fix(lift-webkit): fix Scala 3 snippet instantiation with type-safe reflection

Resolves snippet class instantiation failures in Scala 3.3.6 that were
causing ClassCastException when constructing snippet instances via reflection.

Root Cause:
Scala 3's stricter type inference would default generic type parameter T
to Nothing when calling makeOne[T] without explicit type context, causing
attempts to cast snippet instances to scala.runtime.Nothing$ which failed.

Solution:
- Changed ConstructorType.makeOne methods to return AnyRef instead of T
- Added targetClass parameter to ConstructorType trait and implementations
- Used Class.cast() for runtime type verification before final cast
- Moved final asInstanceOf[T] to call site where type context is available
- Fixed variable shadowing issue (paramClz vs clz) in constructFrom
- Enhanced error handling for ClassCastException and InvocationTargetException

The fix uses Class.cast() which performs proper runtime type checking,
making the code more type-safe while working correctly in both Scala 2.13
and Scala 3.3.6.

Tests Fixed:
- OneShot: all 6 tests now pass (snippet callbacks execute correctly)
- ToHeadUsages: all 12 tests now pass (snippet rendering works)
- WebSpecSpec: template processing test un-pended and passing
- Full test suite: 281 tests pass on Scala 3.3.6 (previously had failures)
- No regressions: 282 tests pass on Scala 2.13 (unchanged)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: farmdawgnation

web/webkit/src/main/scala/net/liftweb/http/LiftSession.scala

@@ -114,32 +114,32 @@ object LiftSession {
       val const = clz.getDeclaredConstructors()
 
       def nullConstructor(): Box[ConstructorType] =
-        const.find(_.getParameterTypes.length == 0).map(const => UnitConstructor(const))
+        const.find(_.getParameterTypes.length == 0).map(const => UnitConstructor(const, clz))
 
       pp match {
-        case Full(ParamPair(value, clz)) =>
+        case Full(ParamPair(value, paramClz)) =>
           const.find {
             cp => {
               cp.getParameterTypes.length == 2 &&
-                cp.getParameterTypes().apply(0).isAssignableFrom(clz) &&
+                cp.getParameterTypes().apply(0).isAssignableFrom(paramClz) &&
                 cp.getParameterTypes().apply(1).isAssignableFrom(classOf[LiftSession])
             }
           }.
-            map(const => PAndSessionConstructor(const)) orElse
+            map(const => PAndSessionConstructor(const, clz)) orElse
             const.find {
               cp => {
                 cp.getParameterTypes.length == 1 &&
-                  cp.getParameterTypes().apply(0).isAssignableFrom(clz)
+                  cp.getParameterTypes().apply(0).isAssignableFrom(paramClz)
               }
             }.
-              map(const => PConstructor(const)) orElse nullConstructor()
+              map(const => PConstructor(const, clz)) orElse nullConstructor()
 
         case _ =>
           nullConstructor()
       }
     }
 
-    (if (Props.devMode) {
+    val constructorBox = if (Props.devMode) {
       // no caching in dev mode
       calcConstructor()
     } else {
@@ -152,10 +152,12 @@ object LiftSession {
           nv
         }
       }
-    }).map {
-      case uc: UnitConstructor => uc.makeOne
-      case pc: PConstructor => pc.makeOne(pp.openOrThrowException("It's ok").v)
-      case psc: PAndSessionConstructor => psc.makeOne(pp.openOrThrowException("It's ok").v, session)
+    }
+
+    constructorBox.map {
+      case uc: UnitConstructor => uc.makeOne.asInstanceOf[T]
+      case pc: PConstructor => pc.makeOne(pp.openOrThrowException("It's ok").v).asInstanceOf[T]
+      case psc: PAndSessionConstructor => psc.makeOne(pp.openOrThrowException("It's ok").v, session).asInstanceOf[T]
     }
   }
 
@@ -1355,7 +1357,15 @@ class LiftSession(private[http] val _contextPath: String, val underlyingId: Stri
         c)
 
     } catch {
-      case e: IllegalAccessException => Empty
+      case e: IllegalAccessException =>
+        logger.debug(s"IllegalAccessException instantiating ${c.getName}", e)
+        Empty
+      case e: ClassCastException =>
+        logger.warn(s"ClassCastException instantiating ${c.getName} - reflection type mismatch", e)
+        Empty
+      case e: java.lang.reflect.InvocationTargetException =>
+        logger.warn(s"InvocationTargetException instantiating ${c.getName}", e.getCause)
+        Empty
     }
   }
 

web/webkit/src/main/scala/net/liftweb/http/Templates.scala

@@ -339,28 +339,34 @@ class StateInStatelessException(msg: String) extends SnippetFailureException(msg
   /**
    * a trait that defines some ways of constructing an instance
    */
-  private sealed trait ConstructorType
+  private sealed trait ConstructorType {
+    def targetClass: Class[_]
+  }
 
   /**
    * A unit constructor... just pass in null
    */
-  private final case class UnitConstructor(c: java.lang.reflect.Constructor[_]) extends ConstructorType {
-    def makeOne[T]: T = c.newInstance().asInstanceOf[T]
+  private final case class UnitConstructor(c: java.lang.reflect.Constructor[_], targetClass: Class[_]) extends ConstructorType {
+    def makeOne: AnyRef = {
+      targetClass.asInstanceOf[Class[AnyRef]].cast(c.newInstance().asInstanceOf[Object])
+    }
   }
 
   /**
    * A parameter and session constructor
    */
-  private final case class PAndSessionConstructor(c: java.lang.reflect.Constructor[_]) extends ConstructorType {
-    def makeOne[T](p: Any, s: LiftSession): T =
-      c.newInstance(p.asInstanceOf[Object], s).asInstanceOf[T]
+  private final case class PAndSessionConstructor(c: java.lang.reflect.Constructor[_], targetClass: Class[_]) extends ConstructorType {
+    def makeOne(p: Any, s: LiftSession): AnyRef = {
+      targetClass.asInstanceOf[Class[AnyRef]].cast(c.newInstance(p.asInstanceOf[Object], s).asInstanceOf[Object])
+    }
   }
 
   /**
    * A parameter constructor
    */
-  private final case class PConstructor(c: java.lang.reflect.Constructor[_]) extends ConstructorType {
-    def makeOne[T](p: Any): T =
-      c.newInstance(p.asInstanceOf[Object]).asInstanceOf[T]
+  private final case class PConstructor(c: java.lang.reflect.Constructor[_], targetClass: Class[_]) extends ConstructorType {
+    def makeOne(p: Any): AnyRef = {
+      targetClass.asInstanceOf[Class[AnyRef]].cast(c.newInstance(p.asInstanceOf[Object]).asInstanceOf[Object])
+    }
   }
 

web/webkit/src/test/scala-3/net/liftweb/mockweb/WebSpecSpec.scala

@@ -153,9 +153,9 @@ class WebSpecSpec extends WebSpec(WebSpecSpecBoot.boot _) {
       })
     }
 
-    "properly process a template" in {
-      // FIXME: Scala 3 has ClassCastException with snippet instantiation
-      pending("Scala 3 snippet instantiation issue - ClassCastException during snippet loading")
+    "properly process a template" withTemplateFor("http://foo.com/net/liftweb/mockweb/webspecspectemplate") in {
+      case Full(template) => template.toString.contains("Hello, WebSpec!") === true
+      case other =>          failure("Error on template : " + other)
     }
   }
 }