Description

abramobagnara

opened on Nov 19, 2017

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TypeScript Version: 2.7.0-dev.201xxxxx

Code

class C { }

function f<T extends C>(v: T | string): void {
  if (v instanceof C) {
    const x: T = v;
    console.log(x);
  }
}

Expected behavior:

No compile errors:

Actual behavior:

p.ts(5,11): error TS2322: Type 'string | T' is not assignable to type 'T'.
Type 'string' is not assignable to type 'T'.

Activity

1. ahejlsberg commented on Nov 19, 2017

   

   Anders Hejlsberg (ahejlsberg)

   on Nov 19, 2017

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   This was an error in previous releases too because C is an empty object type. However, with #19671 we can (and should) get this right.
2. Anders Hejlsberg (ahejlsberg)

   self-assigned this

   on Nov 19, 2017
3. Anders Hejlsberg (ahejlsberg)

   added

   BugA bug in TypeScriptA bug in TypeScript

   on Nov 19, 2017
4. abramobagnara commented on Nov 19, 2017

   

   abramobagnara

   on Nov 19, 2017

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   From last part of comment above it seems you imply that master branch after #19671 merge get this right, but this is not the case. Am I missing something?

   I can confirm that if C is not an empty object type Typescript 2.6.1 compile the source without errors.
5. Anders Hejlsberg (ahejlsberg)

   added this to the TypeScript 2.7 milestone on Nov 19, 2017
6. ahejlsberg commented on Nov 19, 2017

   

   Anders Hejlsberg (ahejlsberg)

   on Nov 19, 2017

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   abramobagnara I meant that with the approach we introduced in #19671 we can, but currently don't, get it right even when C is an empty class type (as in your example). You are correct that it used to work when C is not an empty class. Both issues are fixed by #20142 that I just put up.
7. Anders Hejlsberg (ahejlsberg)

   added

   FixedA PR has been merged for this issueA PR has been merged for this issue

   on Nov 19, 2017
8. Anders Hejlsberg (ahejlsberg)

   closed this as completedin #20142on Nov 20, 2017
9. microsoft

   locked and limited conversation to collaborators on Jun 14, 2018

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