Merge pull request swiftlang#35100 from tbkka/tbkka/conditional-objc-check-rdar72323929

Author: swift-ci

stdlib/public/runtime/DynamicCast.cpp

@@ -110,6 +110,12 @@ extern "C" const StructDescriptor NOMINAL_TYPE_DESCR_SYM(Sh);
 /// Nominal type descriptor for Swift.String.
 extern "C" const StructDescriptor NOMINAL_TYPE_DESCR_SYM(SS);
 
+// If this returns `true`, then we will call `fatalError` when we encounter a
+// null reference in a storage locaation whose type does not allow null.
+static bool unexpectedNullIsFatal() {
+  return true;  // Placeholder for an upcoming check.
+}
+
 static HeapObject * getNonNullSrcObject(OpaqueValue *srcValue,
                                         const Metadata *srcType,
                                         const Metadata *destType) {
@@ -120,12 +126,21 @@ static HeapObject * getNonNullSrcObject(OpaqueValue *srcValue,
 
   std::string srcTypeName = nameForMetadata(srcType);
   std::string destTypeName = nameForMetadata(destType);
-  swift::fatalError(/* flags = */ 0,
-                    "Found unexpected null pointer value"
+  const char *msg = "Found unexpected null pointer value"
                     " while trying to cast value of type '%s' (%p)"
-                    " to '%s' (%p)\n",
-                    srcTypeName.c_str(), srcType,
-                    destTypeName.c_str(), destType);
+                    " to '%s' (%p)%s\n";
+  if (unexpectedNullIsFatal()) {
+    swift::fatalError(/* flags = */ 0, msg,
+                      srcTypeName.c_str(), srcType,
+                      destTypeName.c_str(), destType,
+                      "");
+  } else {
+    swift::warning(/* flags = */ 0, msg,
+                   srcTypeName.c_str(), srcType,
+                   destTypeName.c_str(), destType,
+                   ": Continuing with null object, but expect problems later.");
+  }
+  return object;
 }
 
 /******************************************************************************/