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Conditional type simplified too early #46761
Description
Bug Report
🔎 Search Terms
conditional tuple extend simplify type parameter constraint
🕗 Version & Regression Information
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about conditional types
⏯ Playground Link
Playground link with relevant code
💻 Code
type Bit = 0 | 1;
type And<A extends Bit, B extends Bit> = [A, B] extends [1, 1] ? 1 : 0;
type Test<A extends Bit, B extends Bit> = And<A extends 1 ? 0 : 1, B extends 1 ? 0 : 1>;
// ^? type Test<I1 extends Bit, I2 extends Bit> = 1
type a = Test<1, 1>; // 1, should be 0🙁 Actual behavior
Test<A, B> is being incorrectly simplified to 1 before being called, and Test<1, 1> is resolving incorrectly to 1.
🙂 Expected behavior
Test<A, B> should not be simplified, and Test<1, 1> should resolve to 0.
Activity
andrewbranch commentedon Nov 10, 2021
Your inner conditional types being fed to
Andare distributive. Making them not distributive fixes it. Still, it feels odd to me. Wesley Wigham (@weswigham) is this expected behavior?webstrand commentedon Nov 10, 2021
Some more weird behavior: If I extract the conditional out into an alias,
Test1<A,B>still gets simplified to1, but if I add another branch to that conditional,Test2<A,B>is not simplified: playground The latter should still be distributive, right?whzx5byb commentedon Nov 10, 2021
If you make
Andlike this,Test<A, B>will resolve to0. That's weird.