State the empty-array arm rule once

The three sites that give an empty `[]` literal a home — a union slot, a
contextual union, and the `??` default — each carried their own copy of
the rule and each tested it slightly differently (element count here, a
`never[]` own type there), so a later fix could drift one from the
others. emptyArrayLiteralArm states it once: an empty literal has no
element to select an arm by, so it builds as the destination's first
array arm. Every site keeps the lowering it had.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/compiler/src/frontend/lowering/expressions/array-literals.ts

@@ -38,6 +38,28 @@ import { arrayValueStore, primitiveElement } from "../array-values.js";
 import { classGeneratorIterable } from "../class-iterables.js";
 import { iteratorCanStep } from "../iterator-consumption.js";
 
+/** The arm an EMPTY `[]` literal builds as: `arms`' first array arm, or
+ * null when the literal is not empty (an element selects the arm) or no
+ * arm is an array.
+ *
+ * An empty literal carries no element of its own — tsc infers `never[]`,
+ * whose f64-element mapping is the uninhabited element's representation,
+ * not one of the destination's arms — and every empty array is the same
+ * value whichever array family stores it, so several array arms raise no
+ * ambiguity. This is the ONE statement of that rule: a union slot, a
+ * contextual union, and the `??` default all ask it. A lone array slot
+ * states the same thing without asking. */
+export function emptyArrayLiteralArm(
+  node: ts.Expression,
+  arms: readonly IrType[],
+): (IrType & { kind: "array" }) | null {
+  let value = node;
+  while (ts.isParenthesizedExpression(value)) value = value.expression;
+  if (!ts.isArrayLiteralExpression(value) || value.elements.nodes.length > 0) return null;
+  const arm = arms.find((candidate) => candidate.kind === "array");
+  return (arm as (IrType & { kind: "array" }) | undefined) ?? null;
+}
+
 /** JavaScript: whether `expr` is or holds an existing checked-dynamic
  * object — a binding stored as a dyn value whose checker type is an
  * object, or an array or object literal with such an element. Building
@@ -206,16 +228,12 @@ export function lowerArrayLiteral(
       mapped = arrayOf(unitOnlyUnion(lowerer.unions));
     // An EMPTY literal in a union slot (`const t: string[] | undefined =
     // []`, the `[]` default argument against `string[] | undefined`) has
-    // no useful own type — tsc infers `never[]`, whose f64-element
-    // mapping is a representation for the uninhabited, not the slot's
-    // arm. Build the literal as the union's FIRST array arm and let the
-    // slot's coercion wrap it: an empty array is the same value whichever
-    // array family stores it, so several array arms raise no ambiguity —
-    // the rule a lone array slot already states.
-    if (ctxUnion && expr.elements.nodes.length === 0) {
+    // no useful own type: it builds as the union's first array arm (the
+    // emptyArrayLiteralArm rule) and the slot's coercion wraps it.
+    if (ctxUnion) {
       const def = lowerer.unions.get(ctxUnion.unionId);
-      const arrayArms = def?.arms.filter((a) => a.kind === "array") ?? [];
-      if (arrayArms.length >= 1) mapped = arrayArms[0]!;
+      const arm = emptyArrayLiteralArm(expr, def?.arms ?? []);
+      if (arm) mapped = arm;
     }
     // A NON-EMPTY literal whose own type has no static home (the JS dyn
     // fallback — a null/dyn mapping, or an inference that degraded to a

packages/compiler/src/frontend/lowering/expressions/binary.ts

@@ -73,6 +73,7 @@ import { nsWritableTarget } from "../lower-namespaces.js";
 import { arrayValueStore, tryLowerBytesReadNumberEquality } from "../array-values.js";
 import { cjsExportMemberWriteGlobalOf, cjsInFileExportGlobalOf } from "../cjs-exports-object.js";
 import { lowerUnionFieldWrite } from "./union-fields.js";
+import { emptyArrayLiteralArm } from "./array-literals.js";
 import { lowerAbstractEquality } from "../abstract-equality.js";
 import { lowerInExpression } from "./relational.js";
 import { lowerDynTypeofTest, lowerErrorCodeTypeofTest } from "./typeof.js";
@@ -380,19 +381,6 @@ function untypedCollectionFallback(lowerer: Lowerer, node: ts.Expression): boole
   });
 }
 
