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diff --git a/core/fxcrt/fx_fallthrough.h b/core/fxcrt/fx_fallthrough.h
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+// Copyright 2018 PDFium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CORE_FXCRT_FX_FALLTHROUGH_H_
+#define CORE_FXCRT_FX_FALLTHROUGH_H_
+
+// When clang suggests inserting [[clang::fallthrough]], it first checks if
+// it knows of a macro expanding to it, and if so suggests inserting the
+// macro. This means that this macro must be used only in code internal
+// to PDFium, so that PDFium's user code doesn't end up getting suggestions
+// for FX_FALLTHROUGH instead of the user-specific fallthrough macro.
+// So do not include this header in any of PDFium's public headers.
+#if defined(__clang__)
+#define FX_FALLTHROUGH [[clang::fallthrough]]
+#else
+#define FX_FALLTHROUGH
+#endif
+
+#endif // CORE_FXCRT_FX_FALLTHROUGH_H_