Don't lambda expand and/or types over named type parameters

Do it only if at least one of the types has unnamed parameters. This is
a fundamental conflict with how we deal with intersections and unions.

1 files changed, 16 insertions, 2 deletions

diff --git a/src/dotty/tools/dotc/core/TypeComparer.scala b/src/dotty/tools/dotc/core/TypeComparer.scala
index 4e7a4a75d..833adf008 100644
--- a/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -1075,15 +1075,29 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
     }
   }
 
-  /** `op(tp1, tp2)` unless `tp1` and `tp2` are type-constructors.
+  /** `op(tp1, tp2)` unless `tp1` and `tp2` are type-constructors with at least
+   *  some unnamed type parameters.
    *  In the latter case, combine `tp1` and `tp2` under a type lambda like this:
    *
    *    [X1, ..., Xn] -> op(tp1[X1, ..., Xn], tp2[X1, ..., Xn])
+   *
+   *  Note: There is a tension between named and positional parameters here, which
+   *  is impossible to resolve completely. Say you have
+   *
+   *    C[type T], D[type U]
+   *
+   *  Then do you expand `C & D` to `[T, U] -> C[T, U] & D[T, U]` or not? Under the named
+   *  type parameter interpretation, this would be wrong whereas under the traditional
+   *  higher-kinded interpretation this would be required. The problem arises from
+   *  allowing both interpretations. A possible remedy is to be somehow stricter
+   *  in where we allow which interpretation.
    */
   private def liftIfHK(tp1: Type, tp2: Type, op: (Type, Type) => Type) = {
     val tparams1 = tp1.typeParams
     val tparams2 = tp2.typeParams
-    if (tparams1.isEmpty || tparams2.isEmpty) op(tp1, tp2)
+    def onlyNamed(tparams: List[TypeSymbol]) = tparams.forall(!_.is(ExpandedName))
+    if (tparams1.isEmpty || tparams2.isEmpty ||
+        onlyNamed(tparams1) && onlyNamed(tparams2)) op(tp1, tp2)
     else if (tparams1.length != tparams2.length) mergeConflict(tp1, tp2)
     else hkCombine(tp1, tp2, tparams1, tparams2, op)
   }