From c544e4128931bba8650bd61d2d29131f75426ccb Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Thu, 2 May 2024 13:23:31 +0200
Subject: [PATCH 1/2] Harden GADT constraint handling to survive illegal
 F-bounds

---
 compiler/src/dotty/tools/dotc/core/GadtConstraint.scala | 3 ++-
 tests/neg/i20317.scala                                  | 3 +++
 tests/neg/i20317a.scala                                 | 5 +++++
 3 files changed, 10 insertions(+), 1 deletion(-)
 create mode 100644 tests/neg/i20317.scala
 create mode 100644 tests/neg/i20317a.scala

diff --git a/compiler/src/dotty/tools/dotc/core/GadtConstraint.scala b/compiler/src/dotty/tools/dotc/core/GadtConstraint.scala
index 1cbfabc08958..5a8938602523 100644
--- a/compiler/src/dotty/tools/dotc/core/GadtConstraint.scala
+++ b/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),
diff --git a/tests/neg/i20317.scala b/tests/neg/i20317.scala
new file mode 100644
index 000000000000..e9d8599b9fc1
--- /dev/null
+++ b/tests/neg/i20317.scala
@@ -0,0 +1,3 @@
+type Foo[A] = A
+
+def foo[A <: Foo[A]]: Unit = () // error // error
diff --git a/tests/neg/i20317a.scala b/tests/neg/i20317a.scala
new file mode 100644
index 000000000000..d7b8b66eb80e
--- /dev/null
+++ b/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

From 5e408bdf2e9c6b326398d27e6f682187e8db708f Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Mon, 6 May 2024 12:48:45 +0200
Subject: [PATCH 2/2] Add test to best effort blacklist

---
 compiler/test/dotc/neg-best-effort-pickling.blacklist | 1 +
 1 file changed, 1 insertion(+)

diff --git a/compiler/test/dotc/neg-best-effort-pickling.blacklist b/compiler/test/dotc/neg-best-effort-pickling.blacklist
index ff02be107a8a..1c8421b44539 100644
--- a/compiler/test/dotc/neg-best-effort-pickling.blacklist
+++ b/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