diff --git a/core/src/fr/hammons/slinc/CUnion.scala b/core/src/fr/hammons/slinc/CUnion.scala
index d4dd67a5..fa0296fb 100644
--- a/core/src/fr/hammons/slinc/CUnion.scala
+++ b/core/src/fr/hammons/slinc/CUnion.scala
@@ -7,7 +7,7 @@ import fr.hammons.slinc.modules.ReadWriteModule
 import fr.hammons.slinc.modules.DescriptorModule
 import scala.NonEmptyTuple
 
-class CUnion[T <: Tuple](mem: Mem):
+class CUnion[T <: Tuple](private[slinc] val mem: Mem):
   private inline def getHelper[T <: Tuple, A](using
       dO: DescriptorOf[A],
       rwm: ReadWriteModule
diff --git a/core/src/fr/hammons/slinc/DescriptorOf.scala b/core/src/fr/hammons/slinc/DescriptorOf.scala
index e623bc6d..33d1bbf8 100644
--- a/core/src/fr/hammons/slinc/DescriptorOf.scala
+++ b/core/src/fr/hammons/slinc/DescriptorOf.scala
@@ -2,6 +2,8 @@ package fr.hammons.slinc
 
 import fr.hammons.slinc.container.*
 import scala.quoted.*
+import scala.compiletime.{summonInline, erasedValue}
+import scala.NonEmptyTuple
 
 /** Typeclass that summons TypeDescriptors
   */
@@ -64,3 +66,14 @@ object DescriptorOf:
       )
 
     '{ $expr.descriptor }
+
+  private inline def helper[B <: Tuple]: Set[TypeDescriptor] =
+    inline erasedValue[B] match
+      case _: (a *: t)   => helper[t] + summonInline[DescriptorOf[a]].descriptor
+      case _: EmptyTuple => Set.empty[TypeDescriptor]
+
+  inline given [A <: NonEmptyTuple]: DescriptorOf[CUnion[A]] =
+    new DescriptorOf[CUnion[A]]:
+      val descriptor: CUnionDescriptor { type Inner = CUnion[A] } =
+        CUnionDescriptor(helper[A])
+          .asInstanceOf[CUnionDescriptor { type Inner = CUnion[A] }]
diff --git a/core/src/fr/hammons/slinc/MacroHelpers.scala b/core/src/fr/hammons/slinc/MacroHelpers.scala
index cdce3bf5..2cc17915 100644
--- a/core/src/fr/hammons/slinc/MacroHelpers.scala
+++ b/core/src/fr/hammons/slinc/MacroHelpers.scala
@@ -1,8 +1,6 @@
 package fr.hammons.slinc
 
 import scala.quoted.*
-import scala.annotation.nowarn
-
 private[slinc] object MacroHelpers:
   def widenExpr(t: Expr[?])(using Quotes) =
     import quotes.reflect.*
diff --git a/core/src/fr/hammons/slinc/Mem.scala b/core/src/fr/hammons/slinc/Mem.scala
index e6c03e60..b2da4e97 100644
--- a/core/src/fr/hammons/slinc/Mem.scala
+++ b/core/src/fr/hammons/slinc/Mem.scala
@@ -1,7 +1,5 @@
 package fr.hammons.slinc
 
-import scala.annotation.nowarn
-
 trait Mem:
   import scala.compiletime.asMatchable
   def offset(bytes: Bytes): Mem
diff --git a/core/src/fr/hammons/slinc/MethodHandleTools.scala b/core/src/fr/hammons/slinc/MethodHandleTools.scala
index a8a47aa5..beaf07e8 100644
--- a/core/src/fr/hammons/slinc/MethodHandleTools.scala
+++ b/core/src/fr/hammons/slinc/MethodHandleTools.scala
@@ -3,7 +3,6 @@ package fr.hammons.slinc
 import scala.quoted.*
 import java.lang.invoke.MethodHandle
 import fr.hammons.slinc.modules.TransitionModule
-import scala.annotation.nowarn
 
