Control flow analysis inference of a generic mapped type

[JasonKleban]

When upgrading to 3.1.3 I get a new error not in 3.0.3:

I see two problems in the type checking of the sample below.

1. before const lo: number = min!;, the return after null test of either identifier means that both will be non-null.
2. Even so, I'm asserting non-null and still it thinks it could be null. The subsequent acceptance of lo2 which includes null as an option suggests that it isn't a problem with the T["min"] and number half.

(There is no error without the generic aspect of this sample.)

interface RangeSelectorStateBase {
    min: number | null;
    max: number | null;
}

type RangeSelectorState<T> = {
    [P in keyof (T & RangeSelectorStateBase)]: (T & RangeSelectorStateBase)[P];
}

function foo<T>(state : RangeSelectorState<T>) {
    const { min, max } = state;

    if (min === null || max === null) {
        return;
    }

    const lo: number = min!; // Type 'T["min"] & null' is not assignable to type 'number'.
    const hi: number = max!; // Type 'T["min"] & null' is not assignable to type 'number'.

    const lo2: number | null = min; // type-checks fine.
    const hi2: number | null = max; // type-checks fine.
}

playground link

Full error text:

Type 'NonNullable<(T & RangeSelectorStateBase)["min"]>' is not assignable to type 'number'.
  Type '(T["min"] & null) | (T["min"] & number)' is not assignable to type 'number'.
    Type 'T["min"] & null' is not assignable to type 'number'.
      Type 'NonNullable<T["min"] & null> | NonNullable<T["min"] & number>' is not assignable to type 'number'.
        Type 'NonNullable<T["min"] & null>' is not assignable to type 'number'.
          Type 'T["min"] & null' is not assignable to type 'number'.

[DanielRosenwasser]

Seems potentially related to the recent indexed access type changes.

[ahejlsberg]

This looks to be the same underlying issue as #22137. Assigning to @weswigham.

[RyanCavanaugh]

Tracking with #22137