Erase existential thrown error types

lib/Sema/TypeCheckEffects.cpp

@@ -800,6 +800,24 @@ static bool isRethrowingDueToAsyncSequence(DeclContext *rethrowsDC) {
   return true;
 }
 
+/// Type-erase the opened archetypes in the given type, if there is one.
+static Type typeEraseOpenedArchetypes(Type type) {
+  if (!type || !type->hasOpenedExistential())
+    return type;
+
+  const OpenedArchetypeType *root = nullptr;
+  type.visit([&](Type type) {
+    if (auto opened = dyn_cast<OpenedArchetypeType>(type.getPointer())) {
+      root = opened->getRoot();
+    }
+  });
+
+  if (!root)
+    return type;
+
+  return constraints::typeEraseOpenedArchetypesWithRoot(type, root);
+}
+
 /// A type expressing the result of classifying whether a call or function
 /// throws or is async.
 class Classification {
@@ -928,7 +946,7 @@ class Classification {
 
     result.ThrowKind = conditionalKind;
     result.ThrowReason = reason;
-    result.ThrownError = thrownError;
+    result.ThrownError = typeEraseOpenedArchetypes(thrownError);
     return result;
   }
 

test/Concurrency/async_sequence_existential.swift

@@ -0,0 +1,18 @@
+// RUN: %target-swift-frontend  -disable-availability-checking %s -emit-sil -o /dev/null -verify
+
+// RUN: %target-swift-frontend  -disable-availability-checking %s -dump-ast 2>&1 | %FileCheck %s
+
+// REQUIRES: concurrency
+
+extension Error {
+  func printMe() { }
+}
+
+func test(seq: any AsyncSequence) async {
+  // CHECK: "error" interface type="any Error"
+  do {
+    for try await _ in seq { }
+  } catch {
+    error.printMe()
+  }
+}