Fails for simple type inference

[kgtkr]

TypeScript Version: 3.6.0-dev.20190726

Search Terms:
type inference

Code

declare function f(x: string | undefined): string;
declare function id<T>(x: T): T;

let state: undefined | string = undefined;
while (true) {
  const x = id(f(state));
  state = x;
}

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.

Expected behavior:
x: string

Actual behavior:
'x' implicitly has type 'any' because it does not have a type annotation and is referenced directly or indirectly in its own initializer.

Playground Link:
https://www.typescriptlang.org/play/#code/CYUwxgNghgTiAEAzArgOzAFwJYHtVIAoAPALngGcMYtUBzeAH3jVERpGAEozLq6BuAFChIsBCnTY88LMAA8AFQB8xMgu7wFQwRBAYKGKBhBkWINqg6MDfegF5mqVu2BCA7gAssu+ASrIQTngAb0F4eDA8SngieAdZAkQCSiNAziFwlOM4mKEAX0EgA

Related Issues:

[ahejlsberg]

This is a design limitation in the control flow analyzer. Ideally the analyzer would realize that id(f(state)) always has type string regardless of the type of state. However, the analyzer attempts to resolve the type of the expression, which entails resolving the type of state, which entails resolving the type of x (because of the state = x assignment and the back edge of the loop), which leads to a circularity and therefore an implicit any.