fix(transformer/typescript): keep enum IIFE when a non-inlinable value reference remains

[Dunqing]

Refs vitejs/vite#22452 (comment).

The optimize pass dropped the enum declaration whenever the members were all evaluable — regardless of whether some non-inlinable value reference still named the enum at runtime. The result referenced a binding that no longer existed:

const enum Phase { one = 'one' }
export default Phase;
// -> export default Phase;  // ReferenceError at runtime

enum Direction { Up }
Direction.Up.toString();
// -> Direction.Up.toString();  // ReferenceError at runtime

Fix

Decide based on the remaining resolved references: drop only when none of them is a value reference (Read/Write). Type references (as E, : E) are kept here and stripped later by annotations.rs, so they don't block removal. typeof E in JS expression position is Read and correctly keeps the IIFE; typeof E in TS type position is ValueAsType (not in Value) and doesn't.

Unifies the rule for regular and const enums — the previous asymmetry (const always dropped, regular dropped only when no refs at all) had no principled basis.

Compatibility

Diverges from Babel on two optimize-const-enums/* fixtures where the input has a bare value reference to the enum that we can't inline. Three engines disagree on what to do:

Engine	Inline E.X member accesses	Preserve E binding (IIFE)	Result for E.unknown / bare E
Babel transform-typescript (optimize-const-enums)	yes	no	runtime ReferenceError / TypeError
tsc --isolatedModules	no	yes	undefined / resolves
oxc after this PR	yes	yes	undefined / resolves

We now hybridize: inline what's inlinable (Babel) and keep the binding (tsc). Local overrides under tasks/transform_conformance/overrides/ record the new expected output for the two affected Babel fixtures.

optimize-const-enums/local

Input:

const enum A {
  x, y
}

A.x;
A["y"];
A.z;
A;

Babel (and oxc before this PR):

0;
1;
A.z;
A;

A.z is TypeError: cannot read properties of undefined; A; is ReferenceError.

tsc (--isolatedModules):

var A;
(function (A) {
    A[A["x"] = 0] = "x";
    A[A["y"] = 1] = "y";
})(A || (A = {}));
A.x;
A["y"];
A.z;
A;

Keeps the IIFE binding, no member-access inlining. A.z evaluates to undefined; A; is a no-op.

oxc after this PR (overrides/.../optimize-const-enums/local/output.mjs):

var A = /* @__PURE__ */ function(A) {
  A[A["x"] = 0] = "x";
  A[A["y"] = 1] = "y";
  return A;
}(A || {});
0;
1;
A.z;
A;

Hybrid: inlines A.x / A["y"] like Babel, and keeps the IIFE binding like tsc — so A.z evaluates to undefined (not crash) and A; resolves.

optimize-const-enums/merged

Input:

const enum A {
  x, y
}

const enum A {
  z
}

A.x;
A["y"];
A.z;
A.w;
A;

Babel (and oxc before this PR):

0;
1;
0;
A.w;
A;

A.w is TypeError: cannot read properties of undefined; A; is ReferenceError.

tsc (--isolatedModules):

var A;
(function (A) {
    A[A["x"] = 0] = "x";
    A[A["y"] = 1] = "y";
})(A || (A = {}));
(function (A) {
    A[A["z"] = 0] = "z";
})(A || (A = {}));
A.x;
A["y"];
A.z;
A.w;
A;

Both IIFEs emitted, no inlining. A.w is undefined; A; resolves.

oxc after this PR (overrides/.../optimize-const-enums/merged/output.mjs):

var A = /* @__PURE__ */ function(A) {
  A[A["x"] = 0] = "x";
  A[A["y"] = 1] = "y";
  return A;
}(A || {});
A = /* @__PURE__ */ function(A) {
  A[A["z"] = 0] = "z";
  return A;
}(A || {});
0;
1;
0;
A.w;
A;

Hybrid: both enum bodies lower (the second reuses the first's binding via A = (function(A) { … })(A || {}) — assignment, not var) and members are inlined like Babel. A.w evaluates to undefined; A; resolves.

Verification

New fixtures cover the bug shapes:

• babel-plugin-transform-typescript/const-enum-value-ref-kept/ — export default Phase
• babel-plugin-transform-typescript/const-enum-mixed-refs/ — member access + bare ref combined

Existing optimize-enums/type-cast updated to drop the IIFE for the all-type-refs case (the regular-enum side of the unified rule).

[Dunqing]

• fix(transformer): sync scoping after lowering an enum to IIFE #22502
• fix(transformer/typescript): keep enum IIFE when a non-inlinable value reference remains #22501 👈 (View in Graphite)
• fix(transformer/typescript): inline const enum members through type-cast wrappers #22500
• main

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[codspeed-hq]

Merging this PR will not alter performance

✅ 44 untouched benchmarks
⏩ 7 skipped benchmarks¹

---

_{Comparing fix/transformer-const-enum-value-ref-kept (c723603) with fix/transformer-const-enum-type-cast (62376dd)}

Footnotes

1. 7 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Dunqing]

Merge activity

• May 18, 2:18 AM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• May 18, 2:18 AM UTC: Dunqing added this pull request to the Graphite merge queue.
• May 18, 2:24 AM UTC: Merged by the Graphite merge queue.