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

Playground の再現例から始め、レポートで示されている型チェックの経路を追跡します: getInferredType、inferReverseMappedType、getTypeFromInference。reverse-mapped 推論に候補がない場合に制約型がどのように処理されるかを調査します。完了の条件は、提示された setup call が、示されている制約エラーなしで正常に推論されることです。

🔎 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.