Fix #1442: add new phase, SelectStatic

GenBCode has an implicit assumption that I wasn't aware of:
  GetStatic should not be emitted against a valid selector.
If it is, GenBCode messes up the stack by not pop-ing the selector.

Surprisingly, this transformation is perfumed in nsc by flatten.

Author: DarkDimius

src/dotty/tools/dotc/Compiler.scala

@@ -91,6 +91,7 @@ class Compiler {
            new Flatten,             // Lift all inner classes to package scope
            new RestoreScopes),      // Repair scopes rendered invalid by moving definitions in prior phases of the group
       List(new ExpandPrivate,       // Widen private definitions accessed from nested classes
+           new SelectStatic,        // get rid of selects that would be compiled into GetStatic
            new CollectEntryPoints,  // Find classes with main methods
            new CollectSuperCalls,   // Find classes that are called with super
            new MoveStatics,         // Move static methods to companion classes

src/dotty/tools/dotc/transform/SelectStatic.scala

@@ -0,0 +1,37 @@
+package dotty.tools.dotc
+package transform
+
+import dotty.tools.dotc.ast.tpd
+import dotty.tools.dotc.core.Contexts.Context
+import dotty.tools.dotc.core.DenotTransformers.IdentityDenotTransformer
+import dotty.tools.dotc.core.Flags._
+import dotty.tools.dotc.core.Symbols._
+import dotty.tools.dotc.core._
+import dotty.tools.dotc.transform.TreeTransforms._
+
+/** Removes selects that would be compiled into GetStatic
+ * otherwise backend needs to be aware that some qualifiers need to be dropped.
+ * Similar transformation seems to be performed by flatten in nsc
+ *
+ * @author Dmytro Petrashko
+ */
+class SelectStatic extends MiniPhaseTransform with IdentityDenotTransformer { thisTransform =>
+  import ast.tpd._
+
+  override def phaseName: String = "selectStatic"
+  private val isPackage = FlagConjunction(PackageCreationFlags.bits)
+
+  override def transformSelect(tree: tpd.Select)(implicit ctx: Context, info: TransformerInfo): tpd.Tree = {
+    val sym = tree.symbol
+    if (!sym.is(isPackage) && !sym.owner.is(isPackage) &&
+      (
+         ((sym is Flags.Module) && sym.owner.isStaticOwner) ||
+         (sym is Flags.JavaStatic) ||
+         (sym.owner is Flags.ImplClass) ||
+         sym.hasAnnotation(ctx.definitions.ScalaStaticAnnot)
+        )
+    )
+      Block(List(tree.qualifier), ref(sym))
+    else tree
+  }
+}