Tasty: Fix unpickling position of SHARED nodes

Author: smarter

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -1090,7 +1090,19 @@ class TreeUnpickler(reader: TastyReader,
       if (ctx.mode.is(Mode.ReadPositions)) {
         posUnpicklerOpt match {
           case Some(posUnpickler) =>
-            posUnpickler.posAt(addr)
+            val addrPos = posUnpickler.posAt(addr)
+            if (addrPos == NoPosition && bytes(index(addr)) == SHARED) {
+              // Note: A tree can only be represented by a SHARED node if it's
+              // identical to some other tree, so you might wonder why we even
+              // bother checking addrPos for such nodes. It's because TypeTrees
+              // can be pickled as just a type and still have their own position,
+              // even if the type is just a SHARED node.
+
+              val sharedFork = forkAt(addr)
+              sharedFork.reader.readByte()
+              posAt(sharedFork.reader.readAddr())
+            }
+            else addrPos
           case _  =>
             NoPosition
         }