Recursive Mapped Conditional Type inferring top-level children to any

[osdiab]

TypeScript Version: 3.1.0-dev.20180721

Search Terms: any recursive mapped conditional inference

Code

    interface MyNumbers {
      cool: {
        mysteriouslySo: number;
        comfortablySo: number;
      };
      scary: {
        ominouslySo: number;
      };
    }
    
    type MapLeaves<OldShape, NewLeaf> = OldShape extends {}
      ? { [k in keyof OldShape]: MapLeaves<OldShape[k], NewLeaf> }
      : NewLeaf;
    
    // This is inferring as { cool: any, scary: any }, not what I expected!
    type MyStrings = MapLeaves<MyNumbers, string>;

Expected behavior:

I'm not sure if I'm misusing recursive mapped conditional types here, but I expected MyStrings to be

    interface MyNumbers {
      cool: {
        mysteriouslySo: string;
        comfortablySo: string;
      };
      scary: {
        ominouslySo: string;
      };
    }

Actual behavior:
It's inferred as

{ cool: any, scary: any }

Playground Link:
Link

[ahejlsberg]

Duplicate of #23897. The types are actually computed correctly, but our type display logic doesn't currently descend into recursive mapped types because they could potentially be infinite.

type MapLeaves<OldShape, NewLeaf> = OldShape extends object
  ? { [k in keyof OldShape]: MapLeaves<OldShape[k], NewLeaf> }
  : NewLeaf;

type ShouldBeString = MapLeaves<MyNumbers, string>["cool"]["comfortablySo"];

Here, ShouldBeString is indeed of type string. By the way, note that I had to change {} to object in your conditional type. The {} type is actually a supertype of every type but null or undefined, whereas object is a supertype of only objects (and not primitive types).

[osdiab]

Ah, interesting. Sounds like displaying it with some kind of ellipsis or placeholder type would be more ideal. I'll post to that issue.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.