[SR-4685] Can't infer associatedtypes with where clause

[rintaro]

Previous ID	SR-4685
Radar	rdar://problem/39040799
Original Reporter	@rintaro
Type	Bug

Environment

Swift version 3.1-dev (LLVM 1067090d70, Clang 5e8cb0e694, Swift f83d1a7)
Target: x86_64-unknown-linux-gnu

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, TypeChecker
Assignee	@DougGregor
Priority	Medium

md5: 3fae0cb45075895bc8535a198c649e47

is duplicated by:

• [SR-7304] typealias overriding associated type ... is better expressed as same-type constraint #49852

Issue Description:

The current master doesn't compile this.

protocol MySequence {
  associatedtype Element
  associatedtype Iterator: IteratorProtocol where Iterator.Element == Element
  func makeIterator() -> Iterator
}

struct Foo {}

struct FooIterator : IteratorProtocol {
  func next() -> Foo? { return Foo() }
}

struct FooSequence : MySequence {
  func makeIterator() -> FooIterator {
    return FooIterator()
  }
}

results:

test.swift:13:8: error: type 'FooSequence' does not conform to protocol 'MySequence'
struct FooSequence : MySequence {
       ^
test.swift:2:18: note: protocol requires nested type 'Element'; do you want to add it?
  associatedtype Element
                 ^
test.swift:3:18: note: protocol requires nested type 'Iterator'; do you want to add it?
  associatedtype Iterator: IteratorProtocol where Iterator.Element == Element
                 ^

This is basically the same structure as #8939.

[rintaro]

@DougGregor Could you take a look?
When tested #8939 locally, this compiles:

struct Foo {}

struct FooIterator : IteratorProtocol {
  func next() -> Foo? { return Foo() }
}

struct FooSequence : Sequence {
  func makeIterator() -> FooIterator {
    return FooIterator()
  }
}

print(FooSequence.Element.self) // "Foo"

I don't understand what the difference is 🙁

[DougGregor]

Inference isn't taking those same-type constraints into account properly. It's "on the list" to fix.

[slavapestov]

This was fixed by #70375.