func copyType<I,O> (_ function: (I) -> (O)) -> (I) -> (O) {
    fatalError("Never called")
}

class Class {
    lazy var storage = copyType(takeFunction) // storage is "autoclosured"
    func takeFunction(_ autoString: @autoclosure () -> String, other: String) {
    }
}

let c = Class()
c.storage = { _ in }

print(type(of: c.storage)) // (() -> String, String) -> ()
// c.storage(({"A"},"b")) // error: function produces expected type 'String'; did you mean to call it with '()'?
c.storage(("A","b")) // only that compiles

$ /Library/Developer/Toolchains/swift-5.0-DEVELOPMENT-SNAPSHOT-2019-02-20-a.xctoolchain/usr/bin/swift autoclosure.swift 
Assertion failed: (flags.isNone()), function appendTypeList, file /Users/buildnode/jenkins/workspace/oss-swift-5.0-package-osx/swift/lib/AST/ASTMangler.cpp, line 1829.
Stack dump:
0.  Program arguments: /Library/Developer/Toolchains/swift-5.0-DEVELOPMENT-SNAPSHOT-2019-02-20-a.xctoolchain/usr/bin/swift -frontend -interpret autoclosure.swift -enable-objc-interop -sdk /Applications/Xcode_1010.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk -color-diagnostics -module-name autoclosure 
0  swift                    0x0000000108c11c85 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  swift                    0x0000000108c10f25 llvm::sys::RunSignalHandlers() + 85
2  swift                    0x0000000108c12272 SignalHandler(int) + 258
3  libsystem_platform.dylib 0x00007fff69079b5d _sigtramp + 29
4  libsystem_malloc.dylib   0x00007fff69040796 tiny_malloc_from_free_list + 444
5  libsystem_c.dylib        0x00007fff68f396a6 abort + 127
6  libsystem_c.dylib        0x00007fff68f0220d basename_r + 0
7  swift                    0x00000001062c11bb swift::Mangle::ASTMangler::appendTypeList(swift::Type) + 523
8  swift                    0x00000001062bd1cf swift::Mangle::ASTMangler::appendType(swift::Type) + 1055
9  swift                    0x00000001062c373c swift::Mangle::ASTMangler::appendTypeListElement(swift::Identifier, swift::Type, swift::ParameterTypeFlags) + 140
10 swift                    0x00000001062c3582 swift::Mangle::ASTMangler::appendFunctionInputType(llvm::ArrayRef<swift::AnyFunctionType::Param>) + 146
11 swift                    0x00000001062c33ea swift::Mangle::ASTMangler::appendFunctionSignature(swift::AnyFunctionType*) + 58
12 swift                    0x00000001062c18f2 swift::Mangle::ASTMangler::appendFunctionType(swift::AnyFunctionType*, bool) + 50
13 swift                    0x00000001062bf6b0 swift::Mangle::ASTMangler::appendDeclType(swift::ValueDecl const*, bool) + 192
14 swift                    0x00000001062bacfe swift::Mangle::ASTMangler::appendAccessorEntity(llvm::StringRef, swift::AbstractStorageDecl const*, bool) + 142
15 swift                    0x00000001062b9db0 swift::Mangle::ASTMangler::mangleEntity(swift::ValueDecl const*, bool, swift::Mangle::ASTMangler::SymbolKind) + 48
16 swift                    0x0000000105dfd24f swift::SILDeclRef::mangle(swift::SILDeclRef::ManglingKind) const + 1775
17 swift                    0x0000000105e4c677 swift::SILFunctionBuilder::getOrCreateFunction(swift::SILLocation, swift::SILDeclRef, swift::ForDefinition_t, swift::ProfileCounter) + 55
18 swift                    0x00000001058b5f9c swift::Lowering::SILGenModule::getFunction(swift::SILDeclRef, swift::ForDefinition_t) + 348
19 swift                    0x000000010599c47e swift::Lowering::SILGenModule::emitVTableMethod(swift::ClassDecl*, swift::SILDeclRef, swift::SILDeclRef) + 430
20 swift                    0x000000010599eab1 (anonymous namespace)::SILGenType::emitType() + 1025
21 swift                    0x000000010599e6a9 swift::Lowering::SILGenModule::visitNominalTypeDecl(swift::NominalTypeDecl*) + 25
22 swift                    0x00000001058bbe16 swift::Lowering::SILGenModule::emitSourceFile(swift::SourceFile*) + 822
23 swift                    0x00000001058bcd78 swift::SILModule::constructSIL(swift::ModuleDecl*, swift::SILOptions&, swift::FileUnit*) + 376
24 swift                    0x00000001058bd270 swift::performSILGeneration(swift::ModuleDecl*, swift::SILOptions&) + 16
25 swift                    0x0000000105046717 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 9943
26 swift                    0x00000001050432d8 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3000
27 swift                    0x0000000104ff5152 main + 690
28 libdyld.dylib            0x00007fff68e943ed start + 1
29 libdyld.dylib            0x000000000000000a start + 2534849566
Abort trap: 6

func mock<I,O> (for function: (I) -> (O), alwaysReturn: O) -> (I) -> (O) {
    return { p in
        return alwaysReturn
    }
}

class Class {
    lazy var storage = mock(for: takeFunction, alwaysReturn: Void())
    func takeFunction(_ autoString: @autoclosure () -> String, other: String) {
        storage((autoString(), other))
    }
}

class ArrayClass<T> {
    var s:[T] = []
}
func spyMockCalls<I,O>(of stub: inout (I) -> (O)) -> ArrayClass<I> {
    let records = ArrayClass<I>()
    let originalStub = stub
    stub = { [weak weakRecords = records] arg in
        weakRecords?.s.append(arg)
        return originalStub(arg)
    }
    return records
}

let c = Class()
let args = spyMockCalls(of: &c.storage)
c.takeFunction("example", other: "other")
print(args.s[0].0())// Crash