Ignore virtual source files in Trees and Symbols

If a tree or symbol is created with a virtual source file,
use the context's source instead for erro reporting. Without
this tweak, replTests fails. It seems the REPL does not follow
the assumption that on tree or symbol creation the context always
has the right source file (including contents). We should fix
that, and then revert this commit.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Trees.scala

@@ -70,7 +70,9 @@ object Trees {
 
     def sourcePos(implicit ctx: Context): SourcePosition = {
       val src = source
-      val srcCtx = ctx.withSource(src)
+      val srcCtx =
+        if (src.file.isVirtual) ctx // don't trust virtual files for tree creation - replTest fails otherwise
+        else ctx.withSource(src)
       Decorators.sourcePos(pos)(srcCtx)
     }
 

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

@@ -659,7 +659,9 @@ object Symbols {
     final def sourcePos(implicit ctx: Context): SourcePosition = {
       val source = {
         val f = sourceFile
-        if (f == null) ctx.source
+        if (f == null || f.isVirtual)
+          // don't trust virtual files for tree creation - replTest fails otherwise
+          ctx.source
         else ctx.getSource(f)
       }
       SourcePosition(source, pos)