SI-8892 Fix incorrect qualification in error message

Since beadafa2, 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.

3 files changed, 10 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala b/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
index 65dea3178b..5f2643cb25 100644
--- a/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
+++ b/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
diff --git a/test/files/neg/t8892.check b/test/files/neg/t8892.check
new file mode 100644
index 0000000000..5930be58c5
--- /dev/null
+++ b/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
diff --git a/test/files/neg/t8892.scala b/test/files/neg/t8892.scala
new file mode 100644
index 0000000000..f857e6f115
--- /dev/null
+++ b/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 }