Throw a type error iwhen using hk-types in unions or intersections

Throw a type error instead of crashing when using higher-kinded types in unions or intersections.

We check kindedness only in PostTyper (to avoid cycles), which means we might get into situations
where we coming a higher-kinded type in a union or intersection, which is illegal. In this case
we now diagnose the problem with a TypeError instead of failing an assert. However, after Typer
we do fail since by then such situations should have been checked by then.

Fixeds #16696

Author: odersky

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

@@ -1071,7 +1071,7 @@ object Types {
      *  @param relaxedCheck   if true type `Null` becomes a subtype of non-primitive value types in TypeComparer.
      *  @param matchLoosely   if true the types `=> T` and `()T` are seen as overriding each other.
      *  @param checkClassInfo if true we check that ClassInfos are within bounds of abstract types
-     * 
+     *
      *  @param isSubType      a function used for checking subtype relationships.
      */
     final def overrides(that: Type, relaxedCheck: Boolean, matchLoosely: => Boolean, checkClassInfo: Boolean = true,
@@ -3322,11 +3322,11 @@ object Types {
   final class CachedAndType(tp1: Type, tp2: Type) extends AndType(tp1, tp2)
 
   object AndType {
-    def apply(tp1: Type, tp2: Type)(using Context): AndType = {
-      assert(tp1.isValueTypeOrWildcard &&
-             tp2.isValueTypeOrWildcard, i"$tp1 & $tp2 / " + s"$tp1 & $tp2")
+    def apply(tp1: Type, tp2: Type)(using Context): AndType =
+      def where = i"in intersection $tp1 & $tp2"
+      expectValueTypeOrWildcard(tp1, where)
+      expectValueTypeOrWildcard(tp2, where)
       unchecked(tp1, tp2)
-    }
 
     def balanced(tp1: Type, tp2: Type)(using Context): AndType =
       tp1 match
@@ -3366,7 +3366,7 @@ object Types {
       TypeComparer.liftIfHK(tp1, tp2, AndType.make(_, _, checkValid = false), makeHk, _ | _)
   }
 
-  abstract case class OrType(tp1: Type, tp2: Type) extends AndOrType {
+  abstract case class OrType protected(tp1: Type, tp2: Type) extends AndOrType {
     def isAnd: Boolean = false
     def isSoft: Boolean
     private var myBaseClassesPeriod: Period = Nowhere
@@ -3399,9 +3399,6 @@ object Types {
         myFactorCount
       else 1
 
-    assert(tp1.isValueTypeOrWildcard &&
-           tp2.isValueTypeOrWildcard, s"$tp1 $tp2")
-
     private var myJoin: Type = _
     private var myJoinPeriod: Period = Nowhere
 
@@ -3476,6 +3473,9 @@ object Types {
   object OrType {
 
     def apply(tp1: Type, tp2: Type, soft: Boolean)(using Context): OrType = {
+      def where = i"in union $tp1 | $tp2"
+      expectValueTypeOrWildcard(tp1, where)
+      expectValueTypeOrWildcard(tp2, where)
       assertUnerased()
       unique(new CachedOrType(tp1, tp2, soft))
     }
@@ -3506,6 +3506,11 @@ object Types {
       TypeComparer.liftIfHK(tp1, tp2, OrType(_, _, soft = true), makeHk, _ & _)
   }
 
+  def expectValueTypeOrWildcard(tp: Type, where: => String)(using Context): Unit =
+    if !tp.isValueTypeOrWildcard then
+      assert(!ctx.isAfterTyper, where) // we check correct kinds at PostTyper
+      throw TypeError(em"$tp is not a value type, cannot be used in $where")
+
   /** An extractor object to pattern match against a nullable union.
    *  e.g.
    *

tests/neg/i16696.check

@@ -0,0 +1,12 @@
+-- Error: tests/neg/i16696.scala:7:29 ----------------------------------------------------------------------------------
+7 |  val boom1 = BoxMaker[Some].make1 // error
+  |                             ^
+  |                             Some is not a value type, cannot be used in in intersection Some & Int
+-- Error: tests/neg/i16696.scala:8:29 ----------------------------------------------------------------------------------
+8 |  val boom2 = BoxMaker[Some].make2 // error
+  |                             ^
+  |                             Some is not a value type, cannot be used in in union Some | Int
+-- Error: tests/neg/i16696.scala:20:27 ---------------------------------------------------------------------------------
+20 |  val boom = BoxMaker[Foo].make(_.foo) // error
+   |                           ^
+   |                           test2.Foo is not a value type, cannot be used in in intersection R & test2.Foo

tests/neg/i16696.scala

@@ -0,0 +1,20 @@
+object test1:
+  class BoxMaker[T] {
+    def make1: T & Int = ???
+    def make2: T | Int = ???
+  }
+
+  val boom1 = BoxMaker[Some].make1 // error
+  val boom2 = BoxMaker[Some].make2 // error
+
+object test2:
+  class Box[R]
+
+  class BoxMaker[T] {
+    def make[R <: T](f: T => Box[R]): Box[R & T] = ???
+  }
+
+  trait Foo[A]{
+    def foo: Box[Foo[Unit]]
+  }
+  val boom = BoxMaker[Foo].make(_.foo) // error