Annotation of boolean type ignore in favor of narrowed type

[drew-gross]

TypeScript Version: 3.5.1

Search Terms:

Boolean explicit annotation ignore

Code

// Errors as expected
let bothInferred = true;
if (bothInferred == false) { }

// Doesn't error as expected 
let bothExplicit: boolean = true as boolean;
if (bothExplicit == false) { }

// Doesn't error as expected
let leftInferredRightExplicit = true as boolean;
if (leftInferredRightExplicit == false) { }

// Why error? Narrow rhs overrides explicit declaration.
let rigthInferredLeftExplicit: boolean = true;
if (rigthInferredLeftExplicit == false) { }

Expected behavior:

4th example above shouldn't error.

Actual behavior:

Error: This condition will always return 'false' since the types 'true' and 'false' have no overlap.

Playground Link:

https://www.typescriptlang.org/play/#src=%2F%2F%20Errors%20as%20expected%0D%0Alet%20bothInferred%20%3D%20true%3B%0D%0Aif%20(bothInferred%20%3D%3D%20false)%20%7B%20%7D%0D%0A%0D%0A%2F%2F%20Doesn't%20error%20as%20expected%20%0D%0Alet%20bothExplicit%3A%20boolean%20%3D%20true%20as%20boolean%3B%0D%0Aif%20(bothExplicit%20%3D%3D%20false)%20%7B%20%7D%0D%0A%0D%0A%2F%2F%20Doesn't%20error%20as%20expected%0D%0Alet%20leftInferredRightExplicit%20%3D%20true%20as%20boolean%3B%0D%0Aif%20(leftInferredRightExplicit%20%3D%3D%20false)%20%7B%20%7D%0D%0A%0D%0A%2F%2F%20Why%20error%3F%0D%0Alet%20rigthInferredLeftExplicit%3A%20boolean%20%3D%20true%3B%0D%0Aif%20(rigthInferredLeftExplicit%20%3D%3D%20false)%20%7B%20%7D%0D%0A

Related Issues:

Tried similar patterns with other types, could not find an error.

[ajafff]

Union types are narrowed on assignment. boolean is a union type true | false behind the scenes

[drew-gross]

Ah yeah I could reproduce the error with a different union type

type U = "a" | "b";

let u: U = "a";

if (u == "b") {
    console.log();
}

Seems like an explicit annotation should override narrowing though? Explicit seems like it should always win over any type of inference.

[RyanCavanaugh]

Seems like an explicit annotation should override narrowing though?

Intuitively, there should really not be a difference between

let u: U = "a";
/* some statement */

and

let u: U;
u = "a";
/* some statement */

[drew-gross]

Huh, intuitively I think of the things on the left of an assignment as happening after the things on the right. Maybe this is my C++ background coming through (https://godbolt.org/z/VvVzbE). I guess for javascript both ways are equally valid though.

[jcalz]

Cross-linking to #8513, of which this issue is arguably a duplicate

[matt-tingen]

This behavior is unintuitive to me, as well. In particular, with aliased object types, adding a widening type annotation can actually result in narrowing:

type Open = { isOpen: true };
type Closed = { isOpen: false };
type Status = Open | Closed;

const openBare = { isOpen: true };
const openAnnotated: Status = { isOpen: true };

type Bare = typeof openBare; // { isOpen: boolean }
type Annotated = typeof openAnnotated; // { isOpen: true }