Incorrect optional inference on trunk

[Steelskin]

Description

Code that used to compile with older compiler versions fails to compile with a compiler built from trunk. This seems to be due to some incorrect type inferrence.

Reproduction

extension AsyncStream {
    public static var finished: Self {
        Self { $0.finish() }
    }
}

public class Sthg {
    public func stream() -> AsyncStream<Int> {
        let stream = AsyncStream { continuation in
            for i in 1...9 {
                continuation.yield(i)
            }
            continuation.finish()
        }
        return stream
    }
    func fetch() async throws {
        await { [weak self] in
            for await i in self?.stream() ?? .finished {
                print(i)
            }
        }()
    }
}

Expected behavior

The above code compiles. It compiles fine with Swift 6.0.3.

Environment

Apple Swift version 6.2-dev (LLVM 86709865028dc89, Swift 2e6d7f71608bbad)
Target: arm64-apple-macosx15.0
Build config: +assertions

Additional information

Error message:

$ ~/Library/Developer/Toolchains/swift-DEVELOPMENT-SNAPSHOT-2025-04-02-a.xctoolchain/usr/bin/swiftc main.swift
main.swift:19:43: error: for-in loop requires 'AsyncStream<Int>?' to conform to 'AsyncSequence'; did you mean to unwrap optional?
17 |     func fetch() async throws {
18 |         await { [weak self] in
19 |             for await i in self?.stream() ?? .finished {
   |                                           `- error: for-in loop requires 'AsyncStream<Int>?' to conform to 'AsyncSequence'; did you mean to unwrap optional?
20 |                 print(i)
21 |             }

[Steelskin]

Smaller reproducer:

extension AsyncStream {
    public static var finished: Self { fatalError() }
}

extension Int {
    public func stream() -> AsyncStream<Int> { fatalError() }
}

extension Optional<Int> {
    func fetch() async throws {
        for await i in self?.stream() ?? .finished {
            print(i)
        }
    }
}

EDIT: Even smaller:

extension AsyncStream {
    public static var finished: Self { fatalError() }
}

nonisolated var defaultStream: AsyncStream<Int> { fatalError() }

func fetch() async throws {
    for await i in (defaultStream ?? .finished) {}
}

Godbolt link: https://godbolt.org/z/7dGnhE9dj

[jumhyn-browser]

Problem occurs even with a custom operator:

extension AsyncStream {
    public static var finished: Self { fatalError() }
}

nonisolated var defaultStream: AsyncStream<Int> { fatalError() }

infix operator ???

func ???<T>(_ lhs: T?, rhs: T) -> T { fatalError() }

func fetch() async throws {
    for await _ in (defaultStream ??? .finished) {}
}

with this case, the error is:

error: type 'AsyncSequence' has no member 'finished'
10 | 
11 | func fetch() async throws {
12 |     for await _ in (defaultStream ??? .finished) {}
   |                                        `- error: type 'AsyncSequence' has no member 'finished'
13 | }

So it appears something changed with how the types are getting inferred for the generic args here. cc @xedin

[jumhyn-browser]

For some reason on main we are not discovering the binding for $T1 := AsyncStream<Int> until we already bind $T3 := AsyncSequence.

---Constraint solving at [<source>:12:5 - line:12:48]---
(increasing 'sync-in-asynchronous' score by 1 @ locator@0x632803988190 [DeclRef@<source>:12:35])
  (overload set choice binding $T0 := ($T1?, $T1) -> $T1 [T := $T1])
  (overload set choice binding $T2 := AsyncStream<Int>)
(increasing 'value to optional promotion' score by 1 @ locator@0x6328039887c8 [Binary@<source>:12:35 → apply argument] → comparing call argument #0 to parameter #0)

---Initial constraints for the given expression---
(call_expr implicit type="$T8" location=<source>:12:48 range=[<source>:12:20 - line:12:48] isolation_crossing="none"
  (unresolved_dot_expr implicit type="$T7" location=<source>:12:48 range=[<source>:12:20 - line:12:48] field="makeAsyncIterator()" function_ref=single apply
    (paren_expr type="$T1" location=<source>:12:35 range=[<source>:12:20 - line:12:48]
      (binary_expr type="$T1" location=<source>:12:35 range=[<source>:12:21 - line:12:40] isolation_crossing="none"
        (declref_expr type="($T1?, $T1) -> $T1" location=<source>:12:35 range=[<source>:12:35 - line:12:35] decl="output.(file).???@<source>:9:6" function_ref=single apply)
        (argument_list implicit
          (argument
            (declref_expr type="AsyncStream<Int>" location=<source>:12:21 range=[<source>:12:21 - line:12:21] decl="output.(file).defaultStream@<source>:5:17" function_ref=unapplied))
          (argument
            (unresolved_member_chain_expr implicit type="$T5" location=<source>:12:40 range=[<source>:12:39 - line:12:40]
              (unresolved_member_expr type="$T4" location=<source>:12:40 range=[<source>:12:39 - line:12:40] name="finished" function_ref=unapplied)))))))
  (argument_list implicit))

