Guard against recursive lower bounds in constraints

We could get an indirect recursion going through a singleton type before.

Fixes #21535

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

@@ -295,11 +295,24 @@ trait ConstraintHandling {
   end legalBound
 
   protected def addOneBound(param: TypeParamRef, rawBound: Type, isUpper: Boolean)(using Context): Boolean =
+
+    // Replace top-level occurrences of `param` in `bound` by `Nothing`
+    def sanitize(bound: Type): Type =
+      if bound.stripped eq param then defn.NothingType
+      else bound match
+        case bound: AndOrType =>
+          bound.derivedAndOrType(sanitize(bound.tp1), sanitize(bound.tp2))
+        case _ =>
+          bound
+
     if !constraint.contains(param) then true
-    else if !isUpper && param.occursIn(rawBound) then
-      // We don't allow recursive lower bounds when defining a type,
-      // so we shouldn't allow them as constraints either.
-      false
+    else if !isUpper && param.occursIn(rawBound.widen) then
+      val rawBound1 = sanitize(rawBound.widenDealias)
+      if param.occursIn(rawBound1) then
+        // We don't allow recursive lower bounds when defining a type,
+        // so we shouldn't allow them as constraints either.
+        false
+      else addOneBound(param, rawBound1, isUpper)
     else
 
       // Narrow one of the bounds of type parameter `param`

tests/neg/i21535.check

@@ -0,0 +1,11 @@
+-- [E007] Type Mismatch Error: tests/neg/i21535.scala:7:4 --------------------------------------------------------------
+3 |  (if (true) then
+4 |    new A(66)
+5 |  else
+6 |    m1()
+7 |  ).m2(p1 = p); // error
+  |  ^
+  |  Found:    (Int | Short) @uncheckedVariance
+  |  Required: Int & Short
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i21535.scala

@@ -0,0 +1,16 @@
+def test() = {
+  val p = 10.toShort
+  (if (true) then
+    new A(66)
+  else
+    m1()
+  ).m2(p1 = p); // error
+
+}
+
+def m1(): A[Short] = new A(10)
+
+class A[D](var f: D) {
+
+  def m2(p1: D = f, p2: D = f): Unit = {}
+}