Do not search contextual function type in union types in general (#24393)

Being not able to assign `null` to `((a: String) ?=> Int) | Null` seems
weird to me.

The example is from
https://users.scala-lang.org/t/type-a-parameter-as-either-a-contextual-function-null

Author: noti0na1

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -2002,11 +2002,13 @@ class Definitions {
    *   - the upper bound of a TypeParamRef in the current constraint
    */
   def asContextFunctionType(tp: Type)(using Context): Type =
-    tp.stripNull().stripTypeVar.dealias match
+    tp.stripTypeVar.dealias match
       case tp1: TypeParamRef if ctx.typerState.constraint.contains(tp1) =>
         asContextFunctionType(TypeComparer.bounds(tp1).hiBound)
       case tp1 @ PolyFunctionOf(mt: MethodType) if mt.isContextualMethod =>
         tp1
+      case tp: FlexibleType =>
+        asContextFunctionType(tp.hi)
       case tp1 =>
         if tp1.typeSymbol.name.isContextFunction && isFunctionNType(tp1) then tp1
         else NoType

tests/explicit-nulls/pos/union-contains-contextual-function.scala

@@ -0,0 +1,9 @@
+class Test:
+  val x: ((a: String) ?=> Int) | Null = null
+
+  val x2: ((a: String) ?=> Int) | Null = (s: String) ?=> 42
+
+  val y: ((a: String) ?=> Int) | String = "hello"
+
+  def f(g: ((a: String) ?=> Int) | Null = null) = ???
+