 object MethodHandleTools:
   def exprNameMapping(expr: Expr[Any])(using Quotes): String =
diff --git a/core/src/fr/hammons/slinc/SlincImpl.scala b/core/src/fr/hammons/slinc/SlincImpl.scala
index 13c7226a..1185da53 100644
--- a/core/src/fr/hammons/slinc/SlincImpl.scala
+++ b/core/src/fr/hammons/slinc/SlincImpl.scala
@@ -2,7 +2,6 @@ package fr.hammons.slinc
 
 import scala.annotation.StaticAnnotation
 import scala.quoted.*
-import scala.annotation.nowarn
 
 class SlincImpl(val version: Int) extends StaticAnnotation
 
diff --git a/core/src/fr/hammons/slinc/Struct.scala b/core/src/fr/hammons/slinc/Struct.scala
index 26616aad..89c32a88 100644
--- a/core/src/fr/hammons/slinc/Struct.scala
+++ b/core/src/fr/hammons/slinc/Struct.scala
@@ -8,7 +8,6 @@ import fr.hammons.slinc.modules.TransitionModule
 import fr.hammons.slinc.modules.ReadWriteModule
 import fr.hammons.slinc.modules.Reader
 import fr.hammons.slinc.modules.Writer
-import scala.annotation.nowarn
 
 trait Struct[A <: Product] extends DescriptorOf[A]
 
@@ -110,12 +109,10 @@ object Struct:
         val reader = readGen[A]
         val writer = writeGen[A]
 
-        @nowarn("msg=unused implicit parameter")
         override val returnTransition = returnValue =>
           val mem = summon[TransitionModule].memReturn(returnValue)
           summon[ReadWriteModule].read(mem, Bytes(0), this)
 
-        @nowarn("msg=unused implicit parameter")
         override val argumentTransition = argument =>
           val mem = summon[Allocator].allocate(this, 1)
           summon[ReadWriteModule].write(mem, Bytes(0), this, argument)
diff --git a/core/src/fr/hammons/slinc/TypeDescriptor.scala b/core/src/fr/hammons/slinc/TypeDescriptor.scala
index 0107568d..4355e2e7 100644
--- a/core/src/fr/hammons/slinc/TypeDescriptor.scala
+++ b/core/src/fr/hammons/slinc/TypeDescriptor.scala
@@ -8,11 +8,11 @@ import fr.hammons.slinc.modules.{
   ArrayReader,
   readWriteModule
 }
-import scala.annotation.nowarn
 import scala.reflect.ClassTag
 import scala.quoted.*
 import fr.hammons.slinc.modules.TransitionModule
 import fr.hammons.slinc.modules.{ArgumentTransition, ReturnTransition}
+import scala.NonEmptyTuple
 
 /** Describes types used by C interop
   */
@@ -106,31 +106,23 @@ case object LongDescriptor extends BasicDescriptor:
 
 case object FloatDescriptor extends BasicDescriptor:
   type Inner = Float
-  @nowarn("msg=unused implicit parameter")
   val reader = readWriteModule.floatReader
-  @nowarn("msg=unused implicit parameter")
   val writer = readWriteModule.floatWriter
 
 case object DoubleDescriptor extends BasicDescriptor:
   type Inner = Double
-  @nowarn("msg=unused implicit parameter")
   val reader = readWriteModule.doubleReader
-  @nowarn("msg=unused implicit parameter")
   val writer = readWriteModule.doubleWriter
 
 case object PtrDescriptor extends TypeDescriptor:
   type Inner = Ptr[?]
-  @nowarn("msg=unused implicit parameter")
   override val reader = (mem, offset) =>
     Ptr(readWriteModule.memReader(mem, offset), Bytes(0))
-  @nowarn("msg=unused implicit parameter")
   override val writer = (mem, offset, a) =>
     readWriteModule.memWriter(mem, offset, a.mem)
 
-  @nowarn("msg=unused implicit parameter")
   override val argumentTransition = _.mem.asAddress
 
-  @nowarn("msg=unused implicit parameter")
   override val returnTransition = o =>
     Ptr[Any](summon[TransitionModule].addressReturn(o), Bytes(0))
 
