Fix #7811: Harden mapArgs in TypeMap

Author: odersky

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

@@ -4599,7 +4599,7 @@ object Types {
 
         case tp: AppliedType =>
           def mapArgs(args: List[Type], tparams: List[ParamInfo]): List[Type] = args match {
-            case arg :: otherArgs =>
+            case arg :: otherArgs if tparams.nonEmpty =>
               val arg1 = arg match {
                 case arg: TypeBounds => this(arg)
                 case arg => atVariance(variance * tparams.head.paramVariance)(this(arg))

tests/neg/i7811.scala

@@ -0,0 +1,8 @@
+trait A {
+    type Type[X,]  // error
+    def a[X]: Type[X,]  // error
+}
+class B extends A {
+    type Type[X]
+    var a = 1
+}