-/** An empty `[]` literal whose own type is `never[]` (no contextual array
- * type supplied one): the `??` default of an optional array read. */
-function emptyNeverArrayLiteral(lowerer: Lowerer, node: ts.Expression): boolean {
-  let value = node;
-  while (ts.isParenthesizedExpression(value)) value = value.expression;
-  if (!ts.isArrayLiteralExpression(value) || value.elements.nodes.length > 0) return false;
-  const own = lowerer.typeOf(value);
-  return (
-    lowerer.checker.isArrayType(own) &&
-    (lowerer.checker.getTypeArguments(own as ts.TypeReference)[0]!.flags & ts.TypeFlags.Never) !== 0
-  );
-}
-
 export function lowerNullishPair(
   lowerer: Lowerer,
   expr: ts.BinaryExpression,
@@ -445,21 +433,21 @@ export function lowerNullishPair(
     return { kind: "nullish", left, right, type: rest[0]!, loc };
   }
   // An EMPTY array-literal default (`''.match(re) ?? []`) has no element of
-  // its own: tsc infers `never[]`, whose f64-element mapping the whole
-  // expression keeps as a second array arm (`RegExpMatchArray | number[]`),
-  // a result no single array slot can receive. It adopts the left's present
-  // array arm instead — the empty-literal rule the union slot and the
-  // ternary arm already state — so `?? []` answers the left's array type.
-  // A JavaScript `[]` is `any[]` and keeps the dynamic path.
+  // its own, so it builds as the left's array arm (the emptyArrayLiteralArm
+  // rule the union slot and the contextual union share) and the whole
+  // expression answers the left's array type instead of the checker's
+  // `never[]`-flavored second arm. A JavaScript `[]` is `any[]` and keeps
+  // the dynamic path.
   if (
     rest.length === 1 &&
     rest[0]!.kind === "array" &&
-    !isJsSourceFile(ts.getSourceFileOfNode(expr.right)) &&
-    emptyNeverArrayLiteral(lowerer, expr.right)
+    !isJsSourceFile(ts.getSourceFileOfNode(expr.right))
   ) {
-    const right = lowerer.lowerExprExpecting(expr.right, rest[0]!);
-    if (typeEquals(right.type, rest[0]!))
-      return { kind: "nullish", left, right, type: rest[0]!, loc };
+    const arm = emptyArrayLiteralArm(expr.right, rest);
+    if (arm) {
+      const right = lowerer.lowerExprExpecting(expr.right, arm);
+      if (typeEquals(right.type, arm)) return { kind: "nullish", left, right, type: arm, loc };
+    }
   }
   let type = lowerer.irTypeOf(expr);
   // A runtime-optional read can reach the default even when declarations

packages/compiler/src/frontend/lowering/lowerer.ts

@@ -781,6 +781,7 @@ import { lowerRecordFieldCall, lowerObjectMethodCall } from "./lower-calls.js";
 import { fenceCrossBlockNsRef, nsPathPrefix } from "./lower-namespaces.js";
 import { isSafeToRepeat } from "./expressions/evaluation-safety.js";
 import { partialAssertionFields } from "./partial-assertions.js";
+import { emptyArrayLiteralArm } from "./expressions/array-literals.js";
 import { lowerTernary } from "./lower-exprs.js";
 import {
   maybePromiseReturnStmt,
@@ -14044,15 +14045,10 @@ export class Lowerer {
             );
           }
         }
-        // An EMPTY literal carries no element to select an arm by: tsc
-        // infers `never[]`, whose f64-element mapping is the uninhabited
-        // element's representation, not one of the slot's arms. Every empty
-        // array is the same value whichever array arm stores it, so the
-        // literal is built as the union's FIRST array arm — the empty
-        // literal rule a lone array slot already states, and the same one
-        // the ternary arm and the `?? []` default apply.
-        if (!arm && arms.length > 1 && x.elements.nodes.length === 0)
-          arm = arms.find((candidate) => candidate.kind === "array");
+        // An EMPTY literal carries no element to select an arm by: it
+        // builds as the union's first array arm (the emptyArrayLiteralArm
+        // rule the contextual union and the `?? []` default share).
+        if (!arm) arm = emptyArrayLiteralArm(x, arms) ?? undefined;
         if (arm) {
           const built = this.lowerArrayLiteral(
             x,