Fix #6064: Hardening in the presence of kind mismatches

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

@@ -58,7 +58,7 @@ trait Substituters { this: Context =>
         val sym = tp.symbol
         var fs = from
         var ts = to
-        while (fs.nonEmpty) {
+        while (fs.nonEmpty && ts.nonEmpty) {
           if (fs.head eq sym) return ts.head
           fs = fs.tail
           ts = ts.tail
@@ -203,7 +203,7 @@ trait Substituters { this: Context =>
         val sym = tp.symbol
         var fs = from
         var ts = to
-        while (fs.nonEmpty) {
+        while (fs.nonEmpty && ts.nonEmpty) {
           if (fs.head eq sym)
             return ts.head match {
               case TypeBounds(lo, hi) => range(lo, hi)

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

@@ -4791,10 +4791,7 @@ object Types {
 
       case tp @ AppliedType(tycon, args) =>
         @tailrec def foldArgs(x: T, tparams: List[ParamInfo], args: List[Type]): T =
-          if (args.isEmpty) {
-            assert(tparams.isEmpty)
-            x
-          }
+          if (args.isEmpty || tparams.isEmpty) x
           else {
             val tparam = tparams.head
             val acc = args.head match {

tests/neg/i6063.scala

@@ -0,0 +1,12 @@
+object i0 {
+trait I1[i0, I1]
+def I1[I1[I1]](I1: I1[ ;  // error
+I1[I1]
+}
+
+object test {
+object I0 {
+trait i2[I1, i2]
+def i2[i2[i2]](i2: i2[ ]) = i2  // error
+i2[i2]}
+}

tests/neg/i6064.scala

@@ -0,0 +1,4 @@
+trait Trait[X] {
+    def f[C[_]](arg: C[X]): Int
+    val a = f[(Int, String)] // error
+}