Index: clang-tools-extra/clang-tidy/misc/CMakeLists.txt
===================================================================
--- clang-tools-extra/clang-tidy/misc/CMakeLists.txt
+++ clang-tools-extra/clang-tidy/misc/CMakeLists.txt
@@ -2,6 +2,7 @@
 
 add_clang_library(clangTidyMiscModule
   DefinitionsInHeadersCheck.cpp
+  DispatchOnceAssignmentCheck.cpp
   MiscTidyModule.cpp
   MisplacedConstCheck.cpp
   NewDeleteOverloadsCheck.cpp
Index: clang-tools-extra/clang-tidy/misc/DispatchOnceAssignmentCheck.h
===================================================================
--- /dev/null
+++ clang-tools-extra/clang-tidy/misc/DispatchOnceAssignmentCheck.h
@@ -0,0 +1,36 @@
+//===--- DispatchOnceAssignmentCheck.h - clang-tidy -------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_DISPATCHONCEASSIGNMENTCHECK_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_DISPATCHONCEASSIGNMENTCHECK_H
+
+#include "../ClangTidyCheck.h"
+
+namespace clang {
+namespace tidy {
+namespace misc {
+
+/// Finds explicit assignment to variables of type dispatch_once_t, which should
+/// not be assigned values directly in application code.
+///
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/misc-dispatch-once-assignment.html
+class DispatchOnceAssignmentCheck : public ClangTidyCheck {
+public:
+  DispatchOnceAssignmentCheck(StringRef Name, ClangTidyContext *Context)
+      : ClangTidyCheck(Name, Context) {}
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+};
+
+} // namespace misc
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_DISPATCHONCEASSIGNMENTCHECK_H
Index: clang-tools-extra/clang-tidy/misc/DispatchOnceAssignmentCheck.cpp
===================================================================
--- /dev/null
+++ clang-tools-extra/clang-tidy/misc/DispatchOnceAssignmentCheck.cpp
@@ -0,0 +1,40 @@
+//===--- DispatchOnceAssignmentCheck.cpp - clang-tidy ---------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "DispatchOnceAssignmentCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/AST/Expr.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace misc {
+
+void DispatchOnceAssignmentCheck::registerMatchers(MatchFinder *Finder) {
+  // Find assignments where the LHS has type dispatch_once_t.
+  Finder->addMatcher(
+      binaryOperator(isAssignmentOperator(),
+                     hasLHS(hasType(asString("dispatch_once_t"))))
+          .bind("assignment"),
+      this);
+}
+
+void DispatchOnceAssignmentCheck::check(
+    const MatchFinder::MatchResult &Result) {
+  if (const auto *Op = Result.Nodes.getNodeAs<BinaryOperator>("assignment")) {
+    diag(Op->getExprLoc(),
+         "do not assign to dispatch_once_t variables");
+  }
+}
+
+} // namespace misc
+} // namespace tidy
+} // namespace clang
Index: clang-tools-extra/clang-tidy/misc/MiscTidyModule.cpp
===================================================================
--- clang-tools-extra/clang-tidy/misc/MiscTidyModule.cpp
+++ clang-tools-extra/clang-tidy/misc/MiscTidyModule.cpp
@@ -10,6 +10,7 @@
 #include "../ClangTidyModule.h"
 #include "../ClangTidyModuleRegistry.h"
 #include "DefinitionsInHeadersCheck.h"
+#include "DispatchOnceAssignmentCheck.h"
 #include "MisplacedConstCheck.h"
 #include "NewDeleteOverloadsCheck.h"
 #include "NonCopyableObjects.h"
@@ -32,6 +33,8 @@
   void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
     CheckFactories.registerCheck<DefinitionsInHeadersCheck>(
         "misc-definitions-in-headers");
+    CheckFactories.registerCheck<DispatchOnceAssignmentCheck>(
+        "misc-dispatch-once-assignment");
     CheckFactories.registerCheck<MisplacedConstCheck>("misc-misplaced-const");
     CheckFactories.registerCheck<NewDeleteOverloadsCheck>(
         "misc-new-delete-overloads");
Index: clang-tools-extra/docs/ReleaseNotes.rst
===================================================================
--- clang-tools-extra/docs/ReleaseNotes.rst
+++ clang-tools-extra/docs/ReleaseNotes.rst
@@ -73,6 +73,11 @@
   Finds instances where variables with static storage are initialized
   dynamically in header files.
 
+- New :doc:`misc-dispatch-once-assignment
+  <clang-tidy/checks/misc-dispatch-once-assignment>` check.
+
+  Finds instances of explicitly writing to ``dispatch_once_t`` variables.
+
 - New :doc:`linuxkernel-must-use-errs
   <clang-tidy/checks/linuxkernel-must-use-errs>` check.
 
Index: clang-tools-extra/docs/clang-tidy/checks/list.rst
===================================================================
--- clang-tools-extra/docs/clang-tidy/checks/list.rst
+++ clang-tools-extra/docs/clang-tidy/checks/list.rst
@@ -278,6 +278,7 @@
    llvm-prefer-register-over-unsigned
    llvm-twine-local
    misc-definitions-in-headers
+   misc-dispatch-once-assignment
    misc-misplaced-const
    misc-new-delete-overloads
    misc-non-copyable-objects
Index: clang-tools-extra/docs/clang-tidy/checks/misc-dispatch-once-assignment.rst
===================================================================
--- /dev/null
+++ clang-tools-extra/docs/clang-tidy/checks/misc-dispatch-once-assignment.rst
@@ -0,0 +1,9 @@
+.. title:: clang-tidy - misc-dispatch-once-assignment
+
+misc-dispatch-once-assignment
+=============================
+
+Finds assignments to variables of type ``dispatch_once_t`` and warns to avoid
+assigning to them. Libdispatch intends that such writes should be guarded
+and making such direct writes may potentially violate the run-once protections
+intended by the library.
Index: clang-tools-extra/test/clang-tidy/misc-dispatch-once-assignment.cpp
===================================================================
--- /dev/null
+++ clang-tools-extra/test/clang-tidy/misc-dispatch-once-assignment.cpp
@@ -0,0 +1,15 @@
+// RUN: %check_clang_tidy %s misc-dispatch-once-assignment %t
+
+typedef int dispatch_once_t;
+extern void dispatch_once(dispatch_once_t *pred, void(^block)(void));
+
+static dispatch_once_t onceToken;
+
+void DoOnce(void(^block)(void)) {
+  dispatch_once(&onceToken, block);
+}
+
+void ResetOnce() {
+  onceToken = 0;
+  // CHECK-MESSAGES: :[[@LINE-1]]:13: warning: do not assign to dispatch_once_t variables [misc-dispatch-once-assignment]
+}