Sema: Partially revert existential opening fix

Selectively revert 36683a8 to resolve
a source compatibility regression. See inline comment for use case. We
are going to consider acknowledging this use case in the rules in a
future release.

Author: AnthonyLatsis

lib/Sema/OpenedExistentials.cpp

@@ -259,11 +259,22 @@ findGenericParameterReferencesRec(CanGenericSignature genericSig,
   if (auto *pack = type->getAs<PackType>()) {
     auto info = GenericParameterReferenceInfo();
 
-    for (auto arg : pack->getElementTypes()) {
-      info |= findGenericParameterReferencesRec(
-          genericSig, origParam, openedParam, arg,
-          TypePosition::Invariant, /*canBeCovariantResult=*/false);
-    }
+    // FIXME: Source compatibility remedy to allow existential opening in
+    // the following case:
+    // ```
+    // protocol P {}
+    // struct S<each T> {}
+    // func foo<T: P>(_: T, _: S<T>? = nil) {}
+    // let p: any P
+    // foo(p)
+    // ```
+    //
+    // for (auto arg : pack->getElementTypes()) {
+    //   info |= findGenericParameterReferencesRec(
+    //       genericSig, origParam, openedParam, arg,
+    //       TypePosition::Invariant, /*canBeCovariantResult=*/false);
+    // }
+    (void)pack;
 
     return info;
   }

test/Constraints/opened_existentials_default_arg.swift

@@ -0,0 +1,11 @@
+// RUN: %target-typecheck-verify-swift -target %target-swift-5.9-abi-triple
+
+do {
+  protocol P {}
+  struct S<each T> {}
+
+  func foo<T: P>(_: T, _: Optional<S<T>> = nil) {}
+
+  let p: any P
+  foo(p) // OK
+}