Fix type mismatch error confusion between types with same simple name (#19204)

Fixes #18678 

Accidentally closed the original PR for this here: #19088

Author: nicolasstucki

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

@@ -193,7 +193,7 @@ class PlainPrinter(_ctx: Context) extends Printer {
         "<overloaded " ~ toTextRef(tp) ~ ">"
       case tp: TypeRef =>
         if (printWithoutPrefix.contains(tp.symbol))
-          toText(tp.name)
+          selectionString(tp)
         else
           toTextPrefixOf(tp) ~ selectionString(tp)
       case tp: TermParamRef =>

compiler/src/dotty/tools/dotc/reporting/Message.scala

@@ -88,7 +88,7 @@ object Message:
       def followAlias(e1: Recorded): Recorded = e1 match {
         case e1: Symbol if e1.isAliasType =>
           val underlying = e1.typeRef.underlyingClassRef(refinementOK = false).typeSymbol
-          if (underlying.name == e1.name) underlying else e1
+          if (underlying.name == e1.name) underlying else e1.namedType.dealias.typeSymbol
         case _ => e1
       }
       val key = SeenKey(str, isType)

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