sbt.ExtractDependencies: avoid false dependencies

Type#member might return a denotation that doesn't "really exists" (as
defined by TypeAssigner#reallyExists), in some circumstance this
denotation can refer to a symbol in a class that is in the classpath but
that is not used by this file, so using addDependency on the result of
Type#member might add a false dependency. We avoid this by using
Type#select instead which will internally do the right thing.

This issue was discovered while compiling the bootstrapped projects
which would sometimes force a full recompilation for no reason.

Author: smarter

compiler/src/dotty/tools/dotc/sbt/ExtractDependencies.scala

@@ -175,7 +175,7 @@ private class ExtractDependenciesCollector(implicit val ctx: Context) extends tp
   override def traverse(tree: Tree)(implicit ctx: Context): Unit = {
     tree match {
       case Import(expr, selectors) =>
-        def lookupImported(name: Name) = expr.tpe.member(name).symbol
+        def lookupImported(name: Name) = expr.tpe.select(name).typeSymbol
         def addImported(name: Name) = {
           // importing a name means importing both a term and a type (if they exist)
           addDependency(lookupImported(name.toTermName))