Fix treatment of bottom types in OrType#join and baseType computations

Author: odersky

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

@@ -2098,25 +2098,27 @@ object SymDenotations {
             computeTypeProxy
 
           case tp: AndOrType =>
-            def computeAndOrType = {
+            def computeAndOrType: Type =
               val tp1 = tp.tp1
               val tp2 = tp.tp2
+              if !tp.isAnd then
+                if tp1.isBottomType && (tp1 frozen_<:< tp2) then return recur(tp2)
+                if tp2.isBottomType && (tp2 frozen_<:< tp1) then return recur(tp1)
               val baseTp =
-                if (symbol.isStatic && tp.derivesFrom(symbol) && symbol.typeParams.isEmpty)
+                if symbol.isStatic && tp.derivesFrom(symbol) && symbol.typeParams.isEmpty then
                   symbol.typeRef
-                else {
+                else
                   val baseTp1 = recur(tp1)
                   val baseTp2 = recur(tp2)
                   val combined = if (tp.isAnd) baseTp1 & baseTp2 else baseTp1 | baseTp2
-                  combined match {
+                  combined match
                     case combined: AndOrType
                     if (combined.tp1 eq tp1) && (combined.tp2 eq tp2) && (combined.isAnd == tp.isAnd) => tp
                     case _ => combined
-                  }
-                }
+
               if (baseTp.exists && inCache(tp1) && inCache(tp2)) record(tp, baseTp)
               baseTp
-            }
+
             computeAndOrType
 
           case JavaArrayType(_) if symbol == defn.ObjectClass =>

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

@@ -3148,7 +3148,10 @@ object Types {
     /** Replace or type by the closest non-or type above it */
     def join(using Context): Type = {
       if (myJoinPeriod != ctx.period) {
-        myJoin = TypeOps.orDominator(this)
+        myJoin =
+          if tp1 frozen_<:< tp2 then tp2
+          else if tp2 frozen_<:< tp1 then tp1
+          else TypeOps.orDominator(this)
         core.println(i"join of $this == $myJoin")
         assert(myJoin != this)
         myJoinPeriod = ctx.period

tests/pos/i11968.scala

@@ -0,0 +1,10 @@
+class C {
+    def get(): Int = 0
+}
+
+def g = {
+    val s: String | Null = ???
+    val l = s.length // ok
+    val c: C | Null = ???
+    c.get() // error: value get is not a member of C | Null
+}