Weird behavior with generics in generic tuple types

[maxludovicohofer]

TypeScript Version: 4.1.2

Search Terms:
Tuple generics
Generics in function tuple types

Code

type Act<S extends object> = {
  act<I extends ((shape: S) => object)[]>(
    ...intentions: I
  ): Act<S> & S & UnionToIntersection<ReturnType<I[number]>>;
};

Expected behavior:
Once S is provided, The 'act' function should automatically infer the specific type of S, which is constant for every element of the tuple type (I), and make the 'shape' parameter typesafe.
This behavior works correctly when:

// Without tuple type
type Act<S extends object> = {
  act<I extends object>(
    ...intentions: ((shape: S) => I)[]
  ): Act<S> & S & I;
};

Actual behavior:
S is inferred as object, instead of its specific type. The return type is inferred correctly.

Playground Link: https://www.typescriptlang.org/play?#code/C4TwDgpgBAqgdgSwPZwCpIJJ2BATgZwgGNhk4AeGAPigF4oAKGKCADxzgBN8oBDOEFAD8jMAC5YASjo0AbkgScoEuBFl5pbDt0YAoKFHFQEcAGZ4oGXdNpyFnfcMuOVavAG5Hu0JCgBBEnIAZRZ2CC4eJAAjACtiYBp6AG9HXkCMUO0eBgZ8AAteSAkgmxpouJJJAG0AXSoGRwMAOhaTDlIUfAkrA0kJAOBgmgAyKBDR+DJ0LBwCeLJyACUIYABXXDRwCHIMKrhVgFsovDqqTwBfdyA

Related Issues:

[RyanCavanaugh]

We don't support anything that involves UnionToIntersection; this is an ill-defined and will frequently produce "surprising" results.

I don't understand what you mean by "S is inferred as object" - it's just a type parameter. It sounds like you're referring to some specific instantiation of Act but there's just declarations in the repro?