diff --git a/clang-tools-extra/test/clang-tidy/checkers/Inputs/readability-identifier-naming/system/coroutines.h b/clang-tools-extra/test/clang-tidy/checkers/Inputs/readability-identifier-naming/system/coroutines.h
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/Inputs/readability-identifier-naming/system/coroutines.h
@@ -0,0 +1,34 @@
+#pragma once
+
+namespace std {
+namespace experimental {
+
+template <typename ret_t, typename... args_t>
+struct coroutine_traits {
+  using promise_type = typename ret_t::promise_type;
+};
+
+template <class promise_t>
+struct coroutine_handle {
+  static constexpr coroutine_handle from_address(void *addr) noexcept { return {}; };
+};
+
+} // namespace experimental
+} // namespace std
+
+struct never_suspend {
+  bool await_ready() noexcept { return false; }
+  template <typename coro_t>
+  void await_suspend(coro_t handle) noexcept {}
+  void await_resume() noexcept {}
+};
+
+struct task {
+  struct promise_type {
+    task get_return_object() noexcept { return {}; }
+    never_suspend initial_suspend() noexcept { return {}; }
+    never_suspend final_suspend() noexcept { return {}; }
+    void return_void() {}
+    void unhandled_exception() {}
+  };
+};
diff --git a/clang-tools-extra/test/clang-tidy/checkers/readability-identifier-naming.cpp b/clang-tools-extra/test/clang-tidy/checkers/readability-identifier-naming.cpp
--- a/clang-tools-extra/test/clang-tidy/checkers/readability-identifier-naming.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/readability-identifier-naming.cpp
@@ -81,13 +81,14 @@
 // RUN:     {key: readability-identifier-naming.LocalPointerPrefix, value: 'l_'}, \
 // RUN:     {key: readability-identifier-naming.LocalConstantPointerCase, value: CamelCase}, \
 // RUN:     {key: readability-identifier-naming.LocalConstantPointerPrefix, value: 'lc_'}, \
-// RUN:   ]}' -- -fno-delayed-template-parsing -Dbad_macro \
+// RUN:   ]}' -- -fno-delayed-template-parsing -Dbad_macro -std=c++17 -fcoroutines-ts \
 // RUN:   -I%S/Inputs/readability-identifier-naming \
 // RUN:   -isystem %S/Inputs/readability-identifier-naming/system
 
 // clang-format off
 
 #include <system-header.h>
+#include <coroutines.h>
 #include "user-header.h"
 // NO warnings or fixes expected from declarations within header files without
 // the -header-filter= option
@@ -287,7 +288,7 @@
   // Overriding a badly-named base isn't a new violation.
   void BadBaseMethod() override {}
   // CHECK-FIXES: {{^}}  void v_Bad_Base_Method() override {}
-  
+
   void foo() {
     BadBaseMethod();
     // CHECK-FIXES: {{^}}    v_Bad_Base_Method();
@@ -614,3 +615,14 @@
 auto GetRes(type_t& Param) -> decltype(Param.res());
 // CHECK-MESSAGES: :[[@LINE-1]]:21: warning: invalid case style for parameter 'Param'
 // CHECK-FIXES: auto GetRes(type_t& a_param) -> decltype(a_param.res());
+
+// Check implicit declarations in coroutines
+
+struct async_obj {
+public:
+  never_suspend operator co_await() const noexcept;
+};
+
+task ImplicitDeclTest(async_obj &a_object) {
+  co_await a_object;  // CHECK-MESSAGES-NOT: warning: invalid case style for local variable
+}
diff --git a/clang/lib/Sema/SemaCoroutine.cpp b/clang/lib/Sema/SemaCoroutine.cpp
--- a/clang/lib/Sema/SemaCoroutine.cpp
+++ b/clang/lib/Sema/SemaCoroutine.cpp
@@ -544,6 +544,7 @@
   auto *VD = VarDecl::Create(Context, FD, FD->getLocation(), FD->getLocation(),
                              &PP.getIdentifierTable().get("__promise"), T,
                              Context.getTrivialTypeSourceInfo(T, Loc), SC_None);
+  VD->setImplicit();
   CheckVariableDeclarationType(VD);
   if (VD->isInvalidDecl())
     return nullptr;
@@ -1577,6 +1578,7 @@
       S.Context, &FD, FD.getLocation(), FD.getLocation(),
       &S.PP.getIdentifierTable().get("__coro_gro"), GroType,
       S.Context.getTrivialTypeSourceInfo(GroType, Loc), SC_None);
+  GroDecl->setImplicit();
 
   S.CheckVariableDeclarationType(GroDecl);
   if (GroDecl->isInvalidDecl())