Better position and associated file for copied symbols

When a copy of a symbol is created with a new owner, the position and
associated file used to be kept as-is, but this can lead to confusing
error messages referencing the original definition which might be
unrelated to the current error, instead we now copy these fields from
the new owner.

Author: smarter

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

@@ -797,19 +797,27 @@ object Symbols {
   NoDenotation // force it in order to set `denot` field of NoSymbol
 
   implicit class Copier[N <: Name](sym: Symbol { type ThisName = N })(implicit ctx: Context) {
-    /** Copy a symbol, overriding selective fields */
+    /** Copy a symbol, overriding selective fields.
+     *  Note that `coord` and `associatedFile` will be set from the fields in `owner`, not
+     *  the fields in `sym`.
+     */
     def copy(
         owner: Symbol = sym.owner,
         name: N = (sym.name: N), // Dotty deviation: type ascription to avoid leaking private sym (only happens in unpickling), won't be needed once #1723 is fixed
         flags: FlagSet = sym.flags,
         info: Type = sym.info,
         privateWithin: Symbol = sym.privateWithin,
-        coord: Coord = sym.coord,
-        associatedFile: AbstractFile = sym.associatedFile): Symbol =
+        coord: Coord = NoCoord, // Can be `= owner.coord` once we boostrap
+        associatedFile: AbstractFile = null // Can be `= owner.associatedFile` once we boostrap
+    ): Symbol = {
+      val coord1 = if (coord == NoCoord) owner.coord else coord
+      val associatedFile1 = if (associatedFile == null) owner.associatedFile else associatedFile
+
       if (sym.isClass)
-        ctx.newClassSymbol(owner, name.asTypeName, flags, _ => info, privateWithin, coord, associatedFile)
+        ctx.newClassSymbol(owner, name.asTypeName, flags, _ => info, privateWithin, coord1, associatedFile1)
       else
-        ctx.newSymbol(owner, name, flags, info, privateWithin, coord)
+        ctx.newSymbol(owner, name, flags, info, privateWithin, coord1)
+    }
   }
 
   /** Makes all denotation operations available on symbols */