Rule out nonsensical bounds for capture variables

Author: bracevac

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3053,8 +3053,15 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     if sym.isOpaqueAlias then
       checkFullyAppliedType(rhs1, "Opaque type alias must be fully applied, but ")
       checkNoContextFunctionType(rhs1)
-    if Feature.ccEnabled && rhs1.tpe.derivesFrom(defn.Caps_CapSet) then
-      rhs1.putAttachment(CaptureVar, ())
+    if Feature.ccEnabled then
+      val isCap = tdef.hasAttachment(CaptureVar)
+      rhs1 match
+        case TypeBoundsTree(lo, hi, _) =>
+          if !isCap && (lo.tpe.derivesFrom(defn.Caps_CapSet) ^ hi.tpe.derivesFrom(defn.Caps_CapSet)) then
+            report.error(em"Illegal type bounds: >: $lo <: $hi. Capture-set bounds cannot be mixed with type bounds of other kinds", tdef.srcPos)
+          if isCap && !(lo.tpe.derivesFrom(defn.Caps_CapSet) && hi.tpe.derivesFrom(defn.Caps_CapSet)) then
+            report.error(em"Illegal type bounds: >: $lo <: $hi. $name^ can only have capture sets as bounds", tdef.srcPos)
+        case _ =>
     assignType(cpy.TypeDef(tdef)(name, rhs1), sym)
   }
 

tests/neg-custom-args/captures/capset-members5.scala

@@ -24,4 +24,8 @@ def test =
     type M >: {x, y, O.z} <: {C}
     type N >: {x} <: {x}
     type O >: {O.z} <: {O.z}
-    type P >: {B,D} // error
+    type P >: {B,D} // error
+    type Q >: {E} <: Int // error
+    type R^ >: {E} <: Int // error
+    type S^ >: Nothing <: Int // error
+    type T >: Nothing <: Int