Enabling the InferSendableFromCaptures upcoming feature flag can break code that involves C functions

[groue]

Description

When I enable the upcoming feature flag InferSendableFromCaptures (from SE-0418), the Swift 6 compiler emits errors:

A C function pointer can only be formed from a reference to a 'func' or a literal closure.

Those errors are not emitted when InferSendableFromCaptures is not enabled.

Reproduction

import SQLite3

func f(_ cFunction: @convention(c) (OpaquePointer?) -> UnsafeMutablePointer<CChar>?) { }

func g() {
  // A C function pointer can only be formed from a reference to a 'func' or a literal closure
  f(sqlite3_expanded_sql)
}

Expected behavior

No compiler error.

Environment

$ /Library/Developer/Toolchains/swift-6.0-DEVELOPMENT-SNAPSHOT-2024-04-22-a.xctoolchain/usr/bin/swift --version
Apple Swift version 6.0-dev (LLVM 94447cf359772fc, Swift f36098a14a0d48b)
Target: arm64-apple-macosx14.0

Additional information

I've been using this technique for many years, with great success, inspired by this post on the Swift forums.

[jckarter]

#73436 should fix this.