bug: operations failed unless separated

[KiaraGrouwstra]

TypeScript Version: 2.3.1-insiders.20170416 (Playground, strictNullChecks off)

Code

Omit type for #12215:

// helpers...
type Obj<T> = { [k: string]: T };
type SafeObj<O extends { [k: string]: any }, Name extends string, Param extends string> = O & Obj<{[K in Name]: Param }>;
type SwitchObj<Param extends string, Name extends string, O extends Obj<any>> = SafeObj<O, Name, Param>[Param];
type Not<T extends string> = SwitchObj<T, 'InvalidNotParam', {
 '1': '0';
 '0': '1';
}>;
type UnionHas<Union extends string, K extends string> = ({[S in Union]: '1' } & { [k: string]: '0' })[K];
type Obj2Keys<T> = {[K in keyof T]: K } & { [k: string]: undefined };

// data...
type Item1 = { a: string, b: number, c: boolean };

// okay, Omit, let's go.
type Omit_<T extends { [s: string]: any }, K extends keyof T> =
 {[P2 in keyof T]: { 1: Obj2Keys<T>[P2], 0: undefined }[Not<UnionHas<K, P2>>]}
type T1 = Omit_<Item1, "a">;
// intermediary result: { a: undefined; b: "b"; c: "c"; }
type T2 = {[P1 in T1[keyof Item1] ]: Item1[P1]}; // { b: number, c: boolean };
// ^ great, the result we want!
// now let's combine the steps?!
type Omit<T extends { [s: string]: any }, K extends keyof T> =
 {[P1 in {[P2 in keyof T]: { 1: Obj2Keys<T>[P2], 0: undefined }[Not<UnionHas<K, P2>>]}[keyof T] ]: T[P1]};
type T3 = Omit<Item1, "a">;
// ^ yields { a: string, b: number, c: boolean }, not { b: number, c: boolean }
// uh, could we instead do the next step in a separate wrapper type?:
type Omit2<T extends { [s: string]: any }, K extends keyof T> = Omit_<T, K>[keyof T];
// ^ not complete yet, but this is the minimum repro of the part that's going wrong
type T4 = Omit2<Item1, "a">;
// ^ nope, foiled again! 'a'|'b'|'c' instead of 'b'|'c'... wth? 
// fails unless this access step is done after the result is calculated, dunno why

Similar issue encountered in my attempt to calculate union differences to resolve #13470.

Expected behavior:
Operations can be combined so as to make more sophisticated types.

Actual behavior:
The compiler gets confused if the access step directly follows the previous half of the computation.

Edit: it appears #16018 is closer to a minimum repro of this same issue.

[KiaraGrouwstra]

Fixed by nir (@nirendy) at #12215. 😃

[KiaraGrouwstra]

Here's another occurrence that doesn't appears solved with those defaults:

export type Obj<T> = { [k: string]: T };
export type TupleHasIndex<Arr extends any[], I extends number> = ({[K in keyof Arr]: '1' } & Array<'0'>)[I];
// ^ #15768, TS2536 `X cannot be used to index Y` on generic
type ObjectHasKey<O extends {}, K extends string> =
 ({[K in keyof O]: '1' } & Obj<'0'>)[K];
type NumberToString = ['0','1','2','3','4','5','6','7','8','9','10','11','12','13','14','15','16','17','18','19','20','21','22','23','24','25','26','27','28','29','30','31','32','33','34','35','36','37','38','39','40','41','42','43','44','45','46','47','48','49','50','51','52','53','54','55','56','57','58','59','60','61','62','63','64','65','66','67','68','69','70','71','72','73','74','75','76','77','78','79','80','81','82','83','84','85','86','87','88','89','90','91','92','93','94','95','96','97','98','99','100','101','102','103','104','105','106','107','108','109','110','111','112','113','114','115','116','117','118','119','120','121','122','123','124','125','126','127','128','129','130','131','132','133','134','135','136','137','138','139','140','141','142','143','144','145','146','147','148','149','150','151','152','153','154','155','156','157','158','159','160','161','162','163','164','165','166','167','168','169','170','171','172','173','174','175','176','177','178','179','180','181','182','183','184','185','186','187','188','189','190','191','192','193','194','195','196','197','198','199','200','201','202','203','204','205','206','207','208','209','210','211','212','213','214','215','216','217','218','219','220','221','222','223','224','225','226','227','228','229','230','231','232','233','234','235','236','237','238','239','240','241','242','243','244','245','246','247','248','249','250','251','252','253','254','255'];
type Inc = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134, 135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162, 163, 164, 165, 166, 167, 168, 169, 170, 171, 172, 173, 174, 175, 176, 177, 178, 179, 180, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210, 211, 212, 213, 214, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234, 235, 236, 237, 238, 239, 240, 241, 242, 243, 244, 245, 246, 247, 248, 249, 250, 251, 252, 253, 254, 255, 256];
type Overwrite<K, T> = {[P in keyof T | keyof K]: { 1: T[P], 0: K[P] }[ObjectHasKey<T, P>]};

type TupleToObject<R extends any[], I extends number = 0, Acc = {}> =
 { 1: TupleToObject<R, Inc[I], Overwrite<Acc, { [P in NumberToString[I]]: R[I] }>>, 0: Acc }[TupleHasIndex<R, I>];

type TestArr = ['a', 'b', 'c'];
type Arr2ObjTest = TupleToObject<TestArr>; // { 0: "a"; 1: "b"; 2: "c"; }
type keys = keyof Arr2ObjTest // "0" | "1" | "2"

// type TupleIndicesToUnion<R extends any[]> = keyof TupleToObject<R>;
type TupleIndicesToUnion<R extends any[], O extends TupleToObject<R, 0> = TupleToObject<R, 0>> = keyof O;
// ^ split into steps using defaults following @niirendy in #12215, same result
type ArrKeys2UnionTest = TupleIndicesToUnion<TestArr>;
// ^ expected to equal `keys`, but gives `never` (O yield `{}`)

[KiaraGrouwstra]

Worked around this one by typing the last one differently altogether, making the root cause less urgent again. I'll re-close until a new blocker comes in I guess.