Merge pull request swiftlang#58418 from philipturner/patch-35

[AutoDiff][RequirementMachine] Regression test for S4TF RNN/Sequential crasher caused by GenericSignatureBuilder

Author: swift-ci

test/AutoDiff/compiler_crashers_fixed/sr15884-generic-signature-builder-keypath-iterable.swift

@@ -0,0 +1,47 @@
+// RUN: %target-swift-frontend -emit-sil -verify %s
+
+// SR-15884: Crash while compiling Swift for TensorFlow. This was caused by a
+// bug in GenericSignatureBuilder, similar to one involving `CaseIterable`. In
+// this reproducer, `KeyPathIterabe` is similar enough to `CaseIterable` to 
+// cause the GSB crash. It was fixed by RequirementMachine abstract signatures.
+
+import _Differentiation
+@_spi(Reflection) import Swift
+
+struct RNNCellInput<Input>: Differentiable {}
+struct RNNCellOutput<Output>: Differentiable {}
+
+protocol Layer: Differentiable {
+  associatedtype Input
+  associatedtype Output: Differentiable
+  
+  @differentiable(reverse)
+  func callAsFunction(_ input: Input) -> Output
+}
+
+public protocol KeyPathIterable {
+  associatedtype AllKeyPaths: Sequence
+    where AllKeyPaths.Element == PartialKeyPath<Self>
+}
+
+protocol RecurrentLayerCell: Layer, KeyPathIterable
+where
+  Input == RNNCellInput<TimeStepInput>,
+  Output == RNNCellOutput<TimeStepOutput>
+{
+  associatedtype TimeStepInput
+  associatedtype TimeStepOutput: Differentiable
+}
+
+struct RecurrentLayer<Cell: RecurrentLayerCell>: Layer {
+  typealias Input = Cell.TimeStepInput
+  typealias Output = Cell.TimeStepOutput
+
+  var cell: Cell
+
+  @differentiable(reverse)
+  func callAsFunction(_ inputs: Cell.TimeStepInput) -> Cell.TimeStepOutput {
+    // expected-warning @+1 {{function call causes an infinite recursion}}
+    return self(inputs)
+  }
+}