extension Array where Element: Differentiable {
func derivativeOfDifferentiableReduce<Result: Differentiable>(
    _ initialResult: Result,
    _ nextPartialResult: @differentiable (Result, Element) -> Result
) -> (
    value: Result,
    differential: (Array.TangentVector, Result.TangentVector) -> Result.TangentVector
) {
    var differentials: [(Result.TangentVector, Element.TangentVector) -> Result.TangentVector] = []
    let count = self.count
    differentials.reserveCapacity(count)
    var result = initialResult
    for element in self {
        let (y, df) = Swift.valueWithDifferential(at: result, element, in: nextPartialResult)
        result = y
        differentials.append(df)
    }
    return (value: result, differential: { dSelf, dInitial in
        zip(dSelf.base, differentials).reduce(dInitial) { dResult, next in
            let (dElement, df) = next
            return df(dResult, dElement)
        }
    })
}
}

CompileSwift normal x86_64 /Users/rxwei/Development/Swift/Incremental/Sources/Incremental/main.swift (in target 'Incremental' from project 'Incremental')
    cd /Users/rxwei/Development/Swift/Incremental
    /Library/Developer/Toolchains/swift-tensorflow-LOCAL-2020-01-16-a.xctoolchain/usr/bin/swift -frontend -c -primary-file /Users/rxwei/Development/Swift/Incremental/Sources/Incremental/main.swift -emit-module-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main\~partial.swiftmodule -emit-module-doc-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main\~partial.swiftdoc -emit-module-source-info-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main\~partial.swiftsourceinfo -serialize-diagnostics-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main.dia -emit-dependencies-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main.d -emit-reference-dependencies-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main.swiftdeps -target x86_64-apple-macos10.10 -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -I /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Products/Debug -F /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Products/Debug -F /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/Frameworks -enable-testing -g -module-cache-path /Users/rxwei/Library/Developer/Xcode/DerivedData/ModuleCache.noindex -swift-version 5 -enforce-exclusivity=checked -Onone -enable-experimental-forward-mode-differentiation -D SWIFT_PACKAGE -D DEBUG -D Xcode -serialize-debugging-options -enable-anonymous-context-mangled-names -Xllvm -enable-experimental-cross-file-derivative-registration -Xcc -I/Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Products/Debug/include -Xcc -I/Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/DerivedSources-normal/x86_64 -Xcc -I/Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/DerivedSources/x86_64 -Xcc -I/Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/DerivedSources -Xcc -DSWIFT_PACKAGE -Xcc -DDEBUG=1 -Xcc -working-directory/Users/rxwei/Development/Swift/Incremental -module-name Incremental -o /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main.o -index-store-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Index/DataStore -index-system-modules

Remangling failed:
original     = $s13TangentVectors14DifferentiablePQz_AaCQyd__AaCQzAEIegnnr_tD
remangled    = $s13TangentVectors14DifferentiablePQz_AaCQyd__AdEIegnnr_tD
Stack dump:
0.  Program arguments: /Library/Developer/Toolchains/swift-tensorflow-LOCAL-2020-01-16-a.xctoolchain/usr/bin/swift -frontend -c -primary-file /Users/rxwei/Development/Swift/Incremental/Sources/Incremental/main.swift -emit-module-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main~partial.swiftmodule -emit-module-doc-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main~partial.swiftdoc -emit-module-source-info-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main~partial.swiftsourceinfo -serialize-diagnostics-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main.dia -emit-dependencies-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main.d -emit-reference-dependencies-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main.swiftdeps -target x86_64-apple-macos10.10 -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk -I /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Products/Debug -F /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Products/Debug -F /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/Library/Frameworks -enable-testing -g -module-cache-path /Users/rxwei/Library/Developer/Xcode/DerivedData/ModuleCache.noindex -swift-version 5 -enforce-exclusivity=checked -Onone -enable-experimental-forward-mode-differentiation -D SWIFT_PACKAGE -D DEBUG -D Xcode -serialize-debugging-options -enable-anonymous-context-mangled-names -Xllvm -enable-experimental-cross-file-derivative-registration -Xcc -I/Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Products/Debug/include -Xcc -I/Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/DerivedSources-normal/x86_64 -Xcc -I/Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/DerivedSources/x86_64 -Xcc -I/Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/DerivedSources -Xcc -DSWIFT_PACKAGE -Xcc -DDEBUG=1 -Xcc -working-directory/Users/rxwei/Development/Swift/Incremental -module-name Incremental -o /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Build/Intermediates.noindex/Incremental.build/Debug/Incremental.build/Objects-normal/x86_64/main.o -index-store-path /Users/rxwei/Library/Developer/Xcode/DerivedData/Incremental-dnoshtbbaqrzhbfdaqfgsrdzjvwo/Index/DataStore -index-system-modules 
1.  Swift version 5.2-dev (Swift ccfceeadd5)
2.  While emitting IR SIL function "@$sSa11Incrementals14DifferentiableRzlE012derivativeOfB6Reduceyqd__5value_13TangentVectorQyd__Sa0B4ViewVyAEQz_G_AFtc12differentialtqd___qd__qd___xtXFtsABRd__lFAfJ_AFtcfU_A2F_AI_A2F_AItcttXEfU_".
