enable ReadPositions when Ysemanticdb is set

Author: bishabosha

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -146,6 +146,11 @@ class Compiler {
 
   def newRun(implicit ctx: Context): Run = {
     reset()
-    new Run(this, ctx)
+    val rctx =
+      if ctx.settings.Ysemanticdb.value
+        ctx.addMode(Mode.ReadPositions)
+      else
+        ctx
+    new Run(this, rctx)
   }
 }

tests/neg/mixin-forwarder-clash2.check

@@ -3,6 +3,6 @@
 1 |class Bar2 extends Bar1 with Two[Foo] // error
   |      ^
   |      Name clash between inherited members:
-  |      def concat(suffix: Int): X in trait One and
-  |      def concat: [Dummy](suffix: Int): Y in trait Two
+  |      def concat(suffix: Int): X in trait One at line 4 and
+  |      def concat: [Dummy](suffix: Int): Y in trait Two at line 8
   |      have the same type after erasure.