Type inference of function return type has failed with overloads and tagged union types

[falsandtru]

This problem is caused by the order of overload.

TypeScript Version: master

Code

declare function f<P, R, S>(process: Supervisor.Process<P, R, S>): void;
declare function f<P, R, S>(process: Supervisor.Process.Call<P, R, S>): void;
namespace Supervisor {
  export type Process<P, R, S> = {
    readonly init: Process.Init<S>;
    readonly call: Process.Call<P, R, S>;
    readonly exit: Process.Exit<S>;
  };
  export namespace Process {
    export type Init<S> = (state: S) => S;
    export type Call<P, R, S> = (param: P, state: S) => [R, S] | PromiseLike<[R, S]>;
    export type Exit<S> = (state: S, reason: any) => void;
  }
}
f<void, void, void>(() => [undefined, undefined]);

Expected behavior:

ok

Actual behavior:

$ node built/local/tsc.js -t es6 index.ts
index.ts(15,21): error TS2345: Argument of type '() => any[]' is not assignable to parameter of type 'Call<void, void, void>'.
  Type 'any[]' is not assignable to type '[void, void] | PromiseLike<[void, void]>'.
    Type 'any[]' is not assignable to type 'PromiseLike<[void, void]>'.
      Property 'then' is missing in type 'any[]'.

If you swap the order of the overload, this error will be resolved.

declare function f<P, R, S>(process: Supervisor.Process.Call<P, R, S>): void;
declare function f<P, R, S>(process: Supervisor.Process<P, R, S>): void;

[RyanCavanaugh]

Overload order does matter