true extends false

[AnyhowStep]

TypeScript Version: 3.2.0-dev.20181117

EDIT: I just tested with TS 3.2.0-rc and it has this bug

Search Terms: true extends false

Code

I feel like I'm losing my mind here. Or maybe I'm tired because it's 3.34am. Maybe I need to take a break. Am I missing something obvious?

//OK
//Expected: "n"
//Actual: "n"
type no = true extends false ? "y" : "n";

type G<DataT extends { b : boolean }> = (
    true extends DataT["b"]?
    ["Actual", "true extends", DataT["b"]] :
    "Expected"
)

//Wat?
//Expected: "Expected"
//Actual: ["Actual", "true extends", false]
type g = G<{ b : false }>;

Expected behavior:

g to be of type "Expected".

Intuitively,

1. DataT is of type { b : false }
2. DataT["b"] is of type false
3. true extends false should be... false

Actual behavior:

g is of type ["Actual", "true extends", false]

Playground Link: Here

---

EDIT: I just tested and this is not a problem in TS 3.0.1

I'm not crazy, phew.

[AnyhowStep]

This has no problems in either TS 3.2 or TS 3.0.1

type G<B extends boolean> = (
    true extends B?
    ["Actual", "true extends", B] :
    "Expected"
)

//OK
//Expected: "Expected"
//Actual: "Expected"
type g = G<false>;

[jack-williams]

I think this is happening because of #27470

In G the constraint of DataT is being pulled down in the access expression DataT["b"]. So it is not that true extends false, rather it is eagerly using the base constraint in DataT so you get true extends boolean. This is not covered by the fix #27490 because "b" is not a generic type. You can see the correct behaviour if you defer the check using a parameter.

type G2<DataT extends { b : boolean }, B extends keyof DataT> = (
    true extends DataT[B]?
    ["Actual", "true extends", DataT[B]] :
    "Expected"
)
// g2: "Expected"
type g2 = G2<{ b: false }, "b">;

or even doing:

type G2<DataT extends { b : boolean }, B extends "b"> = (
    true extends DataT[B]?
    ["Actual", "true extends", DataT[B]] :
    "Expected"
)
// g2: "Expected"
type g2 = G2<{ b: false }, "b">;

[AnyhowStep]

Yikes. I hope this is fixed. This breaks a lot of my code that was working in 3.0.1.

Using your workaround would make the type declarations even more verbose and complex =(

---

Thank you for looking into it! I had this gnawing at me even as I tried to sleep.

I guess I'll hold off on upgrading TS for now :x

[Jessidhia]

This might be related to some problems I found while working on the React types; proving that 'x' extends keyof T is not sufficient to let you index T with 'x'.

[ahejlsberg]

Simplified repro with expected outcomes:

type Foo<T extends { b: boolean }> = true extends T["b"] ? "yes" : "no";

type T0 = Foo<{ b: never }>     // "no"
type T1 = Foo<{ b: false }>;    // "no"
type T2 = Foo<{ b: true }>;     // "yes"
type T3 = Foo<{ b: boolean }>;  // "yes"

Currently Foo is eagerly evaluated to "yes" so every outcome is "yes".

[AnyhowStep]

Thank you for fixing this so quickly!

I just updated from 3.0.1 to 3.3.0-dev.20181129 and nothing broke!