Nonisolated nonsending closure called from nonisolated nonsending function does not inherit caller actor

[AnthonyLatsis]

Description

No response

Reproduction

nonisolated(nonsending) func g() async {
  MainActor.assertIsolated()
}

nonisolated(nonsending)
func f() async {
  await g() // okay
  let closure = g 
  await closure() // boom
}

@main struct Foo {
  static func main() async throws {
    await f()
  }
}

Expected behavior

g does not switch off the main actor.

Environment

Apple Swift version 6.2 (swiftlang-6.2.0.19.8 clang-1700.3.19.1)

Additional information

No response

[jamieQ]

in case it's of any help, a couple things i've noticed:

1. the issue in the reproducer does not occur without specifying -swift-version 6
2. the errant hop to the concurrent executor seems like it is introduced during mandatory inlining when a reabstraction thunk helper is inlined and carries with it the nil actor

SIL diff before/after mandatory inlining

--- sil-hop-before.sil	2025-08-19 20:00:03
+++ sil-hop-after.sil	2025-08-19 20:00:10
@@ -1,8 +1,8 @@
-  *** SIL function after  #73, stage Mandatory Diagnostic Passes + Enabling Optimization Passes, pass 25: MoveOnlyTypeEliminator (sil-move-only-type-eliminator)
+  *** SIL module after #86, stage Mandatory Diagnostic Passes + Enabling Optimization Passes, pass 26: MandatoryInlining (mandatory-inlining)
 // f()
 // Isolation: caller_isolation_inheriting
 sil hidden [ossa] @$s6output1fyyYaF : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> () {
-// %0                                             // users: %14, %13, %4, %3, %1
+// %0                                             // users: %11, %12, %4, %3, %1
 bb0(%0 : @guaranteed $Optional<any Actor>):
   hop_to_executor %0                              // id: %1
   // function_ref g()
@@ -10,20 +10,14 @@
   %3 = apply %2(%0) : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()
   hop_to_executor %0                              // id: %4
   // function_ref g()
-  %5 = function_ref @$s6output1gyyYaF : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> () // user: %7
-  // function_ref thunk for @escaping @convention(thin) @async (@guaranteed Actor?) -> ()
-  %6 = function_ref @$sScA_pSgIetHgIL_IegH_TR : $@convention(thin) @async (@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()) -> () // user: %7
-  %7 = partial_apply [callee_guaranteed] %6(%5) : $@convention(thin) @async (@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()) -> () // user: %9
-  // function_ref thunk for @escaping @callee_guaranteed @async () -> ()
-  %8 = function_ref @$sIegH_ScA_pSgIeghHgIL_TR : $@convention(thin) @Sendable @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>, @guaranteed @async @callee_guaranteed () -> ()) -> () // user: %9
-  %9 = partial_apply [callee_guaranteed] %8(%7) : $@convention(thin) @Sendable @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>, @guaranteed @async @callee_guaranteed () -> ()) -> () // user: %10
-  %10 = move_value [lexical] [var_decl] %9        // users: %12, %16, %11
-  debug_value %10, let, name "closure"            // id: %11
-  %12 = copy_value %10                            // users: %13, %15
-  %13 = apply %12(%0) : $@Sendable @async @callee_guaranteed (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()
-  hop_to_executor %0                              // id: %14
-  destroy_value %12                               // id: %15
-  destroy_value %10                               // id: %16
-  %17 = tuple ()                                  // user: %18
-  return %17                                      // id: %18
+  %5 = function_ref @$s6output1gyyYaF : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> () // user: %8
+  %6 = enum $Optional<any Actor>, #Optional.none!enumelt // users: %8, %7
+  hop_to_executor %6                              // id: %7
+  %8 = apply %5(%6) : $@convention(thin) @async (@sil_isolated @sil_implicit_leading_param @guaranteed Optional<any Actor>) -> ()
+  %9 = tuple ()
+  %10 = tuple ()
+  hop_to_executor %0                              // id: %11
+  hop_to_executor %0                              // id: %12
+  %13 = tuple ()                                  // user: %14
+  return %13                                      // id: %14
 } // end sil function '$s6output1fyyYaF'

godbolt: https://swift.godbolt.org/z/q9e5ccMKM