Try to avoid static symbols if leaving them would make a leak

Possible fix for #9314

Typer.ensureNoLocalRefs doesn't avoid local static refs like anonymous
class instances to allow typing more specific expected types with e.g. refinements.
Unfortunately this can cause leaks when expected types have to be
inferred. This PR tries to fix this problem by adding a fallback that
avoids all local symbols if this leak were to happen.

Author: KacperFKorban

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

@@ -507,12 +507,12 @@ object TypeOps:
    *  does not update `ctx.nestingLevel` when entering a block so I'm leaving
    *  this as Future Work™.
    */
-  def avoid(tp: Type, symsToAvoid: => List[Symbol])(using Context): Type = {
+  def avoid(tp: Type, symsToAvoid: => List[Symbol], avoidStatic: Boolean = false)(using Context): Type = {
     val widenMap = new AvoidMap {
       @threadUnsafe lazy val forbidden = symsToAvoid.toSet
       def toAvoid(tp: NamedType) =
         val sym = tp.symbol
-        !sym.isStatic && forbidden.contains(sym)
+        (!sym.isStatic || avoidStatic) && forbidden.contains(sym)
 
       override def apply(tp: Type): Type = tp match
         case tp: TypeVar if mapCtx.typerState.constraint.contains(tp) =>

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

@@ -1115,8 +1115,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       fullyDefinedType(tree.tpe, "block", tree.srcPos)
       var avoidingType = TypeOps.avoid(tree.tpe, localSyms)
       val ptDefined = isFullyDefined(pt, ForceDegree.none)
-      if (ptDefined && !(avoidingType.widenExpr <:< pt)) avoidingType = pt
-      val tree1 = ascribeType(tree, avoidingType)
+      var tree1 = ascribeType(tree, avoidingType)
+      if ptDefined && !(avoidingType.widenExpr <:< pt) then
+        tree1 = ascribeType(tree, pt)
+      else if !noLeaks(tree1) then
+        tree1 = ascribeType(tree, TypeOps.avoid(tree.tpe, localSyms, avoidStatic = true))
       assert(ptDefined || noLeaks(tree1) || tree1.tpe.isErroneous,
           // `ptDefined` needed because of special case of anonymous classes
           i"leak: ${escapingRefs(tree1, localSyms).toList}%, % in $tree1")

tests/pos/i9314.scala

@@ -0,0 +1,4 @@
+final class fooAnnot[T](member: T) extends scala.annotation.StaticAnnotation // must have type parameter
+
+@fooAnnot(new RecAnnotated {}) // must pass instance of anonymous subclass
+trait RecAnnotated