Score: [sync-in-asynchronous(s), weight: 17, impact: 1] [value to optional promotion(s), weight: 7, impact: 1]
Contextual Type: AsyncSequence at Paren@<source>:12:35
Type Variables:
  $T0 as ($T1?, $T1) -> $T1 @ locator@0x632803988190 [DeclRef@<source>:12:35]
  $T1 [can bind to: hole] [attributes: potentially_incomplete] [adjacent to: $T5] [potential bindings: (supertypes of) AsyncStream<Int>] @ locator@0x6328039881e0 [DeclRef@<source>:12:35 → generic parameter 'T']
  $T2 as AsyncStream<Int> @ locator@0x632803988480 [DeclRef@<source>:12:21]
  $T3 [can bind to: noescape, hole] [attributes: hole, delayed] [defaultable bindings: 1] [potential bindings: <none>] @ locator@0x6328039884d0 [UnresolvedMember@<source>:12:40 → member reference base]
  $T4 [can bind to: lvalue, noescape] [attributes: delayed] [adjacent to: $T5] [potential bindings: <none>] @ locator@0x6328039884f8 [UnresolvedMember@<source>:12:40 → unresolved member]
  $T5 [can bind to: lvalue, noescape, hole] [attributes: hole] [adjacent to: $T1, $T4] [defaultable bindings: 1] [potential bindings: <none>] @ locator@0x6328039885e8 [UnresolvedMemberChainResult@<source>:12:40 → unresolved chain result]
  $T6 equivalent to $T1 @ locator@0x6328039886e8 [Binary@<source>:12:35 → function result]
  $T7 [can bind to: lvalue, noescape] [attributes: delayed] [potential bindings: <none>] @ locator@0x6328039889b8 [UnresolvedDot@<source>:12:48 → member]
  $T8 [can bind to: noescape] [attributes: delayed] [potential bindings: <none>] @ locator@0x632803988a90 [Call@<source>:12:48 → function result]
Active Constraints:
  $T1 conforms to Copyable @ locator@0x632803988268 [DeclRef@<source>:12:35 → opened generic → type parameter requirement #0 (conformance)]
  $T1 conforms to Escapable @ locator@0x6328039882e0 [DeclRef@<source>:12:35 → opened generic → type parameter requirement #1 (conformance)]
  AsyncStream<Int> operator arg conv $T1 @ locator@0x632803988828 [Binary@<source>:12:35 → apply argument → comparing call argument #0 to parameter #0 → optional injection]
  $T5 operator arg conv $T1 @ locator@0x6328039888b0 [Binary@<source>:12:35 → apply argument → comparing call argument #1 to parameter #1]
Inactive Constraints:
  $T3.Type[(implicit) .finished: value] == $T4 @ locator@0x6328039884f8 [UnresolvedMember@<source>:12:40 → unresolved member]
  $T4 conv $T5 @ locator@0x6328039885e8 [UnresolvedMemberChainResult@<source>:12:40 → unresolved chain result]
  $T5 unresolved member chain base $T3 @ locator@0x6328039885e8 [UnresolvedMemberChainResult@<source>:12:40 → unresolved chain result]
  $T1[.makeAsyncIterator(): value] == $T7 @ locator@0x6328039889b8 [UnresolvedDot@<source>:12:48 → member]
  () -> $T8 applicable fn $T7 @ locator@0x632803988b10 [Call@<source>:12:48 → apply function]
Resolved overloads:
  selected overload set choice defaultStream: $T2 == AsyncStream<Int> for locator@0x632803988480 [DeclRef@<source>:12:21]
  selected overload set choice ???: $T0 == ($T1?, $T1) -> $T1 for locator@0x632803988190 [DeclRef@<source>:12:35]
Opened types:
  locator@0x632803988190 [DeclRef@<source>:12:35] opens 'T' (T) -> $T1

