fix: better inference for function types when union with Null

Author: hamzaremmal

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

@@ -1922,7 +1922,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         NoType
     }
 
-    pt match {
+    pt.stripNull() match {
       case pt: TypeVar
       if untpd.isFunctionWithUnknownParamType(tree) && !calleeType.exists =>
         // try to instantiate `pt` if this is possible. If it does not

tests/pos/i24206.scala

@@ -13,4 +13,7 @@ class DispatchQuery:
 trait Result
 def getAll(nameFilter: Option[String => Boolean]): List[Result] = ???
 def get(collectionName: String): List[Result] =
-    getAll(Option(_.startsWith(collectionName)))
+    getAll(Option(_.startsWith(collectionName)))
+
+def f[T](x: T | Null): T = ???
+val _: Any => Any = f(x => x)