Fix scala#4372: Add regression test

Probably fixed by scala#4385

Author: nicolasstucki

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

@@ -164,6 +164,7 @@ class CompilationTests extends ParallelTesting {
     compileFile("tests/neg-custom-args/i3627.scala", allowDeepSubtypes) +
     compileFile("tests/neg-custom-args/matchtype-loop.scala", allowDeepSubtypes) +
     compileFile("tests/neg-custom-args/completeFromSource/nested/Test1.scala", defaultOptions.and("-sourcepath", "tests/neg-custom-args", "-scansource")) +
+    compileFile("tests/neg-custom-args/i4372.scala", defaultOptions.without("-Yno-deep-subtypes")) +
     compileFile("tests/neg-custom-args/repeatedArgs213.scala", defaultOptions.and("-Ynew-collections"))
   }.checkExpectedErrors()
 

tests/neg-custom-args/i4372.scala

@@ -1,3 +1 @@
-object i4372 {
-  class X[A >: X[_ <: X[_]] <: X[A]] // error: too deep
-}
+class X[A >: X[_ <: X[_]] <: X[A]] // error