Generic types combined with & instead of | #48287

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1. whzx5byb commented on Mar 16, 2022

   

   whzx5byb

   on Mar 16, 2022

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   This is a correct error. Here is a simple example in which your expected behavior is unsound:

   class ExampleClass<T> {
       test(value: T): T {
           return value;
       }
   }
   
   class Example1 extends ExampleClass<string> { 
       test(value: string): string {
           return value.toLowerCase();
       }
   }
   
   class Example2 extends ExampleClass<number> { 
       test(value: number): number {
           return Number(value.toFixed(2));
       }
   }
   
   function Example(example: 1|2) // returns `typeof Example1 | typeof Example2`
   {
       return Math.random() > 0.5 ? Example1 : Example2;
   }
   
   const instance = new (Example(1))();
   
   console.log(instance.test('Foo'))
   // You expect it return 'foo', but there's a 50% chance of runtime error

   See: #30769 for more details.
2. RoelVB commented on Mar 16, 2022

   

   Roel (RoelVB)

   on Mar 16, 2022

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   I don't see how this would be correct.
   In test(value: T): T the first T becomes String & Number, the second T becomes String | Number
3. fatcerberus commented on Mar 16, 2022

   

   Bruce Pascoe (fatcerberus)

   on Mar 16, 2022

   Last edited by fatcerberus

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   Yes, because the first T is in an input position (contravariant) and the other is in an output position (covariant). If you have a function and the only thing you know about it is that it accepts either an X or a Y, and will throw if you give it the wrong thing, the only thing you can safely pass to it is X & Y, i.e. a value that satisfies both types. That’s what the error you’re seeing reflects.

   👍React with 👍1Reacted by Martin Johns
4. RoelVB commented on Mar 16, 2022

   

   Roel (RoelVB)

   on Mar 16, 2022

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   Thanks Bruce Pascoe (@fatcerberus), I understand now
5. Roel (RoelVB)

   closed this as completedon Mar 16, 2022
6. microsoft

   locked as resolved and limited conversation to collaborators on Oct 22, 2025