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.

Start with the Playground repro and trace the type-checking paths named in the report: getInferredType, inferReverseMappedType, and getTypeFromInference. Investigate how constraint types are handled when reverse-mapped inference has no candidates; done means the provided setup call infers successfully without the shown constraint error.