Associated Type inference regression with default types

[adam-fowler]

Description

The following code, with a protocol that specialises another protocol with three associated types by providing a default type for the second associated type in the case the first associated type is a certain value compiles when using Swift 5.10. The nightly from 1st March fails to infer the types and cannot compile the code.

Reproduction

protocol PB<A> {
    associatedtype A
}

protocol PC<B> {
    associatedtype B: PB
}

protocol PD {
    associatedtype A
    associatedtype B: PB<A>
    associatedtype C: PC<B>

    func test() -> C
}

protocol PDD: PD where A == Int {
    associatedtype B = SB<Int>
}

struct SB<A>: PB {}

struct SC<B: PB>: PC {}

struct SDD: PDD {
    func test() -> some PC<B> {
        return SC<B>()
    }
}

In Swift 6 produces the following errors

/tmp/vscode-swift/Untitled-15bef939ba85f8.swift:26:28: error: reference to invalid associated type 'B' of type 'SDD'
24 │ 
25 │ struct SDD: PDD {
26 │     func test() -> some PC<B> {
   │                            ╰─ error: reference to invalid associated type 'B' of type 'SDD'
27 │         return SC<B>()
28 │     }

/tmp/vscode-swift/Untitled-15bef939ba85f8.swift:26:20: error: 'some' types are only permitted in properties, subscripts, and functions
24 │ 
25 │ struct SDD: PDD {
26 │     func test() -> some PC<B> {
   │                    ╰─ error: 'some' types are only permitted in properties, subscripts, and functions
27 │         return SC<B>()
28 │     }

/tmp/vscode-swift/Untitled-15bef939ba85f8.swift:25:8: error: type 'SDD' does not conform to protocol 'PDD'
23 │ struct SC<B: PB>: PC {}
24 │ 
25 │ struct SDD: PDD {
   │        ╰─ error: type 'SDD' does not conform to protocol 'PDD'
26 │     func test() -> some PC<B> {
27 │         return SC<B>()

/tmp/vscode-swift/Untitled-15bef939ba85f8.swift:25:8: error: type 'SDD' does not conform to protocol 'PD'
 8 │ 
 9 │ protocol PD {
10 │     associatedtype A
   │                    ╰─ note: protocol requires nested type 'A'; add nested type 'A' for conformance
11 │     associatedtype B: PB<A>
12 │     associatedtype C: PC<B>
   │                    ╰─ note: protocol requires nested type 'C'; add nested type 'C' for conformance
13 │ 
14 │     func test() -> C
   ┆
16 │ 
17 │ protocol PDD: PD where A == Int {
18 │     associatedtype B = SB<Int>
   │                    ╰─ note: protocol requires nested type 'B'; add nested type 'B' for conformance
19 │ }
20 │ 
   ┆
23 │ struct SC<B: PB>: PC {}
24 │ 
25 │ struct SDD: PDD {
   │        ╰─ error: type 'SDD' does not conform to protocol 'PD'
26 │     func test() -> some PC<B> {
27 │         return SC<B>()

Expected behavior

Code compiles

Environment

Swift version 6.0-dev (LLVM aa5a99efe7c5734, Swift 2f6e9b3)
Target: x86_64-unknown-linux-gnu

Additional information

No response