Improve type mismatch errors with fully qualified names for types within the scala package

Author: ausmarton

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -192,9 +192,6 @@ class PlainPrinter(_ctx: Context) extends Printer {
       case tp: TermRef if tp.denot.isOverloaded =>
         "<overloaded " ~ toTextRef(tp) ~ ">"
       case tp: TypeRef =>
-        if (printWithoutPrefix.contains(tp.symbol))
-          toText(tp.name)
-        else
           toTextPrefixOf(tp) ~ selectionString(tp)
       case tp: TermParamRef =>
         ParamRefNameString(tp) ~ lambdaHash(tp.binder) ~ ".type"

tests/neg/i18678b.check

@@ -0,0 +1,10 @@
+-- [E007] Type Mismatch Error: tests/neg/i18678b.scala:3:16 --------------------
+3 |def s: String = "" // error
+  |                ^^
+  |                Found:    ("" : String)
+  |                Required: String²
+  |
+  |                where:    String  is a class in package java.lang
+  |                          String² is a class in the empty package
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i18678b.scala

@@ -0,0 +1,3 @@
+class String
+
+def s: String = "" // error