Support Actual extends Expected in conjunction with Expected extends Actual

[RebeccaStevens]

The Problem

The type DeepEqualAssertion requires that the Expected type extends the Actual type.
However, sometimes we want this relationship to be the other way around.

Consider the following example:

test("my test", (t) => {
  const expected = { foo: [1, 2, 3] } as const;
  const actual = functionToTest(1, 2, 3);

  t.deepEqual(actual, expected);
});

If the type of actual is inferred as { foo: number[] } then this does not extend the type of expected which is { readonly foo: readonly number[] }. Thus TypeScript reports an error.

Suggested Fix

Add a overload to DeepEqualAssertion to handle this case:

export interface DeepEqualAssertion {
	/**
	 * Assert that `actual` is [deeply equal](https://github.com/concordancejs/concordance#comparison-details) to
	 * `expected`, returning a boolean indicating whether the assertion passed.
	 */
	<Actual, Expected extends Actual>(actual: Actual, expected: Expected, message?: string): actual is Expected;

	/**
	 * Assert that `actual` is [deeply equal](https://github.com/concordancejs/concordance#comparison-details) to
	 * `expected`, returning a boolean indicating whether the assertion passed.
	 */
	<Actual extends Expected, Expected>(actual: Actual, expected: Expected, message?: string): expected is Actual;

	/** Skip this assertion. */
	skip(actual: any, expected: any, message?: string): void;
}

[novemberborn]

I think this would be great. Would there be any compatibility issues with existing code?

[RebeccaStevens]

I don't believe this change would introduce any compatibility issues.

[novemberborn]

@RebeccaStevens do you want to submit a PR for this?

[RebeccaStevens]

@novemberborn #2969