Reverse mapped types don't use their constraint types when no candidates are present unlike the regular type parameters

🔎 Search Terms

reverse mapped constraint inference candidates

🕗 Version & Regression Information

• This is the behavior in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?ts=5.3.0-dev.20231028#code/JYOwLgpgTgZghgYwgAgApynAthSVgBeEAJgPIBGAVhAmMgN4BQyyYAngA4QBcyAzmHwgA5gG5myDhmx8A-LwBKNAPZRiAHgFDhAGmQBXEAGsQygO4gAfOIC+jRsRoAbDChiHawZSH659HdQkAFQBhb0gADzAdYIBxfQxiPmQIKIgQJOQlBFUNLVBdNGkcPEISCmpaAG0AIilMLD4agF1kAB8DDIgYUBJLGMsACgB9XiYWdi4+MYkWHPBUsF5Q8MXxFht15GEEtWmGWeQqgGlkUGQjCDZlGGQg+MS+Zt5B+cilu7CFqL16mWWHnsTs0AJTIAC8lmQADdlMBiFtNowbCCAbskuJGPMBMgoBA+PonHRwb4wP5BuNWJx8TMWHNVlFaXT6YYPgBGAAMHJizJsPOQfIkO0eTLmAAsaEYAGLKZQvYZ6ejIGCygVjZWy3j5EQCsGQjXKCHgkk1FXKGr8lhwJxmOBsPhBKD6HjIEZ6qGCZ38wUozF4glE8QAeiDyAAerJ7IwgA

💻 Code

interface ParameterizedObject {
  type: string;
  params?: Record<string, unknown>;
}

declare function setup<
  TContext,
  TGuards extends Record<string, ParameterizedObject["params"] | undefined>,
>(_: {
  types: {
    context: TContext;
  };
  guards: {
    [K in keyof TGuards]: (context: TContext, params: TGuards[K]) => void;
  };
}): TGuards;

const result = setup({
  types: {
    context: {
      count: 100,
    },
  },
  guards: {
    checkFoo: (_, { foo }: { foo: string }) => foo === "foo",
    alwaysTrue: (_) => true,
  },
});

result;
// ^?

🙁 Actual behavior

Type '(_: { count: number; }, { foo }: { foo: string; }) => boolean' is not assignable to type '(context: { count: number; }, params: Record<string, unknown> | undefined) => void'.
  Types of parameters '__1' and 'params' are incompatible.
    Type 'Record<string, unknown> | undefined' is not assignable to type '{ foo: string; }'.
      Type 'undefined' is not assignable to type '{ foo: string; }'.(2322)

🙂 Expected behavior

I'd expect the inference to succeed here

Additional information about the issue

TGuards is inferred as { checkFoo: { foo: string; }; alwaysTrue: unknown; } and thus rejected by the constraint check in getInferredType. alwaysTrue is inferred as unknown because inferReverseMappedType has such a return:

return getTypeFromInference(inference) || unknownType

And getTypeFromInference doesn't attempt to read the type parameter's constraint at all. It simply handles .candidates and .contraCandidates and that's it.

Beginne mit der Playground-Reproduktion und verfolge die im Bericht genannten Pfade der Typprüfung: getInferredType, inferReverseMappedType und getTypeFromInference. Untersuche, wie Constraint-Typen behandelt werden, wenn die Reverse-Mapped-Inferenz keine Kandidaten hat; die Aufgabe ist abgeschlossen, wenn der bereitgestellte Setup-Aufruf erfolgreich und ohne den gezeigten Constraint-Fehler inferiert wird.