public protocol Layer: Differentiable & KeyPathIterable
    where AllDifferentiableVariables: KeyPathIterable {
    /// The input type of the layer.
    associatedtype Input: Differentiable
    /// The output type of the layer.
    associatedtype Output: Differentiable

    /// Returns the output obtained from applying to an input.
    @differentiable(wrt: (self, input))
    func applied(to input: Input) -> Output
}
public struct Dense<Scalar>: Layer
    where Scalar: FloatingPoint & Differentiable & TensorFlowScalar {

    public var weight: Tensor<Scalar>
    public var bias: Tensor<Scalar>
    public typealias Activation = @autodiff (Tensor<Scalar>) -> Tensor<Scalar>
    public let activation: Activation

    // FIXME(SR-9716): Remove this once the bug is fixed or worked around.
    public var allKeyPaths: [PartialKeyPath<Dense>] {
        return [\Dense.weight, \Dense.bias]
    }

    @differentiable(wrt: (self, input))
    public func applied(to input: Tensor<Scalar>) -> Tensor<Scalar> {
        return activation(matmul(input, weight) + bias)
    }
}

/Users/rxwei/Development/swift-apis/Sources/DeepLearning/Layer.swift:56:16: warning: stored property has no derivative because it does not conform to 'Differentiable'; add '@noDerivative' to make it explicit
    public let f: (Float) -> Float
               ^
    @noDerivative 
Assertion failed: (loc.isValid() && "Diagnosing attribute with invalid location"), function diagnoseAndRemoveAttr, file /usr/local/src/swift-build/swift/lib/Sema/TypeCheckAttr.cpp, line 43.
Stack dump:
0.  Program arguments: /Library/Developer/Toolchains/swift-tensorflow-DEVELOPMENT-2019-01-24-a.xctoolchain/usr/bin/swift -frontend -c /Users/rxwei/Development/swift-apis/Sources/DeepLearning/Helpers.swift -primary-file /Users/rxwei/Development/swift-apis/Sources/DeepLearning/Layer.swift /Users/rxwei/Development/swift-apis/Sources/DeepLearning/Loss.swift /Users/rxwei/Development/swift-apis/Sources/DeepLearning/Optimizer.swift -emit-module-path /Users/rxwei/Library/Developer/Xcode/DerivedData/DeepLearning-ecphexrowvsgalflnojluvjbiadn/Build/Intermediates.noindex/DeepLearning.build/Debug/DeepLearning.build/Objects-normal/x86_64/Layer~partial.swiftmodule -emit-module-doc-path /Users/rxwei/Library/Developer/Xcode/DerivedData/DeepLearning-ecphexrowvsgalflnojluvjbiadn/Build/Intermediates.noindex/DeepLearning.build/Debug/DeepLearning.build/Objects-normal/x86_64/Layer~partial.swiftdoc -serialize-diagnostics-path /Users/rxwei/Library/Developer/Xcode/DerivedData/DeepLearning-ecphexrowvsgalflnojluvjbiadn/Build/Intermediates.noindex/DeepLearning.build/Debug/DeepLearning.build/Objects-normal/x86_64/Layer.dia -emit-dependencies-path /Users/rxwei/Library/Developer/Xcode/DerivedData/DeepLearning-ecphexrowvsgalflnojluvjbiadn/Build/Intermediates.noindex/DeepLearning.build/Debug/DeepLearning.build/Objects-normal/x86_64/Layer.d -emit-reference-dependencies-path /Users/rxwei/Library/Developer/Xcode/DerivedData/DeepLearning-ecphexrowvsgalflnojluvjbiadn/Build/Intermediates.noindex/DeepLearning.build/Debug/DeepLearning.build/Objects-normal/x86_64/Layer.swiftdeps -target x86_64-apple-macosx10.10 -enable-objc-interop -sdk /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk -I /Users/rxwei/Library/Developer/Xcode/DerivedData/DeepLearning-ecphexrowvsgalflnojluvjbiadn/Build/Products/Debug -F /Users/rxwei/Library/Developer/Xcode/DerivedData/DeepLearning-ecphexrowvsgalflnojluvjbiadn/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 