Delay widening in derivedSelect when avoiding

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]`.

But also, widen when we're deriving to an intersection prefix...
Test cases: i16105,i16435,i2945,i8900,i8861.

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

Author: dwijnand

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

@@ -504,13 +504,15 @@ object TypeOps:
     override def derivedSelect(tp: NamedType, pre: Type) =
       if (pre eq tp.prefix)
         tp
-      else tryWiden(tp, tp.prefix).orElse {
+      else (if pre.isInstanceOf[AndType] then tryWiden(tp, tp.prefix) else NoType).orElse {
         if (tp.isTerm && variance > 0 && !pre.isSingleton)
           apply(tp.info.widenExpr)
         else if (upper(pre).member(tp.name).exists)
           super.derivedSelect(tp, pre)
-        else
+        else if (pre.isInstanceOf[AndType])
           range(defn.NothingType, defn.AnyType)
+        else
+          tryWiden(tp, tp.prefix).orElse(range(defn.NothingType, defn.AnyType))
       }
   end AvoidMap
 

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

@@ -5866,7 +5866,7 @@ object Types {
      *  underlying bounds to a range, otherwise return the expansion.
      */
     def expandParam(tp: NamedType, pre: Type): Type =
-      tp.argForParam(pre) match {
+      tp.argForParam(pre, widenAbstract = true) match {
         case arg @ TypeRef(pre, _) if pre.isArgPrefixOf(arg.symbol) =>
           arg.info match {
             case argInfo: TypeBounds => expandBounds(argInfo)

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.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