Summary

Fixes #37447. Two paths in the React Compiler collapsed JavaScript's observable distinction between 0 and -0:

1. 
   Constant propagation (Optimization/ConstantPropagation.ts): evaluatePhi compared phi operand constant values with !==. Since 0 !== -0 evaluates to false in JS, a phi merging 0 on one branch and -0 on another was incorrectly folded into a single "constant" value, silently discarding whichever branch didn't "win", regardless of which branch actually executed at runtime.


2. 
   Codegen (ReactiveScopes/CodegenReactiveFunction.ts): codegenValue only emitted a unary negation (-0) when value < 0. Since -0 < 0 is false, a literal -0 was re-emitted as plain 0, losing its sign.

Fix

• Use Object.is() instead of !== when comparing phi constant candidates in evaluatePhi, so operands with different sign but "equal" primitive values (i.e. 0/-0) are correctly treated as non-constant (and thus preserved as a genuine runtime phi instead of being folded to one arbitrary branch's value).


• Emit a unary negation in codegen when Object.is(value, -0) is true, in addition to the existing value < 0 check, so -0 literals round-trip correctly.

Tests

• Added negative-zero-phi-constant-propagation.js/.expect.md: a component whose phi alternates between 0 and -0 across renders. 1 / x is used to observe the sign of zero (Infinity vs -Infinity), and the fixture harness compares compiled vs. uncompiled eval output, verifying Object.is parity is preserved pre/post compile.


• Updated the pre-existing constant-propagation-unary-number snapshot: it already contained a -0 literal that was silently being emitted as 0 in codegen prior to this fix — that regression is now fixed and visible in the updated snapshot (-2, -0, true, ... instead of -2, 0, true, ...).


• Ran the full babel-plugin-react-compiler snap test suite (1817 fixtures) — all pass with only the one expected snapshot update above.


• 
  yarn workspace babel-plugin-react-compiler lint passes.

Test plan

cd compiler/packages/babel-plugin-react-compiler

yarn build

yarn workspace snap run snap

yarn lint