asyncの正体、ここっぽいですねw

 auto refType = FunctionType::get(arg, result, FunctionType::ExtInfoBuilder() .withNoEscape(true) .withAsync(true) // 👈🏻 .withThrows(true, thrownError) .build());

https://github.com/swiftlang/swift/blob/main/lib/Sema/TypeOfReference.cpp#L2278 その後スコアを加算して、最終的にasyncが選ばれています https://github.com/swiftlang/swift/blob/main/lib/Sema/CSSimplify.cpp#L1919 ~~https://github.com/swiftlang/swift/blob/main/lib/Sema/TypeOfReference.cpp#L2486~~

---Solution--- Fixed score: [sync-in-asynchronous(s), weight: 17, impact: 1] Type variables: $T0 as Void @ locator@0x127053000 [DiscardAssignment@79078.swift:4:5] $T1 as (@escaping () async -> Void, (@escaping () async -> Void) async -> Void) async -> Void @ locator@0x127053068 [DeclRef@79078.swift:4:15] $T2 as () async -> Void @ locator@0x1270530b0 [DeclRef@79078.swift:4:15 → function argument] $T3 as () async -> Void @ locator@0x1270530b0 [DeclRef@79078.swift:4:15 → function argument] Overload choices: locator@0x127053068 [DeclRef@79078.swift:4:15] with Swift.(file).withoutActuallyEscaping(_:do:) as withoutActuallyEscaping: ($T2, ($T3) async throws($T5) -> $T4) async throws($T5) -> $T4

withAsyncをtrueにしないようにする方向かなと思っています。 (edited)