SI-8892 Fix incorrect qualification in error message

Since beadafa, we've ended up with nutty error messages for
type errors involving aliases that target types in `java.lang` and
`scala` packages. I think the intent of that change was to force
the error messages to qualify types like `String` when needed, but
to leave them unqualified by default.

However, this led to this flat out wrong message in the enclosed test.

	 found   : B
	 required: C.this.java.lang.B
	    (which expands to)  String

I've changed the heuristic slightly limit this code to aliases
that are eponymous with their targets. Still feels pretty kludgy, but
we can at least book a little progress.

Author: retronym

src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala

@@ -318,7 +318,7 @@ trait TypeDiagnostics {
      */
     def qualifyDefaultNamespaces() = {
       val intersect = Set(trueOwner, aliasOwner) intersect UnqualifiedOwners
-      if (intersect.nonEmpty) preQualify()
+      if (intersect.nonEmpty && tp.typeSymbolDirect.name == tp.typeSymbol.name) preQualify()
     }
 
     // functions to manipulate the name

test/files/neg/t8892.check

@@ -0,0 +1,7 @@
+t8892.scala:2: error: type mismatch;
+ found   : B
+ required: C.this.B
+    (which expands to)  String
+class C[B](x: B) extends A { def f: B = x }
+                                        ^
+one error found

test/files/neg/t8892.scala

@@ -0,0 +1,2 @@
+trait A { type B = String }
+class C[B](x: B) extends A { def f: B = x }