import _Differentiation

func foo<T: Differentiable>(_ x: T) -> (T, T) { (x, x) }

func bar<T: Differentiable>(_ x: T) -> T {
  let (y1, y2) = foo(x)
  return y1
}
print(derivative(at: 3, in: bar))

$ swift -Xfrontend -enable-experimental-forward-mode-differentiation -Xllvm -debug-only=differentiation error.swift
...
[AD] JVPCloner visited:
[ORIG]  %6 = apply %5<T>(%3, %4, %1) : $@convention(thin) <τ_0_0 where τ_0_0 : Differentiable> (@in_guara
nteed τ_0_0) -> (@out τ_0_0, @out τ_0_0)
[AD] JVP-transforming:
  %6 = apply %5<T>(%3, %4, %1) : $@convention(thin) <τ_0_0 where τ_0_0 : Differentiable> (@in_guaranteed
τ_0_0) -> (@out τ_0_0, @out τ_0_0)

[AD] Active indices: params={0}, results={0}
[AD] Applied jvp function
  %13 = apply %10(%3, %4, %1) : $@callee_guaranteed (@in_guaranteed τ_0_0) -> (@out τ_0_0, @out τ_0_0, @o
wned @callee_guaranteed @substituted <τ_0_0, τ_0_1> (@in_guaranteed τ_0_0) -> @out τ_0_1 for <τ_0_0.Tange
ntVector, τ_0_0.TangentVector>)
Assertion failed: (it != bufferMap.end() && "Tangent buffer should already exist"), function getTangentBu
ffer, file /Users/danielzheng/swift-build/swift/lib/SILOptimizer/Differentiation/JVPCloner.cpp, line 316.
Stack dump:
0.      Program arguments: /Users/danielzheng/swift-build/build/Ninja-ReleaseAssert/swift-macosx-x86_64/b
in/swift-frontend -frontend -interpret error.swift -enable-objc-interop -sdk /Applications/Xcode-beta.app
/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk -enable-experimental-forward-mode
-differentiation -Xllvm -debug-only=differentiation -target-sdk-version 10.16 -module-name error
1.      Swift version 5.3-dev (LLVM 19447e27024a8d4, Swift cea678431f903e4)
2.      While evaluating request ExecuteSILPipelineRequest(Run pipelines { Mandatory Diagnostic Passes +
Enabling Optimization Passes } on SIL for error.error)
3.      While running pass #&#8203;45 SILModuleTransform "Differentiation".
4.      While canonicalizing `differentiable_function` SIL node   %24 = differentiable_function [paramete
rs 0] [results 0] %18 : $@callee_guaranteed (@in_guaranteed Double) -> @out Double // user: %25
5.      While ...in SIL function "@main".
6.      While processing // differentiability witness for bar<A>(_:)
sil_differentiability_witness private [parameters 0] [results 0] <τ_0_0 where τ_0_0 : Differentiable> @$s
5error3baryxx16_Differentiation14DifferentiableRzlF : $@convention(thin) <T where T : Differentiable> (@i
n_guaranteed T) -> @out T {
}

 on SIL function "@$s5error3baryxx16_Differentiation14DifferentiableRzlF".
 for 'bar(_:)' (at error.swift:5:1)
7.      While generating JVP and differential for SIL function "@$s5error3baryxx16_Differentiation14DifferentiableRzlF".
 for 'bar(_:)' (at error.swift:5:1)
0  swift-frontend           0x000000010df8c8b5 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 37
1  swift-frontend           0x000000010df8b888 llvm::sys::RunSignalHandlers() + 248
2  swift-frontend           0x000000010df8ce96 SignalHandler(int) + 262
3  libsystem_platform.dylib 0x00007fff6e0115fd _sigtramp + 29
4  libsystem_platform.dylib 000000000000000000 _sigtramp + 18446603338670598688
5  libsystem_c.dylib        0x00007fff6dee7808 abort + 120
6  libsystem_c.dylib        0x00007fff6dee6ac6 err + 0
7  swift-frontend           0x000000010e174113 swift::autodiff::JVPCloner::Implementation::getTangentBuffer(swift::SILBasicBlock*, swift::SILValue) (.cold.1) + 35
8  swift-frontend           0x0000000109fa7c21 swift::autodiff::JVPCloner::Implementation::getTangentBuffer(swift::SILBasicBlock*, swift::SILValue) + 145
9  swift-frontend           0x0000000109fa9081 swift::autodiff::JVPCloner::Implementation::emitTangentForApplyInst(swift::ApplyInst*, swift::SILAutoDiffIndices, swift::CanTypeWrapper<swift::SILFunctionType>) + 257
10 swift-frontend           0x0000000109f9b93b swift::autodiff::JVPCloner::Implementation::visitApplyInst(swift::ApplyInst*) + 6139

for (auto indRes : ai->getIndirectSILResults())
  diffArgs.push_back(getTangentBuffer(bb, indRes));