[CSSimplify] Prevent invalid pack references from causing infinite loops

When `matchTypesBindTypeVar` detects invalid pack reference it should
reset the type to a placeholder and allow binding to go through,
otherwise, if binding comes from inference, returning without binding
would create an infinite loop because type variable would perpetually
be the best choice to attempt.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -4282,16 +4282,17 @@ ConstraintSystem::matchTypesBindTypeVar(
   // pack expansion expression.
   if (!typeVar->getImpl().canBindToPack() &&
       (type->is<PackArchetypeType>() || type->is<PackType>())) {
-    if (shouldAttemptFixes()) {
-      auto *fix = AllowInvalidPackReference::create(*this, type,
-                                                    getConstraintLocator(locator));
-      if (!recordFix(fix)) {
-        recordPotentialHole(typeVar);
-        return getTypeMatchSuccess();
-      }
-    }
+    if (!shouldAttemptFixes())
+      return getTypeMatchFailure(locator);
 
-    return getTypeMatchFailure(locator);
+    auto *fix = AllowInvalidPackReference::create(
+        *this, type, getConstraintLocator(locator));
+    if (recordFix(fix))
+      return getTypeMatchFailure(locator);
+
+    // Don't allow the invalid pack reference to propagate to other
+    // bindings.
+    type = PlaceholderType::get(typeVar->getASTContext(), typeVar);
   }
 
   // Binding to a pack expansion type is always an error in Swift 6 mode.