Improve ClassTag handling to avoid invalid ClassTag generation and inference failure (#16492)

The first commit generalizes the fix for #1730, the second commit
improves inference in a common situation to generate a valid ClassTag
rather than emitting an error, fixing the example from
#16328 (comment).

Author: odersky

compiler/src/dotty/tools/dotc/typer/Synthesizer.scala

@@ -28,13 +28,25 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
   private type SpecialHandlers = List[(ClassSymbol, SpecialHandler)]
 
   val synthesizedClassTag: SpecialHandler = (formal, span) =>
+    def instArg(tp: Type): Type = tp.stripTypeVar match
+      // Special case to avoid instantiating `Int & S` to `Int & Nothing` in
+      // i16328.scala. The intersection comes from an earlier instantiation
+      // to an upper bound.
+      // The dual situation with unions is harder to trigger because lower
+      // bounds are usually widened during instantiation.
+      case tp: AndOrType if tp.tp1 =:= tp.tp2 =>
+        instArg(tp.tp1)
+      case _ =>
+        if isFullyDefined(tp, ForceDegree.all) then tp
+        else NoType // this happens in tests/neg/i15372.scala
+
     val tag = formal.argInfos match
-      case arg :: Nil if isFullyDefined(arg, ForceDegree.all) =>
-        arg match
+      case arg :: Nil =>
+        instArg(arg) match
           case defn.ArrayOf(elemTp) =>
             val etag = typer.inferImplicitArg(defn.ClassTagClass.typeRef.appliedTo(elemTp), span)
             if etag.tpe.isError then EmptyTree else etag.select(nme.wrap)
-          case tp if hasStableErasure(tp) && !defn.isBottomClassAfterErasure(tp.typeSymbol) =>
+          case tp if hasStableErasure(tp) && !tp.isBottomTypeAfterErasure =>
             val sym = tp.typeSymbol
             val classTagModul = ref(defn.ClassTagModule)
             if defn.SpecialClassTagClasses.contains(sym) then

tests/neg/i1730.scala

@@ -0,0 +1,7 @@
+import scala.reflect.ClassTag
+
+@main def Test =
+  val x: Array[? <: String] = Array[Int & Nothing]() // error: No ClassTag available for Int & Nothing
+                                                     // (was: ClassCastException: [I cannot be cast to [Ljava.lang.String)
+  val y: Array[? <: Int] = Array[String & Nothing]() // error: No ClassTag available for String & Nothing
+                                                     // (was: ClassCastException: [Lscala.runtime.Nothing$; cannot be cast to [I)

tests/pos/i16328.scala

@@ -0,0 +1,19 @@
+import scala.reflect.ClassTag
+
+object Test {
+  def getParamType[T: ClassTag](x: T => Int): T = ???
+
+  def id[S](x: S): S = x
+
+  def main(args: Array[String]) = {
+    // worked before
+    val a1 = getParamType((x: Int) => x)
+    val a2: Int = a1 // ensure that we actually got a ClassTag for the right type
+
+    // broken before
+    val b1 = id(getParamType((x: Int) => x)) // was error
+    val b2: Int = b1
+    val c1 = id(id(getParamType((x: Int) => x))) // was error
+    val c2: Int = c1
+  }
+}