[Nathan Shively-Sanders (sandersn)]

This means that, for example, the tuple [number, string?] allows its second element to be narrowed with element access:

export function f(pair: [number, string?]): string {
  return pair[1] ? pair[1] : 'nope';
}

Or you can narrow on properties that don't work with property access:

interface Square {
    ["dash-ok"]: "square";
    ["square-size"]: number;
}
 interface Rectangle {
    ["dash-ok"]: "rectangle";
    width: number;
    height: number;
}
 interface Circle {
    ["dash-ok"]: "circle";
    radius: number;
}
 type Shape = Square | Rectangle | Circle;
function area(s: Shape): number {
    switch(s['dash-ok']) {
        case "square": return s['square-size'] * s['square-size'];
        case "rectangle": return s.width * s['height'];
        case "circle": return Math.PI * s['radius'] * s.radius;
    }
}

This only works for actual literal references, not expressions with literal types, to prevent the binder from making the control flow graph much bigger than it is currently. In other words, this doesn't work:

export function f(pair: [number, string?]): string {
  const one = 1
  return pair[one] ? pair[one] : 'nope';
}

Performance is not significantly different from today, even on the Compiler, Unions Edition, which is full of very large unions:

┌─────────────┬─────────────────────┬─────────────────────┬───────────────────┬──────────┬──────────┐
│ Project     │            Baseline │             Current │             Delta │     Best │    Worst │
╞═════════════╧═════════════════════╧═════════════════════╧═══════════════════╧══════════╧══════════╡
│ Angular - node (v10.7.0, x64)                                                                     │
├─────────────┬─────────────────────┬─────────────────────┬───────────────────┬──────────┬──────────┤
│ Memory used │ 366,503k (±  0.01%) │ 366,586k (±  0.01%) │   +82k (+  0.02%) │ 366,421k │ 366,712k │
│ Parse Time  │    1.46s (±  0.97%) │    1.47s (±  0.97%) │ +0.01s (+  0.48%) │    1.43s │    1.56s │
│ Bind Time   │    0.66s (±  0.86%) │    0.67s (±  1.05%) │ +0.01s (+  1.29%) │    0.65s │    0.69s │
│ Check Time  │    3.44s (±  0.48%) │    3.47s (±  0.60%) │ +0.02s (+  0.71%) │    3.41s │    3.60s │
│ Emit Time   │    4.85s (±  0.59%) │    4.88s (±  0.52%) │ +0.03s (+  0.55%) │    4.82s │    5.05s │
├─────────────┼─────────────────────┼─────────────────────┼───────────────────┼──────────┼──────────┤
│ Total Time  │   10.41s (±  0.29%) │   10.47s (±  0.35%) │ +0.06s (+  0.62%) │   10.37s │   10.65s │
╞═════════════╧═════════════════════╧═════════════════════╧═══════════════════╧══════════╧══════════╡
│ Compiler - node (v10.7.0, x64)                                                                    │
├─────────────┬─────────────────────┬─────────────────────┬───────────────────┬──────────┬──────────┤
│ Memory used │ 198,369k (±  0.33%) │ 197,846k (±  0.22%) │  -523k (-  0.26%) │ 197,377k │ 200,636k │
│ Parse Time  │    0.61s (±  1.05%) │    0.61s (±  1.76%) │ +0.01s (+  0.91%) │    0.59s │    0.69s │
│ Bind Time   │    0.67s (±  1.26%) │    0.67s (±  1.29%) │ +0.00s (+  0.15%) │    0.65s │    0.71s │
│ Check Time  │    2.88s (±  0.61%) │    2.89s (±  0.95%) │ +0.01s (+  0.33%) │    2.82s │    3.00s │
│ Emit Time   │    2.00s (±  0.69%) │    2.01s (±  0.74%) │ +0.00s (+  0.07%) │    1.97s │    2.09s │
├─────────────┼─────────────────────┼─────────────────────┼───────────────────┼──────────┼──────────┤
│ Total Time  │    6.15s (±  0.38%) │    6.17s (±  0.56%) │ +0.02s (+  0.29%) │    6.07s │    6.33s │
╞═════════════╧═════════════════════╧═════════════════════╧═══════════════════╧══════════╧══════════╡
│ Compiler - Unions - node (v10.7.0, x64)                                                           │
├─────────────┬─────────────────────┬─────────────────────┬───────────────────┬──────────┬──────────┤
│ Memory used │ 219,013k (±  0.27%) │ 218,586k (±  0.13%) │  -427k (-  0.20%) │ 218,148k │ 221,204k │
│ Parse Time  │    0.56s (±  1.42%) │    0.56s (±  1.19%) │ -0.00s (-  0.18%) │    0.54s │    0.59s │
│ Bind Time   │    0.62s (±  1.19%) │    0.62s (±  1.09%) │ +0.00s (+  0.65%) │    0.60s │    0.65s │
│ Check Time  │    9.26s (±  0.46%) │    9.27s (±  0.43%) │ +0.01s (+  0.12%) │    9.09s │    9.42s │
│ Emit Time   │    1.92s (±  0.73%) │    1.90s (±  0.91%) │ -0.02s (-  0.81%) │    1.85s │    1.98s │
├─────────────┼─────────────────────┼─────────────────────┼───────────────────┼──────────┼──────────┤
│ Total Time  │   12.36s (±  0.43%) │   12.36s (±  0.34%) │ -0.00s (-  0.00%) │   12.22s │   12.54s │
╞═════════════╧═════════════════════╧═════════════════════╧═══════════════════╧══════════╧══════════╡
│ Monaco - node (v10.7.0, x64)                                                                      │
├─────────────┬─────────────────────┬─────────────────────┬───────────────────┬──────────┬──────────┤
│ Memory used │ 370,896k (±  0.01%) │ 370,943k (±  0.01%) │   +48k (+  0.01%) │ 370,735k │ 371,128k │
│ Parse Time  │    1.18s (±  1.07%) │    1.18s (±  1.30%) │ +0.01s (+  0.47%) │    1.12s │    1.24s │
│ Bind Time   │    0.60s (±  1.26%) │    0.61s (±  0.93%) │ +0.01s (+  1.00%) │    0.59s │    0.63s │
│ Check Time  │    3.45s (±  0.71%) │    3.47s (±  0.65%) │ +0.02s (+  0.61%) │    3.41s │    3.56s │
│ Emit Time   │    2.43s (±  0.68%) │    2.44s (±  0.55%) │ +0.01s (+  0.39%) │    2.40s │    2.50s │
├─────────────┼─────────────────────┼─────────────────────┼───────────────────┼──────────┼──────────┤
│ Total Time  │    7.65s (±  0.50%) │    7.70s (±  0.35%) │ +0.04s (+  0.59%) │    7.61s │    7.82s │
╞═════════════╧═════════════════════╧═════════════════════╧═══════════════════╧══════════╧══════════╡
│ TFS - node (v10.7.0, x64)                                                                         │
├─────────────┬─────────────────────┬─────────────────────┬───────────────────┬──────────┬──────────┤
│ Memory used │ 328,758k (±  0.01%) │ 328,830k (±  0.02%) │   +72k (+  0.02%) │ 328,637k │ 328,995k │
│ Parse Time  │    0.88s (±  1.52%) │    0.87s (±  1.05%) │ -0.01s (-  0.68%) │    0.85s │    0.92s │
│ Bind Time   │    0.54s (±  1.69%) │    0.54s (±  0.86%) │ -0.00s (-  0.65%) │    0.52s │    0.55s │
│ Check Time  │    3.46s (±  0.66%) │    3.46s (±  0.82%) │ +0.01s (+  0.16%) │    3.39s │    3.64s │
│ Emit Time   │    2.56s (±  0.69%) │    2.57s (±  0.80%) │ +0.01s (+  0.35%) │    2.49s │    2.65s │
├─────────────┼─────────────────────┼─────────────────────┼───────────────────┼──────────┼──────────┤
│ Total Time  │    7.44s (±  0.51%) │    7.44s (±  0.55%) │ +0.01s (+  0.09%) │    7.34s │    7.66s │
╞═════════════╧═════════════════════╧═════════════════════╧═══════════════════╧══════════╧══════════╡
│ skype4life - node (v10.7.0, x64)                                                                  │
├─────────────┬─────────────────────┬─────────────────────┬───────────────────┬──────────┬──────────┤
│ Memory used │ 821,863k (±  0.00%) │ 822,017k (±  0.00%) │  +154k (+  0.02%) │ 821,873k │ 822,173k │
│ Parse Time  │    3.48s (±  1.03%) │    3.46s (±  0.68%) │ -0.02s (-  0.56%) │    3.38s │    3.58s │
│ Bind Time   │    1.37s (±  1.76%) │    1.37s (±  1.25%) │ -0.00s (-  0.07%) │    1.32s │    1.46s │
│ Check Time  │    9.76s (±  0.33%) │    9.79s (±  0.45%) │ +0.03s (+  0.32%) │    9.66s │   10.02s │
│ Emit Time   │    7.13s (±  0.58%) │    7.18s (±  0.59%) │ +0.05s (+  0.76%) │    7.05s │    7.39s │
├─────────────┼─────────────────────┼─────────────────────┼───────────────────┼──────────┼──────────┤
│ Total Time  │   21.74s (±  0.29%) │   21.80s (±  0.32%) │ +0.06s (+  0.29%) │   21.54s │   22.18s │
└─────────────┴─────────────────────┴─────────────────────┴───────────────────┴──────────┴──────────┘

