runtime/opcodes/**
• targeted tests in src/__tests__/**

Completion Gate

• Do not call the target complete if the implementation still relies on ES5-style lowering for the feature.
• Do not call the target complete if relevant compiler/runtime tests fail.
• When the change is broad enough to affect branch health, also use $flat-js-branch-working-status and satisfy its CI/web/Jest/Test262 completion gates.