4.2 -enforce-exclusivity=checked -Onone -D SWIFT_PACKAGE -D DEBUG -D Xcode -serialize-debugging-options -Xcc -working-directory -Xcc /Users/rxwei/Development/swift-apis -Xcc -I/Users/rxwei/Library/Developer/Xcode/DerivedData/DeepLearning-ecphexrowvsgalflnojluvjbiadn/Build/Intermediates.noindex/DeepLearning.build/Debug/DeepLearning.build/swift-overrides.hmap -Xcc -I/Users/rxwei/Library/Developer/Xcode/DerivedData/DeepLearning-ecphexrowvsgalflnojluvjbiadn/Build/Products/Debug/include -Xcc -I/Users/rxwei/Library/Developer/Xcode/DerivedData/DeepLearning-ecphexrowvsgalflnojluvjbiadn/Build/Intermediates.noindex/DeepLearning.build/Debug/DeepLearning.build/DerivedSources/x86_64 -Xcc -I/Users/rxwei/Library/Developer/Xcode/DerivedData/DeepLearning-ecphexrowvsgalflnojluvjbiadn/Build/Intermediates.noindex/DeepLearning.build/Debug/DeepLearning.build/DerivedSources -Xcc -DDEBUG=1 -module-name DeepLearning -o /Users/rxwei/Library/Developer/Xcode/DerivedData/DeepLearning-ecphexrowvsgalflnojluvjbiadn/Build/Intermediates.noindex/DeepLearning.build/Debug/DeepLearning.build/Objects-normal/x86_64/Layer.o -index-store-path /Users/rxwei/Library/Developer/Xcode/DerivedData/DeepLearning-ecphexrowvsgalflnojluvjbiadn/Index/DataStore -index-system-modules 
1.  While type-checking 'Dense' (at /Users/rxwei/Development/swift-apis/Sources/DeepLearning/Layer.swift:50:8)
0  swift                    0x000000010597c928 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
1  swift                    0x000000010597bba5 llvm::sys::RunSignalHandlers() + 85
2  swift                    0x000000010597cf32 SignalHandler(int) + 258
3  libsystem_platform.dylib 0x00007fff76edcb3d _sigtramp + 29
4  libsystem_platform.dylib 0x0000000000004098 _sigtramp + 2299688312
5  libsystem_c.dylib        0x00007fff76d9b1c9 abort + 127
6  libsystem_c.dylib        0x00007fff76d63868 basename_r + 0
7  swift                    0x0000000102d24667 void (anonymous namespace)::diagnoseAndRemoveAttr<swift::Diag<>&>(swift::TypeChecker&, swift::Decl*, swift::DeclAttribute*, swift::Diag<>&) + 199
8  swift                    0x0000000102d1fdd1 swift::TypeChecker::checkDeclAttributes(swift::Decl*) + 49
9  swift                    0x0000000102d61a64 (anonymous namespace)::DeclChecker::visitStructDecl(swift::StructDecl*) + 164
10 swift                    0x0000000102d55213 (anonymous namespace)::DeclChecker::visit(swift::Decl*) + 1491
11 swift                    0x0000000102d54c36 swift::TypeChecker::typeCheckDecl(swift::Decl*) + 38
12 swift                    0x0000000102df1948 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int, unsigned int, unsigned int, unsigned int) + 872
13 swift                    0x00000001024bc805 swift::CompilerInstance::parseAndCheckTypesUpTo(swift::CompilerInstance::ImplicitImports const&, swift::SourceFile::ASTStage_t) + 629
14 swift                    0x00000001024bbe08 swift::CompilerInstance::performSemaUpTo(swift::SourceFile::ASTStage_t) + 616
15 swift                    0x0000000101c8ec8d performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 1309
16 swift                    0x0000000101c8d6dd swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 3021
17 swift                    0x0000000101c3f2ce main + 686
18 libdyld.dylib            0x00007fff76cf308d start + 1
19 libdyld.dylib            0x0000000000000041 start + 2301677493