Regression in some error elaborations involving a target type mixing arrays with non-arrays

[Andarist]

🔎 Search Terms

discriminated discrimination array error elaboration single

🕗 Version & Regression Information

• This changed in PR Port "Improve type discrimination algorithm" from tsgo #61828

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=6.0.0-dev.20250807#code/C4TwDgpgBAyglgOwOYBsIHkBOBBTmCGIAPACoB8UAvFCVAD40DaAugNwBQ7okUAksBAC2VdlHpQA3qLEyAzpgDGALigAiAGYB7Tao4yoAX2kMp+sfOVqARvky7pYgx3YLNCWcCiarAKwCMKvDIaFi4BMT8QhTUplAWKqo2AF6qADTsTpyu7p7ePgBMgYioGDh4hESRghQMAK4IACYQ6ogQDVSS0vHW+CnpmS5uHl6+AMxFwaVhFVXi9U0tCG3RneaKCclpGc7Zw3kALBMloeURAsJ1jc2tDTVQ89dL7TFd6z1921lDnraYAbDFEJlcKVc4rWLdRK9LYDXY-PCFAGTE4gqp3B6LNodRjSWJrSxQj6OdJsL45KC-cZI47AmbnOZXTG3bG4hxxN6ErbE9ikwbk36HalA6ZnIQMhY3dGMm4ssR49kEzbpbm87jQAAymiQcAUlVSUHQKwAFIgwLVgCpeABKKgUdDONV8c75ERiExs-EJLQ6PRmOANFQeTDFX2OYyrMzdTXa3VLABuEEw+vqAGsEJoAO4IMihmT+gD8KnjidzAyaChQtmgcKgKC1OpU0Z1RCDxX1CFqgisiZzZL2vgArEdhadQUJ8uDXpY6zHlVB-QlZJpBBBeA0YTtviMfAA2YdTUdVCfix5Yl6e2v1hRzhdqJcrtcbvu5XwAdn3KLp45PTMnF5nOo3gGd7Lqu679Ju5J5AAHB+tKioILqXBKTxSihZ4Rt0AHXtIt6qPeYFPnywy-EOQoHqizorDicpslhV5zmIeEEY+c4GCSkEkXge7kZ+CHHshp7PFANERv+DFssxoGsdI7E8px8KYO+vHwWOiE-pKspiQqKjYYx87Afh0ngbJHHPhSeCwSpIpqUh9zSqhGlPFp8r0bOkmGSxJkqhwQA

💻 Code

type SingleOrArray<T> = T | T[];

type Item =
  | {
      src: "foo";
    }
  | {
      src: "bar";
    };

const obj1: SingleOrArray<Item> = {
  src: "baz", // ✅ errors nicely at property
};

// ⚠️ no change but reported error is at suboptimal position
const obj2: SingleOrArray<Item> | undefined = {
  src: "baz",
};

// ⚠️ no change but reported error is at suboptimal position
const obj3: SingleOrArray<Item | undefined> = {
  src: "baz",
};

// ⚠️ no change but reported error is at suboptimal position
const obj4: SingleOrArray<Item | undefined> | undefined = {
  src: "baz",
};

const arr1: SingleOrArray<Item> = {
  src: "baz", // ✅ errors nicely at property
};

const arr2: SingleOrArray<Item> | undefined = [
  {
    src: "baz", // ✅ errors nicely at property
  },
];

const arr3: SingleOrArray<Item | undefined> = [
  {
    src: "baz", // ✅ errors nicely at property
  },
];

const arr4: SingleOrArray<Item | undefined> | undefined = [
  {
    src: "baz", // ✅ errors nicely at property
  },
];

type Logic<I, O> = (input: I) => O;

type Item2 =
  | {
      src: "foo";
      id: string;
    }
  | {
      src: Logic<never, unknown>;
      id?: never;
    };

declare const logic: Logic<string, number>;

// ❌ error moved from a property to the variable
const obj5: SingleOrArray<Item2> = {
  src: logic,
  id: "someId",
};

// ❌ error moved from a property to the variable
const obj6: SingleOrArray<Item2> | undefined = {
  src: logic,
  id: "someId",
};

// ❌ error moved from a property to the variable
const obj7: SingleOrArray<Item2 | undefined> = {
  src: logic,
  id: "someId",
};

// ❌ error moved from a property to the variable
const obj8: SingleOrArray<Item2 | undefined> | undefined = {
  src: logic,
  id: "someId",
};

// ⚠️ no change but reported error is at suboptimal position
const arr5: SingleOrArray<Item2> = [
  {
    src: logic,
    id: "someId",
  },
];

// ⚠️ no change but reported error is at suboptimal position
const arr6: SingleOrArray<Item2> | undefined = [
  {
    src: logic,
    id: "someId",
  },
];

const arr7: SingleOrArray<Item2 | undefined> = [
  {
    src: logic,
    id: "someId", // ✅ errors nicely at property
  },
];

const arr8: SingleOrArray<Item2 | undefined> | undefined = [
  {
    src: logic,
    id: "someId", // ✅ errors nicely at property
  },
];

🙁 Actual behavior

Some of the errors in the "second group" moved. It is a specific situation with multiple available discriminants.

🙂 Expected behavior

I'd really expect all of the errors within each "group" to be reported at a property. I especially don't see a reason why | undefined would make the error move to one of the parent nodes.

Additional information about the issue

this was caught here: #61828 (comment)

1. is this purely an elaboration issue?
2. or could a heuristic be introduced that would allow for array targets to be eliminated early when discriminating by available object members?