[Sema] Fix compiler error when extending a typealias of a partially specialized generic type

[xavgru12]

Resolves #68212.

[AnthonyLatsis]

@swift-ci please smoke test Linux

[saehejkang]

i was curious to see how this was solved and guess I was overcomplicating the issue 😭

[AnthonyLatsis]

This is far from solved, FWIW 🙂. I only triggered the tests to encourage the author to iterate on failures before any feedback arrives.

[xedin]

Is it possible to make this PR as a draft?

[xavgru12]

I was converting to draft but Anthony was faster

[AnthonyLatsis]

For posterity, the convert to draft button is at the bottom of the Reviewers section.

[xavgru12]

@AnthonyLatsis
I changed the code according review and added test cases. Let me know what you think about this.

[xavgru12]

@AnthonyLatsis reminder

[xavgru12]

You once said a ping every two weeks is fine. So here I am pinging you @AnthonyLatsis

[AnthonyLatsis]

Thank you for the reminders and the patience. Another somewhat random test case I think worth having:

struct S1<T> {
  struct Inner<U> {}
}

struct S2<T> {
  typealias A1<U> = S1<T>.Inner<U>
  typealias A2<U> = S1<T>.Inner<U> where T == Int
}

extension S2<Int>.A1 {
  func foo1() {
    let int: Int
    let _: T = int // OK
  }
}

extension S2.A2 {
  func foo2() {
    let int: Int
    let _: T = int // OK
  }
}

[AnthonyLatsis]

If we remove isInferredType, this can be

Suggested change

	if (!typealiasBoundGenericType->hasTypeParameter()) {
	isInferredType = true;
	} else {
	isInferredType = false;
	break;
	}
	if (typealiasBoundGenericType->hasTypeParameter()) {
	return false;
	}

[xavgru12]

I removed isInferredType. I dislike the new solution because I have to write return true at the end of the function and I would like to have return false as default.

[AnthonyLatsis]

We need to perform this check regardless of whether the original number of generic params matched.

[xavgru12]

Confused here, added a comment above about this

[AnthonyLatsis]

I like how this appears to be unsafe but is actually safe because we are trimming a non-owning view into the array. But this is probably better:

Suggested change

	for (const auto &type : nominalGenericParams) {
	if (type->getDepth() == maxDepth) {
	nominalGenericParams = nominalGenericParams.drop_back();
	}
	}
	while (!nominalGenericParams.empty() &&
	nominalGenericParams.back().getDepth() == maxDepth) {
	nominalGenericParams = nominalGenericParams.drop_back();
	}

Test case for why we need !nominalGenericParams.empty():

struct S1<T> {
  struct Inner<U> {}
}
struct S2 {
  typealias A<T> = S1<T>.Inner<Int>
}
extension S2.A {
  func test() {
    let int: Int
    let _: U = int // error
  }
}

[xavgru12]

I add the test case and changed it to while.
The test does not produce an error.

So the nested struct parameter becomes Int. Then it checks for U which is Int now. Why is it supposed to produce an error?

[xavgru12]

@AnthonyLatsis
I iterated over it and added some comments unresolving threads. Please review again and I hope you are able to keep the overview over this, so let me know if something is unclear.

[xavgru12]

@AnthonyLatsis reminder