@@ -174,11 +166,9 @@ case class AliasDescriptor[A](val real: TypeDescriptor) extends TypeDescriptor:
   val writer: (ReadWriteModule, DescriptorModule) ?=> Writer[Inner] =
     (rwm, _) ?=> (mem, bytes, a) => rwm.write(mem, bytes, real, a)
 
-  @nowarn("msg=unused implicit parameter")
   override val argumentTransition =
     summon[TransitionModule].methodArgument(real, _, summon[Allocator])
 
-  @nowarn("msg=unused implicit parameter")
   override val returnTransition = summon[TransitionModule].methodReturn(real, _)
   override def size(using dm: DescriptorModule): Bytes = dm.sizeOf(real)
   override def alignment(using dm: DescriptorModule): Bytes =
@@ -189,22 +179,36 @@ case class AliasDescriptor[A](val real: TypeDescriptor) extends TypeDescriptor:
 case object VaListDescriptor extends TypeDescriptor:
   type Inner = VarArgs
 
-  @nowarn(TypeDescriptor.unusedImplicit)
   override val reader: (ReadWriteModule, DescriptorModule) ?=> Reader[Inner] =
     (mem, offset) => summon[ReadWriteModule].memReader(mem, offset).asVarArgs
 
-  @nowarn(TypeDescriptor.unusedImplicit)
   override val argumentTransition
       : (TransitionModule, ReadWriteModule, Allocator) ?=> ArgumentTransition[
         Inner
       ] = _.mem.asAddress
 
-  @nowarn(TypeDescriptor.unusedImplicit)
   override val writer: (ReadWriteModule, DescriptorModule) ?=> Writer[Inner] =
     (mem, offset, value) =>
       summon[ReadWriteModule].memWriter(mem, offset, value.mem)
 
-  @nowarn(TypeDescriptor.unusedImplicit)
   override val returnTransition
       : (TransitionModule, ReadWriteModule) ?=> ReturnTransition[Inner] = o =>
     summon[TransitionModule].addressReturn(o).asVarArgs
+
+case class CUnionDescriptor(possibleTypes: Set[TypeDescriptor])
+    extends TypeDescriptor:
+  type Inner = CUnion[? <: NonEmptyTuple]
+
+  override val reader: (ReadWriteModule, DescriptorModule) ?=> Reader[Inner] =
+    ???
+
+  override val returnTransition
+      : (TransitionModule, ReadWriteModule) ?=> ReturnTransition[Inner] = ???
+
+  override val argumentTransition
+      : (TransitionModule, ReadWriteModule, Allocator) ?=> ArgumentTransition[
+        Inner
+      ] = ???
+
+  override val writer: (ReadWriteModule, DescriptorModule) ?=> Writer[Inner] =
+    ???
diff --git a/core/src/fr/hammons/slinc/types/Arch.scala b/core/src/fr/hammons/slinc/types/Arch.scala
index e817d941..a0d1b8f3 100644
--- a/core/src/fr/hammons/slinc/types/Arch.scala
+++ b/core/src/fr/hammons/slinc/types/Arch.scala
@@ -1,9 +1,5 @@
 package fr.hammons.slinc.types
 
-import scala.annotation.nowarn
-
-//todo: remove once https://github.com/lampepfl/dotty/issues/16878 is fixed
-@nowarn("msg=unused explicit parameter")
 private[slinc] enum Arch:
   case I386
   case X64
diff --git a/core/src/fr/hammons/slinc/types/OS.scala b/core/src/fr/hammons/slinc/types/OS.scala
index 22f63d4b..7a397336 100644
--- a/core/src/fr/hammons/slinc/types/OS.scala
+++ b/core/src/fr/hammons/slinc/types/OS.scala
@@ -1,9 +1,5 @@
 package fr.hammons.slinc.types
 
