Associated types can't be inferred from derived protocol

[xwu (Xiaodi Wu)]

See this thread:

Automatically inferring associatedtype when conforming to a refinement

I'm seeing some weird associatedtype inference behavior when conforming to a refinement of a protocol that has an associatedtype: protocol P1 { associatedtype Input func req(_ input: Input) } protocol P2: P1 { func req2(_ input: Input) } extension P2 { func req(_ input: Input) { req2(input) } } struct S: P2 { func req2(_ input: String) { print(input) } } Gives error: badassoc2.swift:14:8: error: type 'S' does not conform to protocol 'P2' struct S: P2 { ^ badassoc2.swift:15…