Nested classes or structs cannot use protocols in methods

class Test {
    func test() {
        struct NestedTest: Comparable {
            static func < (lhs: NestedTest, rhs: NestedTest) -> Bool {
                return false;
            static func == (lhs: NestedTest, rhs: NestedTest) -> Bool {
                return false;

error: Type 'NestedTest' does not conform to protocol 'Comparable'

Is it a bug or does not support this?

This is because the Comparable protocol has a Self constraint. I'm not sure if it is a bug.

if you add this

print("\(String(reflecting: NestedTest.self))") 

to your test method you will get something like this

__lldb_expr_1.(unknown context at 0x1005403bc).NestedTest 

So there is definitely an issue when swift tries to resolve the type. I'm just not sure if it is intended or not.

1 Like

The fourth time this is asked about this week; apparently a recently introduced issue. Here's the SR-8696


Thank you for your answer. I want to try the sample you provided, but can't compile?How to get the output you gave?

why wasn’t this caught in the source compatibility suite?

Types nested in functions that explicitly conform to protocols with an operator function requirement is quite the specific case, I'm not surprised there isn't a compiler test for that. Regarding the source compatibility suite, it could be that the changes in the PR that introduced the bug didn't require testing source compatibility. But I can't be sure there is an appropriate test in the source compatibility suite either; that might also be the case, assuming there is high probability that the source compatibility tests were run at least once after introducing the bug.

I'm pretty sure there's no test for it because it has never worked. I'm not sure why it's getting so much attention right now.