-import scala.annotation.nowarn
-
-//todo: remove once https://github.com/lampepfl/dotty/issues/16878 is fixed
-@nowarn("msg=unused explicit parameter")
 enum OS:
   case Linux
   case Darwin
diff --git a/core/test/src/fr/hammons/slinc/TypeDescriptorSpec.scala b/core/test/src/fr/hammons/slinc/TypeDescriptorSpec.scala
new file mode 100644
index 00000000..72717863
--- /dev/null
+++ b/core/test/src/fr/hammons/slinc/TypeDescriptorSpec.scala
@@ -0,0 +1,27 @@
+package fr.hammons.slinc
+import types.*
+
+class TypeDescriptorSpec extends munit.FunSuite:
+  case class A(a: CInt, b: CInt) derives Struct
+  test("DescriptorOf[CUnion[(Int, Float, A)]] gives appropriate descriptor"):
+      assertEquals(
+        DescriptorOf[CUnion[(CInt, CFloat, A)]]: TypeDescriptor,
+        CUnionDescriptor(
+          Set(IntDescriptor, FloatDescriptor, summon[Struct[A]].descriptor)
+        ): TypeDescriptor
+      )
+
+  test(
+    "DescriptorOf[CUnion[(CUnion[(Int, Float)], CUnion[(Int, Float)], A,A])]] doesn't double descriptors"
+  ):
+      assertEquals(
+        DescriptorOf[
+          CUnion[(CUnion[(Int, Float)], CUnion[(Int, Float)], A, A)]
+        ]: TypeDescriptor,
+        CUnionDescriptor(
+          Set(
+            CUnionDescriptor(Set(IntDescriptor, FloatDescriptor)),
+            summon[Struct[A]].descriptor
+          )
+        ): TypeDescriptor
+      )
diff --git a/core/test/src/fr/hammons/slinc/modules/DescriptorSpec.scala b/core/test/src/fr/hammons/slinc/modules/DescriptorSpec.scala
new file mode 100644
index 00000000..6b993839
--- /dev/null
+++ b/core/test/src/fr/hammons/slinc/modules/DescriptorSpec.scala
@@ -0,0 +1,25 @@
+package fr.hammons.slinc.modules
+
+import fr.hammons.slinc.Slinc
+import fr.hammons.slinc.DescriptorOf
+import fr.hammons.slinc.types.CLongLong
+import fr.hammons.slinc.types.CInt
+import fr.hammons.slinc.types.CFloat
+import fr.hammons.slinc.CUnion
+import fr.hammons.slinc.Struct
+
+trait DescriptorSpec(val slinc: Slinc) extends munit.FunSuite:
+  import slinc.dm
+  case class A(a: CInt, b: CInt, c: CInt, d: CLongLong, e: CLongLong)
+      derives Struct
+  test("CUnionDescriptor.size gives the right size"):
+      assertEquals(
+        DescriptorOf[CUnion[(CInt, A, CFloat)]].size,
+        DescriptorOf[A].size
+      )
+
+  test("CUnionDescriptor.alignment gives the right alignment"):
+      assertEquals(
+        DescriptorOf[CUnion[(CInt, A, CFloat)]].alignment,
+        DescriptorOf[A].alignment
+      )
diff --git a/j17/src/fr/hammons/slinc/Allocator17.scala b/j17/src/fr/hammons/slinc/Allocator17.scala
index fcba4f1e..7e5ae3a6 100644
--- a/j17/src/fr/hammons/slinc/Allocator17.scala
+++ b/j17/src/fr/hammons/slinc/Allocator17.scala
@@ -6,13 +6,12 @@ import jdk.incubator.foreign.{
   MemorySegment,
   ResourceScope,
   CLinker,
-  FunctionDescriptor as JFunctionDescriptor
+  FunctionDescriptor as JFunctionDescriptor,
+  GroupLayout
 }, CLinker.{C_POINTER, C_INT, C_LONG_LONG, C_DOUBLE, VaList}
 import fr.hammons.slinc.modules.{descriptorModule17, transitionModule17}
 import fr.hammons.slinc.modules.LinkageModule17
-import scala.annotation.nowarn
 
