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