  (overload set choice binding $T10 := @lvalue $T8)

---Initial constraints for the given expression---
(await_expr implicit type="$T12" location=<source>:12:5 range=[<source>:12:5 - line:12:5]
  (call_expr implicit type="$T12" location=<source>:12:5 range=[<source>:12:5 - line:12:5] isolation_crossing="none"
    (unresolved_dot_expr implicit type="$T11" location=<source>:12:5 range=[<source>:12:5 - line:12:5] field="next(isolation:)" function_ref=single apply
      (declref_expr implicit type="@lvalue $T8" location=<source>:12:5 range=[<source>:12:5 - line:12:5] decl="output.(file).fetch().$generator@<source>:12:20" function_ref=unapplied))
    (argument_list implicit labels="isolation:"
      (argument label="isolation"
        (current_context_isolation_expr implicit type="(any Actor)?" location=<source>:12:5 range=[<source>:12:5 - line:12:5])))))

Score: [sync-in-asynchronous(s), weight: 17, impact: 1] [value to optional promotion(s), weight: 7, impact: 1]
Contextual Type: AsyncIteratorProtocol at DeclRef@<source>:12:5
Contextual Type: AsyncSequence at Paren@<source>:12:35
Type Variables:
  $T0 as ($T1?, $T1) -> $T1 @ locator@0x632803988190 [DeclRef@<source>:12:35]
  $T1 [can bind to: hole] [attributes: potentially_incomplete] [adjacent to: $T5] [potential bindings: (supertypes of) AsyncStream<Int>] @ locator@0x6328039881e0 [DeclRef@<source>:12:35 → generic parameter 'T']
  $T2 as AsyncStream<Int> @ locator@0x632803988480 [DeclRef@<source>:12:21]
  $T3 [can bind to: noescape, hole] [attributes: hole, delayed] [defaultable bindings: 1] [potential bindings: <none>] @ locator@0x6328039884d0 [UnresolvedMember@<source>:12:40 → member reference base]
  $T4 [can bind to: lvalue, noescape] [attributes: delayed] [adjacent to: $T5] [potential bindings: <none>] @ locator@0x6328039884f8 [UnresolvedMember@<source>:12:40 → unresolved member]
  $T5 [can bind to: lvalue, noescape, hole] [attributes: hole] [adjacent to: $T1, $T4] [defaultable bindings: 1] [potential bindings: <none>] @ locator@0x6328039885e8 [UnresolvedMemberChainResult@<source>:12:40 → unresolved chain result]
  $T6 equivalent to $T1 @ locator@0x6328039886e8 [Binary@<source>:12:35 → function result]
  $T7 [can bind to: lvalue, noescape] [attributes: delayed] [potential bindings: <none>] @ locator@0x6328039889b8 [UnresolvedDot@<source>:12:48 → member]
  $T8 [can bind to: noescape] [attributes: delayed] [potential bindings: <none>] @ locator@0x632803988a90 [Call@<source>:12:48 → function result]
  $T9 equivalent to $T1 @ locator@0x632803988bb0 [Paren@<source>:12:35]
  $T10 as @lvalue $T8 @ locator@0x632803988c40 [DeclRef@<source>:12:5]
  $T11 [can bind to: lvalue, noescape] [attributes: delayed] [potential bindings: <none>] @ locator@0x632803988cd0 [UnresolvedDot@<source>:12:5 → member]
  $T12 [can bind to: noescape] [attributes: delayed] [potential bindings: <none>] @ locator@0x632803988db0 [Call@<source>:12:5 → function result]
Active Constraints:
  $T1 conforms to Copyable @ locator@0x632803988268 [DeclRef@<source>:12:35 → opened generic → type parameter requirement #0 (conformance)]
  $T1 conforms to Escapable @ locator@0x6328039882e0 [DeclRef@<source>:12:35 → opened generic → type parameter requirement #1 (conformance)]
  AsyncStream<Int> operator arg conv $T1 @ locator@0x632803988828 [Binary@<source>:12:35 → apply argument → comparing call argument #0 to parameter #0 → optional injection]
  $T5 operator arg conv $T1 @ locator@0x6328039888b0 [Binary@<source>:12:35 → apply argument → comparing call argument #1 to parameter #1]
  $T1[.makeAsyncIterator(): value] == $T7 @ locator@0x6328039889b8 [UnresolvedDot@<source>:12:48 → member]
  () -> $T8 applicable fn $T7 @ locator@0x632803988b10 [Call@<source>:12:48 → apply function]
Inactive Constraints:
  $T3.Type[(implicit) .finished: value] == $T4 @ locator@0x6328039884f8 [UnresolvedMember@<source>:12:40 → unresolved member]
  $T4 conv $T5 @ locator@0x6328039885e8 [UnresolvedMemberChainResult@<source>:12:40 → unresolved chain result]
  $T5 unresolved member chain base $T3 @ locator@0x6328039885e8 [UnresolvedMemberChainResult@<source>:12:40 → unresolved chain result]
  $T1 conforms to AsyncSequence @ locator@0x632803988168 [Paren@<source>:12:35 → contextual type]
  @lvalue $T8[.next(isolation:): value] == $T11 @ locator@0x632803988cd0 [UnresolvedDot@<source>:12:5 → member]
  ((any Actor)?) -> $T12 applicable fn $T11 @ locator@0x632803988e50 [Call@<source>:12:5 → apply function]
Resolved overloads:
  selected overload set choice $generator: $T10 == @lvalue $T8 for locator@0x632803988c40 [DeclRef@<source>:12:5]
  selected overload set choice defaultStream: $T2 == AsyncStream<Int> for locator@0x632803988480 [DeclRef@<source>:12:21]
  selected overload set choice ???: $T0 == ($T1?, $T1) -> $T1 for locator@0x632803988190 [DeclRef@<source>:12:35]
Opened types:
  locator@0x632803988190 [DeclRef@<source>:12:35] opens 'T' (T) -> $T1

