Try to detecting cycles at instantiation time

Author: smarter

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

@@ -4751,6 +4751,9 @@ object Types {
       assert(!myInst.exists, i"$origin is already instantiated to $myInst but we attempted to instantiate it to $tp")
       typr.println(i"instantiating $this with $tp")
 
+      if origin.occursIn(tp) then
+        throw new TypeError(i"cyclic instantiation of $origin to $tp")
+
       if Config.checkConstraintsSatisfiable then
         assert(currentEntry.bounds.contains(tp),
           i"$origin is constrained to be $currentEntry but attempted to instantiate it to $tp")

tests/neg/indirect-cycle.scala

@@ -0,0 +1,12 @@
+class LT[-X, +Y] // error: cyclic instantiation of T to D[D[T]]
+object LT {
+ given lt[X <: Y, Y]: LT[X, Y] = ???
+}
+
+trait C[X]
+trait D[X] extends C[X]
+trait A { // error: cyclic instantiation of T to D[D[T]]
+  def foo[S <: C[T], T <: C[S]](using LT[D[T], S], LT[D[S], T]): S = ???
+
+  foo
+}