Harden GADT constraint handling to survive illegal F-bounds

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

@@ -193,7 +193,8 @@ sealed trait GadtState {
                 case i => pt.paramRefs(i)
             case tp => tp
         }
-
+        if !param.info.exists then
+          throw TypeError(em"illegal recursive reference involving $param")
         val tb = param.info.bounds
         tb.derivedTypeBounds(
           lo = substDependentSyms(tb.lo, isUpper = false),

compiler/test/dotc/neg-best-effort-pickling.blacklist

@@ -13,6 +13,7 @@ curried-dependent-ift.scala
 i17121.scala
 illegal-match-types.scala
 i13780-1.scala
+i20317a.scala
 
 # semantic db generation fails in the first compilation
 i1642.scala

tests/neg/i20317.scala

@@ -0,0 +1,3 @@
+type Foo[A] = A
+
+def foo[A <: Foo[A]]: Unit = () // error // error

tests/neg/i20317a.scala

@@ -0,0 +1,5 @@
+type SemigroupStructural[A] =
+  A & { def combine(a: A): A }
+def combineAll[A <: SemigroupStructural[A]](
+  i: A, l: List[A]
+): A = l.foldLeft(i)(_.combine(_)) // error