Tweak AvoidMap's derivedSelect (#16563)

Using the example from `pos/i16435.avoid`:

    class Foo:
      type Value
      def test: Option[Value] =
        val scr = {
          val self: Foo.this.type = this
          None: Option[self.Value]
        }
        scr

We want avoidance to return `Option[Value]`, aka
`Option[Foo.this.Value]`, rather than widening to `Option[Any]`.

Using test cases i16105,i16435,i2945,i8900,i8861 we don't tryWiden if
the derived prefix is a singleton.

And fix some kind of race condition in creating files/directories.

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

@@ -504,7 +504,7 @@ object TypeOps:
     override def derivedSelect(tp: NamedType, pre: Type) =
       if (pre eq tp.prefix)
         tp
-      else tryWiden(tp, tp.prefix).orElse {
+      else (if pre.isSingleton then NoType else tryWiden(tp, tp.prefix)).orElse {
         if (tp.isTerm && variance > 0 && !pre.isSingleton)
           apply(tp.info.widenExpr)
         else if (upper(pre).member(tp.name).exists)

compiler/src/dotty/tools/io/AbstractFile.scala

@@ -260,8 +260,10 @@ abstract class AbstractFile extends Iterable[AbstractFile] {
 
         // a race condition in creating the entry after the failed lookup may throw
         val path = jpath.resolve(name)
-        if (isDir) Files.createDirectory(path)
-        else Files.createFile(path)
+        try
+          if (isDir) Files.createDirectory(path)
+          else Files.createFile(path)
+        catch case _: FileAlreadyExistsException => ()
         new PlainFile(new File(path))
       case lookup => lookup
     }

tests/pos/i16435.TreeUnpickler.scala

@@ -0,0 +1,13 @@
+// Derived from the CI failure in compiling TreeUnpickler.scala
+class Foo
+class Bar extends Foo:
+  type Self >: this.type <: Bar
+  final def meth(): Self = this
+
+class Test:
+  def test(cond: Boolean, bar: Bar): Foo =
+    val res = bar
+    if cond then
+      res.meth()
+    else
+      res

tests/pos/i16435.avoid.scala

@@ -0,0 +1,8 @@
+class Foo:
+  type Value
+  def test: Option[Value] =
+    val scr = {
+      val self: Foo.this.type = this
+      None: Option[self.Value]
+    }
+    scr

tests/pos/i16435.scala

@@ -0,0 +1,11 @@
+// scalac: -Werror
+trait Base:
+  type Value
+  inline def oov: Option[Option[Value]] = None
+  def get: Option[Value]
+
+trait X extends Base:
+  override def get: Option[Value] =
+    oov match // was: match may not be exhaustive
+      case Some(ov) => ov
+      case None => None