Fix type inference ordering inconsistency with discriminated unions and flatMap

[Copilot]

When inferring a common supertype from candidates, Go and TypeScript can encounter candidates in different orders due to union constituent ordering differences. This caused flatMap callbacks returning discriminated union branches to fail in Go but succeed in TypeScript.

Root Cause

getSingleCommonSupertype used isTypeSubtypeOf as the fallback tiebreaker. The subtype relation sets requireOptionalProperties = true for non-fresh object types, so a widened { op: "remove" } (which lost ObjectFlagsObjectLiteral after getWidenedTypeOfObjectLiteral) would fail the subtype check against InputOp = { op: "add" } | { op: "remove"; value?: Array<unknown> } — despite being clearly assignable to it. With reversed candidate ordering, Go picked the wrong type.

Fix

Replace the isTypeSubtypeOf fallback with asymmetric assignability (isTypeAssignableTo(s, t) && !isTypeAssignableTo(t, s)):

• One-way assignable pairs (e.g. { op: "remove" } → InputOp) correctly promote to the more general type regardless of ordering.
• Mutually-assignable pairs (e.g. { x: number; y?: number } and { x: number; z?: number }) are symmetric, so selection remains order-based — preserving existing behavior.

export type InputOp = { op: "add" } | { op: "remove"; value?: Array<unknown> };
export type OutputOp = { op: "add" | "remove" };

export function f(operations: InputOp[]): OutputOp[] {
  return operations.flatMap((operation) => {
    if (operation.op === "remove" && operation.value) {
      return [].map(() => ({ op: "remove" }));
    } else {
      return [operation];
    }
  });
}

This follows the approach suggested by @ahejlsberg in the issue.

[jakebailey]

This caused one break: microsoft/TypeScript#63328 (comment)

[typescript-bot]

@RyanCavanaugh Here are the results of running the user tests with tsc comparing main and refs/pull/3301/merge:

There were infrastructure failures potentially unrelated to your change:

• 1 instance of "Git clone failed"

Otherwise...

Everything looks good!