-@nowarn("msg=unused import")
 class Allocator17(
     segmentAllocator: SegmentAllocator,
     scope: ResourceScope,
@@ -60,6 +59,20 @@ class Allocator17(
             .asInstanceOf[Addressable]
         )
       )
+    case (cd: CUnionDescriptor, v: CUnion[?]) =>
+      builder.vargFromSegment(
+        descriptorModule17.toMemoryLayout(cd) match
+          case gl: GroupLayout => gl
+          case _ => throw Error("got a non group layout from CUnionDescriptor")
+        ,
+        v.mem.asBase match
+          case ms: MemorySegment => ms
+          case _ => throw Error("base of mem was not J17 MemorySegment!!")
+      )
+    case (a, d) =>
+      throw Error(
+        s"Unsupported type descriptor/data pairing for VarArgs: $a - $d"
+      )
 
   override def makeVarArgs(vbuilder: VarArgsBuilder): VarArgs =
     VarArgs17(
diff --git a/j17/src/fr/hammons/slinc/VarArgs17.scala b/j17/src/fr/hammons/slinc/VarArgs17.scala
index 6153fe5a..f34182c5 100644
--- a/j17/src/fr/hammons/slinc/VarArgs17.scala
+++ b/j17/src/fr/hammons/slinc/VarArgs17.scala
@@ -30,8 +30,9 @@ class VarArgs17(args: VaList) extends VarArgs:
             )
             .nn
         )
-      case AliasDescriptor(real) => get(real)
-      case VaListDescriptor      => args.vargAsAddress(C_POINTER).nn
+      case AliasDescriptor(real)           => get(real)
+      case VaListDescriptor                => args.vargAsAddress(C_POINTER).nn
+      case CUnionDescriptor(possibleTypes) => get(possibleTypes.maxBy(_.size))
   def get[A](using d: DescriptorOf[A]): A =
     transitionModule17.methodReturn[A](d.descriptor, get(d.descriptor))
 
@@ -46,8 +47,9 @@ class VarArgs17(args: VaList) extends VarArgs:
       case PtrDescriptor    => args.skip(C_POINTER)
       case sd: StructDescriptor =>
         args.skip(descriptorModule17.toGroupLayout(sd))
-      case AliasDescriptor(real) => skip(real)
-      case VaListDescriptor      => args.skip(C_POINTER)
+      case AliasDescriptor(real)           => skip(real)
+      case VaListDescriptor                => args.skip(C_POINTER)
+      case CUnionDescriptor(possibleTypes) => skip(possibleTypes.maxBy(_.size))
 
   def skip[A](using dO: DescriptorOf[A]): Unit = skip(dO.descriptor)
 
diff --git a/j17/src/fr/hammons/slinc/modules/DescriptorModule17.scala b/j17/src/fr/hammons/slinc/modules/DescriptorModule17.scala
index edd1ba48..0e3f0fef 100644
--- a/j17/src/fr/hammons/slinc/modules/DescriptorModule17.scala
+++ b/j17/src/fr/hammons/slinc/modules/DescriptorModule17.scala
@@ -9,8 +9,7 @@ import jdk.incubator.foreign.CLinker.{
   C_DOUBLE,
   C_FLOAT,
   C_LONG_LONG,
-  C_POINTER,
-  C_VA_LIST
+  C_POINTER
 }
 import jdk.incubator.foreign.{
   MemoryLayout,
@@ -18,7 +17,7 @@ import jdk.incubator.foreign.{
   MemorySegment,
   GroupLayout,
   CLinker
-}, CLinker.VaList
+}
 import scala.collection.concurrent.TrieMap
 
 given descriptorModule17: DescriptorModule with
@@ -36,6 +35,7 @@ given descriptorModule17: DescriptorModule with
     case _: StructDescriptor    => classOf[MemorySegment]
     case VaListDescriptor       => classOf[MemoryAddress]
     case ad: AliasDescriptor[?] => toCarrierType(ad.real)
+    case ud: CUnionDescriptor   => classOf[MemorySegment]
 
   def genLayoutList(
       layouts: Seq[MemoryLayout],
@@ -80,11 +80,14 @@ given descriptorModule17: DescriptorModule with
     case sd: StructDescriptor =>
       Bytes(toGroupLayout(sd).byteSize())
     case VaListDescriptor => Bytes(toMemoryLayout(VaListDescriptor).byteSize())
-    case ad: AliasDescriptor[?] => sizeOf(ad.real)
+    case ad: AliasDescriptor[?]          => sizeOf(ad.real)
+    case CUnionDescriptor(possibleTypes) => possibleTypes.map(sizeOf).max
 
