Add a test case for an unresolvable dynamic metadata cycle

test/Interpreter/Inputs/resilient_generic_struct_v1.swift

@@ -0,0 +1,6 @@
+public struct ResilientGenericStruct<T> {
+  public init(value: T) {
+    size = MemoryLayout<T>.size
+  }
+  public var size: Int
+}

test/Interpreter/Inputs/resilient_generic_struct_v2.swift

@@ -0,0 +1,8 @@
+public struct ResilientGenericStruct<T> {
+  public init(value: T) {
+    size = MemoryLayout<T>.size
+    storage = value
+  }
+  public var size: Int
+  private var storage: T
+}

test/Interpreter/unresolvable_dynamic_metadata_cycles.swift

@@ -0,0 +1,47 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-build-swift-dylib(%t/libresil.%target-dylib-extension) -Xfrontend -enable-resilience %S/Inputs/resilient_generic_struct_v1.swift -emit-module -emit-module-path %t/resil.swiftmodule -module-name resil
+// RUN: %target-codesign %t/libresil.%target-dylib-extension
+
+// RUN: %target-build-swift %s -L %t -I %t -lresil -o %t/main -Xlinker -rpath -Xlinker %t
+
+// RUN: %target-build-swift-dylib(%t/libresil.%target-dylib-extension) -Xfrontend -enable-resilience %S/Inputs/resilient_generic_struct_v2.swift -emit-module -emit-module-path %t/resil.swiftmodule -module-name resil
+// RUN: %target-codesign %t/libresil.%target-dylib-extension
+
+// RUN: %target-run %t/main %t/libresil.%target-dylib-extension
+
+import StdlibUnittest
+
+// We build this code against a version of 'resil' where
+// ResilientGenericStruct<T> doesn't store a T, then switch the
+// dynamic library to a new version where it does, introducing
+// an unresolvable dynamic cycle.
+//
+// It would also be sufficient to demonstrate this crash if the
+// compiler *actually* didn't know about the internal implementation
+// details of 'resil' when building this file, but since it currently
+// still does, it'll report a cycle immediately if we don't pull
+// this switcharoo.
+import resil
+
+var DynamicMetadataCycleTests =
+  TestSuite("Unresolvable dynamic metadata cycle tests")
+
+enum test0_Node {
+    case link(ResilientGenericStruct<test0_Node>)
+
+    static func test() -> [test0_Node] {
+        return []
+    }
+}
+DynamicMetadataCycleTests.test("cycle through enum")
+  .crashOutputMatches("runtime error: unresolvable type metadata dependency cycle detected")
+  .crashOutputMatches("  main.test0_Node")
+  .crashOutputMatches("  depends on layout of resil.ResilientGenericStruct<main.test0_Node")
+  .crashOutputMatches("  depends on layout of main.test0_Node")
+  .code {
+    expectCrashLater()
+    _ = test0_Node.test()
+  }
+
+runAllTests()