scala#204 check for global uniqueness of definitions.

Check now fails after erasure.
Should become error after fixed.

Author: DarkDimius

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

@@ -67,19 +67,28 @@ class TreeChecker {
 
   class Checker(phasesToCheck: Seq[Phase]) extends ReTyper {
 
-    val definedSyms = new mutable.HashSet[Symbol]
-
-    def withDefinedSym[T](tree: untpd.Tree)(op: => T)(implicit ctx: Context): T = {
-      if (tree.isDef) {
-        assert(!definedSyms.contains(tree.symbol), i"doubly defined symbol: ${tree.symbol}in $tree")
-        definedSyms += tree.symbol
+    val nowDefinedSyms = new mutable.HashSet[Symbol]
+    val everDefinedSyms = new mutable.HashMap[Symbol, Tree]
+
+    def withDefinedSym[T](tree: untpd.Tree)(op: => T)(implicit ctx: Context): T = tree match {
+      case tree: DefTree =>
+        val sym = tree.symbol
+        everDefinedSyms.get(sym) match {
+          case Some(t)  =>
+            if(t ne tree)
+              ctx.warning(i"symbol ${tree.symbol} is defined at least twice in different parts of AST")
+            // should become an error
+          case None =>
+            everDefinedSyms(sym) = tree
+        }
+        assert(!nowDefinedSyms.contains(tree.symbol), i"doubly defined symbol: ${tree.symbol}in $tree")
+        nowDefinedSyms += tree.symbol
         //println(i"defined: ${tree.symbol}")
         val res = op
-        definedSyms -= tree.symbol
+        nowDefinedSyms -= tree.symbol
         //println(i"undefined: ${tree.symbol}")
         res
-      }
-      else op
+      case _ => op
     }
 
     def withDefinedSyms[T](trees: List[untpd.Tree])(op: => T)(implicit ctx: Context) =
@@ -90,7 +99,7 @@ class TreeChecker {
 
     def assertDefined(tree: untpd.Tree)(implicit ctx: Context) =
       if (tree.symbol.maybeOwner.isTerm)
-        assert(definedSyms contains tree.symbol, i"undefined symbol ${tree.symbol}")
+        assert(nowDefinedSyms contains tree.symbol, i"undefined symbol ${tree.symbol}")
 
     override def typedUnadapted(tree: untpd.Tree, pt: Type)(implicit ctx: Context): tpd.Tree = {
       val res = tree match {