Finally, unions with index signatures behave slightly differently:

type Urg = { 0: string } | { [n: number]: number }
var u: Urg = { 0: 1 }
const n: number = u[0]
u[0] = 'no'

Which is visible in one baseline change. I'll check the user tests and Definitely Typed, then report back.

Fixes #10530 (and probably a few others that are still open; there are tons of duplicates of this one.)

[Nathan Shively-Sanders (sandersn)]

Webpack breaks with this pattern:

// @Filename: declarations.d.ts
declare function mappy(map: { [s: string]: number }): void;
// @Filename: Compilation.js
export class C {
    constructor() {
        /** @type {{ [assetName: string]: number}} */
        this.assets = {};
    }
    m() {
        // Error: '{}' is not assignable to '{ [assetName: string]: number }'
        mappy(this.assets)
    }
}

Note that this is because of our this-property assignment inference; the equivalent Typescript code doesn't treat the initializer of a class' property declaration as a narrowing assignment.

Edit: Never mind, this is not narrowing, and it repros on master. I'll fix it separately.

[Nathan Shively-Sanders (sandersn)]

No breaks from Definitely Typed either, so I'm pretty confident that the Breaking Change label on this PR is only theoretical code that we happen to have in our test suite.

[ulrichb]

Nathan Shively-Sanders (@sandersn) Symbols are still not supported. => Should #23135 be reopened?