Fix #2230: Add regression test.

Author: nicolasstucki

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -151,6 +151,7 @@ class CompilationTests extends ParallelSummaryReport with ParallelTesting {
     compileFile("../tests/neg/customArgs/nopredef.scala", defaultOptions.and("-Yno-predef")) +
     compileFile("../tests/neg/customArgs/noimports.scala", defaultOptions.and("-Yno-imports")) +
     compileFile("../tests/neg/customArgs/noimports2.scala", defaultOptions.and("-Yno-imports")) +
+    compileFile("../tests/neg/customArgs/overloadsOnAbstractTypes.scala", allowDoubleBindings) +
     compileFile("../tests/neg/tailcall/t1672b.scala", defaultOptions) +
     compileFile("../tests/neg/tailcall/t3275.scala", defaultOptions) +
     compileFile("../tests/neg/tailcall/t6574.scala", defaultOptions) +

tests/neg/customArgs/overloadsOnAbstractTypes.scala

@@ -0,0 +1,22 @@
+
+class Test1 {
+  type A
+  type B <: A
+
+  def foo(): A = ???
+  def foo(): A = ??? // error
+
+  def bar(): A = ???
+  def bar(): B = ??? // error
+}
+
+class Test2 {
+  type A
+  type B <: A
+
+  def foo(x: A) = ???
+  def foo(x: A) = ??? // error
+
+  def bar(x: A) = ???
+  def bar(x: B) = ??? // error
+}