Merge pull request #5929 from dotty-staging/test-#5810

Regression test for #5810

Author: odersky

tests/neg/i5810.scala

@@ -0,0 +1,5 @@
+import language.strictEquality
+def f[T](x: T) =
+  if (x == null) ???        // error: cannot be compared
+  else if (x == "abc") ???  // error: cannot be compared
+  else ???