Forbid subclasses of NativePointed to have backing fields

(this crashes the compiler otherwise)

kotlin-native/backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/lower/SpecialBackendChecksTraversal.kt

@@ -321,6 +321,17 @@ private class BackendChecker(val context: Context, val irFile: IrFile) : IrEleme
             reportError(expression, "Native interop types constructors must not be called directly")
     }
 
+    override fun visitField(declaration: IrField) {
+        if (declaration.isFakeOverride) return // Can't happen now, just trying to be future-proof here.
+
+        val parent = declaration.parent
+
+        if (parent is IrClass && parent.defaultType.isNativePointed(symbols) && parent.symbol != symbols.nativePointed) {
+            val nativePointed = symbols.nativePointed.owner.name
+            reportError(declaration, "Subclasses of $nativePointed cannot have properties with backing fields")
+        }
+    }
+
     override fun visitCall(expression: IrCall) {
         expression.acceptChildrenVoid(this)
 

kotlin-native/backend.native/tests/compilerChecks/t60.kt

@@ -0,0 +1,10 @@
+import kotlinx.cinterop.*
+
+class Vertex constructor(rawPtr: NativePtr) : CStructVar(rawPtr) {
+    var x: Float = 0f
+    var y: Float = 0f
+    var r: Float = 0f
+    var g: Float = 0f
+    var b: Float = 0f
+    companion object : CStructVar.Type(40, 8)
+}