fix: better inference for function types when union with Null

Author: hamzaremmal

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

@@ -1932,7 +1932,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       case _ =>
     }
 
-    val (protoFormals, resultTpt) = decomposeProtoFunction(pt, params.length, tree.srcPos)
+    val (protoFormals, resultTpt) = decomposeProtoFunction(pt.stripNull(), params.length, tree.srcPos)
 
     /** Returns the type and whether the parameter is erased */
     def protoFormal(i: Int): (Type, Boolean) =

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)