 for expression at [/Users/rxwei/Development/Swift/Incremental/Sources/Incremental/main.swift:39:57 - line:42:9] RangeText="{ dResult, next in
            let (dElement, df) = next
            return df(dResult, dElement)
        "
3.  While mangling type for debugger type '(Element.TangentVector, @callee_guaranteed (@in_guaranteed τ_1_0.TangentVector, @in_guaranteed τ_0_0.TangentVector) -> @out τ_1_0.TangentVector)'
0  swift                    0x0000000111d83f35 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  swift                    0x0000000111d83145 llvm::sys::RunSignalHandlers() + 85
2  swift                    0x0000000111d84526 SignalHandler(int) + 278
3  libsystem_platform.dylib 0x00007fff71fad42d _sigtramp + 29
4  libsystem_platform.dylib 0x000000000000003e _sigtramp + 2382703662
5  libsystem_c.dylib        0x00007fff71e82a1c abort + 120
6  swift                    0x00000001121fb863 swift::Mangle::Mangler::verify(llvm::StringRef) (.cold.3) + 99
7  swift                    0x000000010f03890c swift::Mangle::Mangler::verify(llvm::StringRef) + 796
8  swift                    0x000000010f0385d3 swift::Mangle::Mangler::finalize() + 211
9  swift                    0x000000010ee64347 swift::Mangle::ASTMangler::mangleTypeForDebugger(swift::Type, swift::DeclContext const*) + 263
10 swift                    0x000000010e1491a5 (anonymous namespace)::IRGenDebugInfoImpl::getOrCreateType(swift::irgen::DebugTypeInfo) + 1973
11 swift                    0x000000010e14393a (anonymous namespace)::IRGenDebugInfoImpl::emitVariableDeclaration(swift::irgen::IRBuilder&, llvm::ArrayRef<llvm::Value*>, swift::irgen::DebugTypeInfo, swift::SILDebugScope const*, swift::ValueDecl*, swift::SILDebugVariable, swift::irgen::IndirectionKind, swift::irgen::ArtificialKind) + 410
12 swift                    0x000000010e17869f void (anonymous namespace)::IRGenSILFunction::emitDebugVariableDeclaration<llvm::Value*>(llvm::Value*, swift::irgen::DebugTypeInfo, swift::SILType, swift::SILDebugScope const*, swift::VarDecl*, swift::SILDebugVariable, swift::irgen::IndirectionKind) + 191
13 swift                    0x000000010e174459 swift::SILInstructionVisitor<(anonymous namespace)::IRGenSILFunction, void>::visit(swift::SILInstruction*) + 57897
14 swift                    0x000000010e162f4f (anonymous namespace)::IRGenSILFunction::emitSILFunction() + 6927
15 swift                    0x000000010e160f64 swift::irgen::IRGenModule::emitSILFunction(swift::SILFunction*) + 1444
16 swift                    0x000000010e0799ab swift::irgen::IRGenerator::emitLazyDefinitions() + 1243
17 swift                    0x000000010e13c8e5 performIRGeneration(swift::IRGenOptions&, swift::ModuleDecl*, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::StringRef, swift::PrimarySpecificPaths const&, llvm::LLVMContext&, swift::SourceFile*, llvm::GlobalVariable**, llvm::StringSet<llvm::MallocAllocator>*) + 1365
18 swift                    0x000000010e13cc8b swift::performIRGeneration(swift::IRGenOptions&, swift::SourceFile&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, llvm::StringRef, swift::PrimarySpecificPaths const&, llvm::LLVMContext&, llvm::GlobalVariable**, llvm::StringSet<llvm::MallocAllocator>*) + 91
19 swift                    0x000000010dfeb18e performCompileStepsPostSILGen(swift::CompilerInstance&, swift::CompilerInvocation&, std::__1::unique_ptr<swift::SILModule, std::__1::default_delete<swift::SILModule> >, bool, llvm::PointerUnion<swift::ModuleDecl*, swift::SourceFile*>, swift::PrimarySpecificPaths const&, bool, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 3390
20 swift                    0x000000010dfdf5d9 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 8521
21 swift                    0x000000010dfdc503 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 5139
22 swift                    0x000000010df783de main + 734
23 libdyld.dylib            0x00007fff71db47fd start + 1
error: Abort trap: 6 (in target 'Incremental' from project 'Incremental')