Exact does not work as expected with type involving Date and union

[hgossler]

In the example below, the cases t7 and t8 cause a TS error.

import type { Exact } from "type-fest";

interface A {
	a: string;
	b?: Date | null;
}

function testArg<T extends Exact<A, T>>(a: T) {
	return a;
}

const t1 = { a: "a" };
testArg(t1); // OK - Type of arg correct, no TS error

const t2 = { a: "a", b: new Date() };
testArg(t2); // OK - Type of arg correct, no TS error

const t3 = { a: "a", b: null };
testArg(t3); // OK - Type of arg correct, no TS error

const t4 = { a: "a", b: undefined };
testArg(t4); // OK - Type of arg correct, no TS error

const t5 = { a: "a", b: new Date() as Date | null | undefined };
testArg(t5); // OK - Type of arg correct, no TS error

const t6 = { a: "a", b: null as null | undefined };
testArg(t6); // OK - Type of arg correct, no TS error

const t7 = { a: "a", b: new Date() as Date | undefined };
testArg(t7); // Bug - Type of arg correct, but TS error

const t8 = { a: "a", b: new Date() as Date | null };
testArg(t8); // Bug - Type of arg correct, but TS error

Error at testArg(t7):

error TS2345: Argument of type '{ a: string; b: Date | undefined; }' is not assignable to parameter of type 'ExactObject<A, { a: string; b: Date | undefined; }>'.
  Type '{ a: string; b: Date | undefined; }' is not assignable to type '{ a: string; b?: ExactObject<Date, Date | undefined> | null | undefined; }'.
    Types of property 'b' are incompatible.
      Type 'Date | undefined' is not assignable to type 'ExactObject<Date, Date | undefined> | null | undefined'.
        Type 'Date' is not assignable to type 'ExactObject<Date, Date | undefined>'.
          Type 'Date' is not assignable to type '{ toString: ExactObject<() => string, never>; toDateString: ExactObject<() => string, never>; toTimeString: ExactObject<() => string, never>; ... 41 more ...; [Symbol.toPrimitive]: ExactObject<...>; }'.
            Types of property 'toString' are incompatible.
              Type '() => string' is not assignable to type 'ExactObject<() => string, never>'.
                Type '() => string' is not assignable to type 'Record<string | number | symbol, never>'.
                  Index signature for type 'string' is missing in type '() => string'.

Error at testArg(t8):

error TS2345: Argument of type '{ a: string; b: Date | null; }' is not assignable to parameter of type 'ExactObject<A, { a: string; b: Date | null; }>'.
  Type '{ a: string; b: Date | null; }' is not assignable to type '{ a: string; b?: ExactObject<Date, Date | null> | null | undefined; }'.
    Types of property 'b' are incompatible.
      Type 'Date | null' is not assignable to type 'ExactObject<Date, Date | null> | null | undefined'.
        Type 'Date' is not assignable to type 'ExactObject<Date, Date | null>'.
          Type 'Date' is not assignable to type '{ toString: ExactObject<() => string, never>; toDateString: ExactObject<() => string, never>; toTimeString: ExactObject<() => string, never>; ... 41 more ...; [Symbol.toPrimitive]: ExactObject<...>; }'.
            Types of property 'toString' are incompatible.
              Type '() => string' is not assignable to type 'ExactObject<() => string, never>'.

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[sindresorhus]

// @zorji

[zorji]

I originally created this Exact type for myself to check API input to avoid excess fields (i.e. the OpenAI spec additionalProperties).

It looks like the implementation is not quite compatible with class with methods etc.

I add a fix here #902

The solution simply only apply Exact on JsonObject, assuming it ignores all types from a TypeScript class.

@hgossler

In additional to your test cases, I think the following should fail

const t4 = { a: "a", b: undefined };
const t5 = { a: "a", b: new Date() as Date | null | undefined };
const t6 = { a: "a", b: null as null | undefined };
const t7 = { a: "a", b: new Date() as Date | undefined };

Allowing optional field does not mean it should accept undefined. It's passing in your side probably because your setting did not enable strict null check https://www.typescriptlang.org/tsconfig/strictNullChecks.html

In the fix, the above case will still be rejected.