is duplicated by:

• SR-15222 Access Violation when running unit test on Windows

Issue Description:

The following differentiable Swift code builds and runs correctly on macOS and Linux, but on Windows when built and run it triggers a runtime crash in swift_getAssociatedTypeWitness:

import _Differentiation

protocol NumericDifferentiable : Numeric, Differentiable {
  @differentiable(reverse) static func *(lhs: Self, rhs: Self) -> Self
}

extension Float: NumericDifferentiable {}

struct Model2<T: NumericDifferentiable>: Differentiable {
  var first: T
  var second: T
}

@differentiable(reverse)
func loss2(model: Model2<Float>, multiplier: Float) -> Float {
  return model.first
}

var model = Model2<Float>(first: 1, second: 3)
let grad = gradient(at: model, 5.0, of: loss2)

print("Gradient: \(grad)")

This occurs on Windows with the Swift 5.4.1 release, Swift 5.5 development branch, and the latest nightly Swift toolchains. To reproduce, place the above in main.swift, build, and run the resulting executable via lldb to catch the runtime crash.

You should see a stop reason of "Exception 0xc0000005 encountered at address 0x7ff8a327ba4c: Access violation reading location 0x00000018" in swiftCore.dll at swift_getAssociatedTypeWitness.

Again, this same code builds and runs correctly on macOS and Linux, the crash only seems to appear on Windows.