Incorrect error on legitimate type overlap

TypeScript Version: 4.1.0-dev-20201102

Search Terms:

This condition will always return 'false' since the types 'X' and 'Y' have no overlap.

Code

function fn<T>(t: T) {
    return t === '' // error
}

fn('') // but here we pass it

https://www.typescriptlang.org/play?jsx=0#code/GYVwdgxgLglg9mABMMAeAKgPgBRQFyLoCUiA3gFCJWIBOAplCDUlIgLweIDkXiA9H0R0aNODXIBfcuRTYeJAYgDuYgNYBnIA

Expected behavior:

It works when we extend unknown. While this works on simple generics, it becomes inconvenient when the type parameter is nested deeper in a type structure.

function fn<T extends unknown>(t: T) {
    return t === '' // works
}

fn('') // works

https://www.typescriptlang.org/play?jsx=0#code/GYVwdgxgLglg9mABMMAeAKogpgDylsAEwGdFwBrMOAdzAD4AKKALkXQEpEBvAKEX8QAnLFBCCkURAF4ZiAORzEAeiWJqcQeWI8Avjx4oGCzirUatQA

Actual behavior:

This condition will always return 'false' since the types 'T' and 'string' have no overlap.

Related issues
https://github.com/microsoft/TypeScript/issues/27910

Riproduci il confronto generico nel compilatore TypeScript usando il codice e i link a Playground forniti, quindi confrontalo con l’esempio T extends unknown. Analizza la diagnostica della sovrapposizione dei tipi e la issue correlata #27910; il lavoro sarà considerato completato quando il confronto legittimo non segnalerà più che restituirà sempre false, mentre i confronti non validi continueranno a essere diagnosticati.