Change in inference from tuple to array

[TimMoore]

The following code is accepted by TypeScript tsc 5.8.3 but fails with tsgo as of version 7.0.0-dev.20250603.1:

type IMapEntries<K, V> = [K, V][];

type IKeyValueMap<V> = {
  [key: string]: V;
};

function testFunction<K, V>(initialValues?: IMapEntries<K, V> | IKeyValueMap<V>): void {}

const obj = {
  key1: 'value1',
  key2: 'value2',
  key3: 'value3',
};

testFunction(Object.entries(obj).map(([key, value]) => [key, value]));

https://tsplay.dev/mbb5dm

The error from tsgo is:

test.ts(15,14): error TS2345: Argument of type 'string[][]' is not assignable to parameter of type 'IKeyValueMap<string> | IMapEntries<string, string> | undefined'.
  Type 'string[][]' is not assignable to type 'IKeyValueMap<string> | IMapEntries<string, string>'.
    Type 'string[][]' is not assignable to type 'IMapEntries<string, string>'.
      Type 'string[]' is not assignable to type '[string, string]'.
        Target requires 2 element(s) but source may have fewer.

Seemingly incidental changes such as inlining either of those type definitions into the function signature resolves the error.

This is a simplification of production code using the MobX observable.map function.

[jakebailey]

This is union order dependent. Swap the order of the two things in your union, and you get the same error in TS 5.8: Playground Link

The Go port has to have a different union ordering than the original code in order to prevent check-order dependent inconsistencies (allowing us to do concurrent checking), and this is unfortunately a case where that matters.

(This reminds me of microsoft/TypeScript#52313)

[jakebailey]

@ahejlsberg I know you marked this as wontfix, but do you think there's likely to be a generic fix to this class of problem? AFAIK this is union subtype reduction, right?