  (considering: $T1 conforms to Copyable @ locator@0x632803988268 [DeclRef@<source>:12:35 → opened generic → type parameter requirement #0 (conformance)]
    (simplification result:
    )
    (outcome: unsolved)
  )
  (considering: $T1 conforms to Escapable @ locator@0x6328039882e0 [DeclRef@<source>:12:35 → opened generic → type parameter requirement #1 (conformance)]
    (simplification result:
    )
    (outcome: unsolved)
  )
  (considering: AsyncStream<Int> operator arg conv $T1 @ locator@0x632803988828 [Binary@<source>:12:35 → apply argument → comparing call argument #0 to parameter #0 → optional injection]
    (simplification result:
    )
    (outcome: unsolved)
  )
  (considering: $T5 operator arg conv $T1 @ locator@0x6328039888b0 [Binary@<source>:12:35 → apply argument → comparing call argument #1 to parameter #1]
    (simplification result:
    )
    (outcome: unsolved)
  )
  (considering: $T1[.makeAsyncIterator(): value] == $T7 @ locator@0x6328039889b8 [UnresolvedDot@<source>:12:48 → member]
    (simplification result:
    )
    (outcome: unsolved)
  )
  (considering: () -> $T8 applicable fn $T7 @ locator@0x632803988b10 [Call@<source>:12:48 → apply function]
    (simplification result:
    )
    (outcome: unsolved)
  )
  (Potential Binding(s): 
    ($T1 [can bind to: hole] [attributes: potentially_incomplete] [adjacent to: $T5] [potential bindings: (supertypes of) AsyncStream<Int>])
    ($T3 [can bind to: noescape, hole] [attributes: potentially_incomplete] [potential bindings: AsyncSequence])
    ($T12 [can bind to: noescape] [attributes: delayed] [adjacent to: $T14] [potential bindings: (subtypes of) $T14?])
  )
  (attempting type variable binding $T3 := AsyncSequence
    (considering: $T3.Type[(implicit) .finished: value] == $T4 @ locator@0x6328039884f8 [UnresolvedMember@<source>:12:40 → unresolved member]
      (simplification result:
        (removed constraint: $T3.Type[(implicit) .finished: value] == $T4 @ locator@0x6328039884f8 [UnresolvedMember@<source>:12:40 → unresolved member])
        (failed constraint $T3.Type[(implicit) .finished: value] == $T4 @ locator@0x6328039884f8 [UnresolvedMember@<source>:12:40 → unresolved member])
      )
      (outcome: error)
    )
  )

---Solver statistics---
Total number of scopes explored: 2
Total number of trail steps: 8
Maximum depth reached while exploring solutions: 2
---Attempting to salvage and emit diagnostics---
...

[jumhyn-browser]

Ah, potentially already fixed by #80443 !

[Steelskin]

I would like to leave this open to confirm that #80443 fixes the issue for us.

[Steelskin]

I can confirm that #80443 fixed the issue. Closing.