5.9 regression: Union types served by mutually exclusive function overloads not working

[hyperair]

🔎 Search Terms

overload overloaded function parameter union type

🕗 Version & Regression Information

• This changed between versions 5.8.3 and 5.9.3
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about _________
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play/?ts=5.9.3#code/GYVwdgxgLglg9mABAIwIYCcAUATVVUBciAguuqgJ4BCIwwApugJRHjb3Axj3YDcAUKEiwEKDDjyFEAVS5QAHKXIUWiNhy48BQ6PCRosufESWUadRogA+MuYrKUmiAN79EidPSgh0YAQF9+QXBdUWA4OAljNTAAazA4AHcwJ1d3GGBETABCKNRELgBnfEh6OEzZMAVTCmsbI3yikogyzJrzBmYnT29fAXcDPKYAoA

💻 Code

function bar(data: ArrayBuffer): undefined;
function bar(data: Uint8Array): undefined;
function bar(data: ArrayBuffer | Uint8Array) {
  return;
}

function foo(data: unknown) {
  if (!(data instanceof Uint8Array || data instanceof ArrayBuffer)) return;
  bar(data);
}

🙁 Actual behavior

Got the following error in 5.9.3, but works perfectly in 5.8.3:

No overload matches this call.
  Overload 1 of 2, '(data: ArrayBuffer): undefined', gave the following error.
    Argument of type 'ArrayBuffer | Uint8Array<ArrayBufferLike>' is not assignable to parameter of type 'ArrayBuffer'.
      Type 'Uint8Array<ArrayBufferLike>' is not assignable to type 'ArrayBuffer'.
        The types of 'slice(...)[Symbol.toStringTag]' are incompatible between these types.
          Type '"Uint8Array"' is not assignable to type '"ArrayBuffer"'.
  Overload 2 of 2, '(data: Uint8Array<ArrayBufferLike>): undefined', gave the following error.
    Argument of type 'ArrayBuffer | Uint8Array<ArrayBufferLike>' is not assignable to parameter of type 'Uint8Array<ArrayBufferLike>'.
      Type 'ArrayBuffer' is missing the following properties from type 'Uint8Array<ArrayBufferLike>': BYTES_PER_ELEMENT, buffer, byteOffset, copyWithin, and 23 more.

🙂 Expected behavior

Compiles without error

Additional information about the issue

This test case was isolated from https://github.com/brianc/node-postgres/blob/77cb77150d6c0f07b0aa2134c6b3ef0cb80335c1/packages/pg-cloudflare/src/index.ts#L156, while working on brianc/node-postgres#3662.

[hyperair]

As a workaround, this works:

    data instanceof Uint8Array ? bar(data) : bar(data)

[MartinJohns]

This has nothing to do with the overload, but just with type narrowing. It's an unfortunate side effect of: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-5-9.html#libdts-changes

Calling function overloads with unions like this was never supported. Just in 5.8 the type data was narrowed to ArrayBuffer, where in 5.9 it's narrowed to a union type.

Given that both overloads are completely identical besides the type of the argument, your best option is to remove the overload and just accept a union type as argument. See also: TypeScript Handbook - Writing Good Overloads

Always prefer parameters with union types instead of overloads when possible

[RyanCavanaugh]

🤖 Thank you for your issue! I've done some analysis to help get you started. This response is automatically generated; feel free to 👍 or 👎 this comment according to its usefulness.

Similar Issues

Here are the most similar issues I found

• (73%) microsoft/typescript#39962: Error in type in overloaded function
• (68%) microsoft/typescript#29969: Overloading function fails with union types
• (68%) microsoft/typescript#16816: Union type conflicts with overloaded function
• (68%) microsoft/typescript#8350: Argument of type 'number' | 'string' is not assignable to overloaded function accepting 'number' or 'string
• (68%) microsoft/typescript#6503: Type inference is overly strict when dealing w/ Union types and overloads
• (67%) microsoft/typescript#21175: Overloads should be merged to consider whether a union-typed argument is valid
• (67%) microsoft/typescript#17471: Allow calls to overloaded functions when all possible combinations of union type parameters resolve to valid overloads
• (67%) microsoft/typescript#55544: Ambiguously call function with overloads.
• (67%) microsoft/typescript#47480: Union type overload inference don't match if types are in different definitions
• (67%) microsoft/typescript#39313: Function types: overload/merging behavior is unclear
• (66%) microsoft/typescript#22471: Overload Regression in TS2.7 -- the first matching overload is not selected
• (66%) microsoft/typescript#55777: Valid filter predicates rejected after 5.2.2 upgrade Add fallback logic for generating signatures for unions of array members #53489
• (66%) microsoft/typescript#17962: UintArray doesn't accept union of types
• (66%) microsoft/typescript#27131: Overload with type unions
• (66%) microsoft/typescript#1723: Overload on constants requires cast for resolution

If your issue is a duplicate of one of these, feel free to close this issue. Otherwise, no action is needed.

[hyperair]

@MartinJohns

Calling function overloads with unions like this was never supported. Just in 5.8 the type data was narrowed to ArrayBuffer, where in 5.9 it's narrowed to a union type.

Thanks for pointing that out, I saw the release notes about the lib.d.ts change, but didn't realize that's what broke this usecase.

Given that both overloads are completely identical besides the type of the argument, your best option is to remove the overload and just accept a union type as argument. See also: TypeScript Handbook - Writing Good Overloads

This is a bit hard, since this is the Buffer.from signature from @types/node we're talking about.