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Unions and intersections of type predicates produce wrong type

See original GitHub issue

TypeScript Version: 2.5.0-dev.20170803


type Foo = typeof ts.isDoStatement | typeof ts.isWhileStatement;

Expected behavior: Resulting type should be (node: ts.Node) => node is (ts.DoStatement | ts.WhileStatement)

Actual behavior: Resulting type is (node: ts.Node) => node is ts.DoStatement

It seems to just take the first one.

Issue Analytics

  • State:open
  • Created 6 years ago
  • Reactions:4
  • Comments:9 (6 by maintainers)

github_iconTop GitHub Comments

SamPrudencommented, Aug 12, 2017

type predicate

I knew using “type guards” like that wasn’t quite right. 😂

aluanhaddadcommented, Aug 12, 2017

It appears to just grab the first type predicate and ignore the rest.

Here is a self-contained repro (2.5.0-dev.20170808)

type IsStringOrNumber = ((x: any) => x is number) | ((x: any) => x is string);

declare const x: any;

if ((((x: any) => true) as IsStringOrNumber)(x)) {

Intersecting type predicates seems to have the same result, only the first signature is considered.

type IsStringAndNumber = ((x: any) => x is number) & ((x: any) => x is string);
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