Harden GADT constraint handling to survive illegal F-bounds (scala#20325

)

This currently fails BestEffortCompilationTests

The problem is that i20317a.scala contains illegal cyclic references.
The cycles cause many different algorithms in the compiler to loop. We
normally reject the program, and that's that. But under best-effort, we
crash with Stackoverflows in the loops later. What to do? It's not
feasible to avoid the looping behavior at acceptable cost. Can we
disable best effort under certain conditions?

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