Harden orDominator

Fixes #13435

Author: odersky

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

@@ -349,7 +349,11 @@ object TypeOps:
       val doms = dominators(commonBaseClasses, Nil)
       def baseTp(cls: ClassSymbol): Type =
         tp.baseType(cls).mapReduceOr(identity)(mergeRefinedOrApplied)
-      doms.map(baseTp).reduceLeft(AndType.apply)
+      def meet(tp1: Type, tp2: Type) =
+        if !tp1.exists then tp2
+        else if !tp2.exists then tp1
+        else AndType(tp1, tp2)
+      doms.map(baseTp).reduceLeft(meet)
     }
 
     tp match {

tests/neg/i13435.check

@@ -0,0 +1,12 @@
+-- [E029] Pattern Match Exhaustivity Warning: tests/neg/i13435.scala:7:2 -----------------------------------------------
+7 |  s match
+  |  ^
+  |  match may not be exhaustive.
+  |
+  |  It would fail on pattern case: (_), ((_, _), (_, _))
+
+longer explanation available when compiling with `-explain`
+-- Error: tests/neg/i13435.scala:8:10 ----------------------------------------------------------------------------------
+8 |    case (dim: Axis, size: Int) => dim // error
+  |          ^^^^^^^^^
+  |          trait Singleton cannot be used in runtime type tests

tests/neg/i13435.scala

@@ -0,0 +1,8 @@
+type Axis = String&Singleton
+type ShapeTuple = Tuple1[(Axis, Int)]|Tuple2[(Axis, Int), (Axis, Int)]
+type Shape = (Axis, Int) |ShapeTuple
+
+
+def mkSchema(s: Shape) =
+  s match
+    case (dim: Axis, size: Int) => dim // error