Fix #10217: Avoid exponential behavior in derivesFrom

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -242,8 +242,9 @@ object Types {
           // If the type is `T | Null` or `T | Nothing`, and `T` derivesFrom the class,
           // then the OrType derivesFrom the class. Otherwise, we need to check both sides
           // derivesFrom the class.
-          if defn.isBottomType(tp.tp1) then loop(tp.tp2)
-          else loop(tp.tp1) && (defn.isBottomType(tp.tp2) || loop(tp.tp2))
+          loop(tp.tp1) && loop(tp.tp2)
+          || tp.tp1.isNullType && loop(tp.tp2)
+          || tp.tp2.isNullType && loop(tp.tp2)
         case tp: JavaArrayType =>
           cls == defn.ObjectClass
         case _ =>

tests/pos/i10217.scala

@@ -0,0 +1,27 @@
+trait A
+trait B
+trait C
+trait D
+trait E
+trait F
+trait G
+trait H
+trait I
+trait J
+trait K
+trait L
+trait M
+trait N
+trait O
+trait P
+trait Q
+trait R
+trait S
+trait T
+trait U
+trait V
+trait W
+
+class Foo[T]
+
+val f = Foo[A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U]