   override def alignmentOf(td: TypeDescriptor): Bytes = td match
     case s: StructDescriptor =>
       s.members.view.map(_.descriptor).map(alignmentOf).max
+    case CUnionDescriptor(possibleTypes) =>
+      possibleTypes.view.map(alignmentOf).max
     case _ => sizeOf(td)
 
   override def memberOffsets(sd: List[TypeDescriptor]): IArray[Bytes] =
@@ -125,6 +128,8 @@ given descriptorModule17: DescriptorModule with
     case VaListDescriptor       => C_POINTER.nn
     case sd: StructDescriptor   => toGroupLayout(sd)
     case ad: AliasDescriptor[?] => toMemoryLayout(ad.real)
+    case CUnionDescriptor(possibleTypes) =>
+      MemoryLayout.unionLayout(possibleTypes.map(toMemoryLayout).toSeq*).nn
 
   def toMemoryLayout(smd: StructMemberDescriptor): MemoryLayout =
     toMemoryLayout(smd.descriptor).withName(smd.name).nn
diff --git a/j17/test/src/fr/hammons/slinc/modules/Descriptor17Spec.scala b/j17/test/src/fr/hammons/slinc/modules/Descriptor17Spec.scala
new file mode 100644
index 00000000..61d36566
--- /dev/null
+++ b/j17/test/src/fr/hammons/slinc/modules/Descriptor17Spec.scala
@@ -0,0 +1,5 @@
+package fr.hammons.slinc.modules
+
+import fr.hammons.slinc.Slinc17
+
+class Descriptor17Spec extends DescriptorSpec(Slinc17.default)
diff --git a/j19/src/fr/hammons/slinc/Allocator19.scala b/j19/src/fr/hammons/slinc/Allocator19.scala
index 2edf3484..1a2e5b21 100644
--- a/j19/src/fr/hammons/slinc/Allocator19.scala
+++ b/j19/src/fr/hammons/slinc/Allocator19.scala
@@ -10,6 +10,7 @@ import java.lang.foreign.ValueLayout
 import scala.jdk.FunctionConverters.*
 import java.lang.foreign.Addressable
 import java.lang.foreign.MemorySegment
+import java.lang.foreign.GroupLayout
 import fr.hammons.slinc.modules.LinkageModule19
 
 class Allocator19(
@@ -95,6 +96,19 @@ class Allocator19(
               .asInstanceOf[Addressable]
           )
         )
+      case (cUnionDescriptor: CUnionDescriptor, c: CUnion[?]) =>
+        builder.addVarg(
+          descriptorModule19.toMemoryLayout(cUnionDescriptor) match
+            case g: GroupLayout => g
+            case _ =>
+              throw Error("CUnionDescriptor didn't resolve to group layout??")
+          ,
+          c.mem.asBase match
+            case ms: MemorySegment => ms
+            case _                 => throw Error("Illegal datatype")
+        )
+      case (td, d) =>
+        throw Error(s"Unsupported datatype for $td - $d")
 
   override def makeVarArgs(vbuilder: VarArgsBuilder): VarArgs =
     import scala.compiletime.asMatchable
diff --git a/j19/src/fr/hammons/slinc/VarArgs19.scala b/j19/src/fr/hammons/slinc/VarArgs19.scala
index 4f6fdf4c..a161a498 100644
--- a/j19/src/fr/hammons/slinc/VarArgs19.scala
+++ b/j19/src/fr/hammons/slinc/VarArgs19.scala
@@ -28,6 +28,8 @@ private class VarArgs19(vaList: VaList) extends VarArgs:
     case PtrDescriptor | VaListDescriptor => vaList.skip(ValueLayout.ADDRESS)
     case sd: StructDescriptor =>
       vaList.skip(descriptorModule19.toGroupLayout(sd))
+    case cd: CUnionDescriptor =>
+      vaList.skip(descriptorModule19.toMemoryLayout(cd))
     case AliasDescriptor(real) => skip(real)
 
