Generic argument type inference regression in TS 3.4

[ulrichb]

TypeScript Version: 3.4.0-dev.20190223

Search Terms: 3.4 inference generic readonly

Code

The following snippet emits an error in TS 3.4 whereas in TS 3.3 it compiles fine.

class ErrorResult {
    readonly isError = true;

    static combine(errorResults: ReadonlyArray<ErrorResult>): ErrorResult {
        return new ErrorResult(/* ... combine errorResults ... */);
    }
}

//

function extractAndCombineErrorResults<T, TErrorResult extends ErrorResult>(
    allResults: ReadonlyArray<T | TErrorResult>,
    combine: (details: ReadonlyArray<TErrorResult>) => TErrorResult,
): ReadonlyArray<T> | TErrorResult {

    return /* dummy result: */ [];
}

//

interface SomeResult {
    isSomeResult: true
}

const allResults: ReadonlyArray<SomeResult | ErrorResult> = [];

// Here the inference doesn't work anymore:
const result1: ReadonlyArray<SomeResult> | ErrorResult =
    extractAndCombineErrorResults(allResults, ErrorResult.combine);

// Explicit generic args work:
const result2: ReadonlyArray<SomeResult> | ErrorResult =
    extractAndCombineErrorResults<SomeResult, ErrorResult>(
        allResults, ErrorResult.combine);

Command line: npx tsc TypeScriptIssue.ts

Expected behavior:
No compile error.

Actual behavior:

TSC emits

TypeScriptIssue30074.ts:28:7 - error TS2322: Type 'ErrorResult | readonly (ErrorResult | SomeResult)[]' is not assignable to type 'ErrorResult | readonly SomeResult[]'.
  Type 'readonly (ErrorResult | SomeResult)[]' is not assignable to type 'ErrorResult | readonly SomeResult[]'.
    Type 'readonly (ErrorResult | SomeResult)[]' is not assignable to type 'readonly SomeResult[]'.
      Type 'ErrorResult | SomeResult' is not assignable to type 'SomeResult'.
        Property 'isSomeResult' is missing in type 'ErrorResult' but required in type 'SomeResult'.

28 const result1: ReadonlyArray<SomeResult> | ErrorResult =
         ~~~~~~~

Playground Link: (works in TS 3.3)

Possibly related: #29435

[ahejlsberg]

This is working as intended and is an effect of #29847. In your example, we now end up making inferences from the allResults parameter which we previously wouldn't do. Therefore, in the result1 assignment we now infer SomeResult | ErrorResult for T, and we'll make that inference even if you omit the type annotation for result1 (where previously we'd then infer {} for T).

[ulrichb]

Hi @ahejlsberg,

ah, now I see that in TS 3.3 the inference comes from the result assignment (without, it inferred ReadonlyArray<SomeResult | ErrorResult> | ErrorResult).

Many thanks for your reply!

---

I've now found an alternative solution to make inference work again (based on the second combine argument, which fixates TErrorResult) using Exclude<T, TErrorResult> for the result.

// ...

function extractAndCombineErrorResults<T, TErrorResult extends ErrorResult>(
    allResults: ReadonlyArray<T>,
    combine: (details: ReadonlyArray<TErrorResult>) => TErrorResult,
): ReadonlyArray<Exclude<T, TErrorResult>> | TErrorResult {

    return /* dummy result: */ [];
}

//

const allResults: ReadonlyArray<SomeResult | ErrorResult> = [];

const result: ReadonlyArray<SomeResult> | ErrorResult =
    extractAndCombineErrorResults(allResults, ErrorResult.combine);

[typescript-bot]

This issue has been marked 'Working as Intended' and has seen no recent activity. It has been automatically closed for house-keeping purposes.