   override def skip[A](using dO: DescriptorOf[A]): Unit = skip(dO.descriptor)
@@ -61,6 +63,8 @@ private class VarArgs19(vaList: VaList) extends VarArgs:
         )
       case AliasDescriptor(real) => as(real)
       case VaListDescriptor      => vaList.nextVarg(ValueLayout.ADDRESS).nn
+      case CUnionDescriptor(possibleTypes) =>
+        as(possibleTypes.maxBy(_.size))
 
   override def get[A](using d: DescriptorOf[A]): A =
     transitionModule19.methodReturn[A](d.descriptor, as(d.descriptor))
diff --git a/j19/src/fr/hammons/slinc/modules/DescriptorModule19.scala b/j19/src/fr/hammons/slinc/modules/DescriptorModule19.scala
index db022315..ecad5a44 100644
--- a/j19/src/fr/hammons/slinc/modules/DescriptorModule19.scala
+++ b/j19/src/fr/hammons/slinc/modules/DescriptorModule19.scala
@@ -7,7 +7,6 @@ import java.lang.foreign.MemoryLayout
 import java.lang.foreign.MemoryAddress
 import java.lang.foreign.MemorySegment
 import java.lang.foreign.GroupLayout
-import java.lang.foreign.VaList
 
 given descriptorModule19: DescriptorModule with
   private val sdt = TrieMap.empty[StructDescriptor, GroupLayout]
@@ -24,6 +23,8 @@ given descriptorModule19: DescriptorModule with
     case VaListDescriptor       => ValueLayout.ADDRESS.nn
     case sd: StructDescriptor   => toGroupLayout(sd)
     case ad: AliasDescriptor[?] => toMemoryLayout(ad.real)
+    case CUnionDescriptor(possibleTypes) =>
+      MemoryLayout.unionLayout(possibleTypes.view.map(toMemoryLayout).toSeq*).nn
 
   def toMemoryLayout(smd: StructMemberDescriptor): MemoryLayout =
     toMemoryLayout(smd.descriptor).withName(smd.name).nn
@@ -85,6 +86,7 @@ given descriptorModule19: DescriptorModule with
     case PtrDescriptor          => classOf[MemoryAddress]
     case _: StructDescriptor    => classOf[MemorySegment]
     case ad: AliasDescriptor[?] => toCarrierType(ad.real)
+    case CUnionDescriptor(_)    => classOf[MemorySegment]
 
   override def memberOffsets(sd: List[TypeDescriptor]): IArray[Bytes] =
     offsets.getOrElseUpdate(
@@ -124,6 +126,7 @@ given descriptorModule19: DescriptorModule with
     case VaListDescriptor       => Bytes(ValueLayout.ADDRESS.nn.byteSize())
     case sd: StructDescriptor   => Bytes(toGroupLayout(sd).byteSize())
     case ad: AliasDescriptor[?] => sizeOf(ad.real)
+    case CUnionDescriptor(possibleTypes) => possibleTypes.view.map(sizeOf).max
 
   override def alignmentOf(td: TypeDescriptor): Bytes =
     import java.lang.foreign.ValueLayout
@@ -140,3 +143,5 @@ given descriptorModule19: DescriptorModule with
         sd.members.view.map(_.descriptor).map(alignmentOf).max
       case ad: AliasDescriptor[?] =>
         alignmentOf(ad.real)
+      case CUnionDescriptor(possibleTypes) =>
+        possibleTypes.view.map(alignmentOf).max
diff --git a/j19/test/src/fr/hammons/slinc/modules/Descriptor19Spec.scala b/j19/test/src/fr/hammons/slinc/modules/Descriptor19Spec.scala
new file mode 100644
index 00000000..473d056b
--- /dev/null
+++ b/j19/test/src/fr/hammons/slinc/modules/Descriptor19Spec.scala
@@ -0,0 +1,5 @@
+package fr.hammons.slinc.modules
+
+import fr.hammons.slinc.Slinc19
+
+class Descriptor19Spec extends DescriptorSpec(Slinc19.default)