Index: clang/lib/Sema/SemaCoroutine.cpp
===================================================================
--- clang/lib/Sema/SemaCoroutine.cpp
+++ clang/lib/Sema/SemaCoroutine.cpp
@@ -1684,30 +1684,38 @@
       return nullptr;
     }
 
-    if (!InStd) {
-      // Found only in std::experimental.
-      Diag(KwLoc, diag::warn_deprecated_coroutine_namespace)
-          << "coroutine_traits";
-    } else if (InExp) {
-      // Found in std and std::experimental.
-      Diag(KwLoc,
-           diag::err_mixed_use_std_and_experimental_namespace_for_coroutine);
-      Diag(KwLoc, diag::warn_deprecated_coroutine_namespace)
-          << "coroutine_traits";
-      return nullptr;
-    }
-
     // Prefer ::std to std::experimental.
     auto &Result = InStd ? ResStd : ResExp;
     CoroTraitsNamespaceCache = InStd ? StdSpace : ExpSpace;
 
     // coroutine_traits is required to be a class template.
-    if (!(StdCoroutineTraitsCache = Result.getAsSingle<ClassTemplateDecl>())) {
+    StdCoroutineTraitsCache = Result.getAsSingle<ClassTemplateDecl>();
+    if (!StdCoroutineTraitsCache) {
       Result.suppressDiagnostics();
       NamedDecl *Found = *Result.begin();
       Diag(Found->getLocation(), diag::err_malformed_std_coroutine_traits);
       return nullptr;
     }
+
+    if (InExp) {
+      // Found in std::experimental
+      Diag(KwLoc, diag::warn_deprecated_coroutine_namespace)
+          << "coroutine_traits";
+      ResExp.suppressDiagnostics();
+      auto *Found = *ResExp.begin();
+      Diag(Found->getLocation(), diag::note_entity_declared_at) << Found;
+
+      if (InStd) {
+        // Also found in std
+        Diag(KwLoc,
+             diag::err_mixed_use_std_and_experimental_namespace_for_coroutine);
+        Diag(StdCoroutineTraitsCache->getLocation(),
+             diag::note_entity_declared_at)
+            << StdCoroutineTraitsCache;
+
+        return nullptr;
+      }
+    }
   }
   Namespace = CoroTraitsNamespaceCache;
   return StdCoroutineTraitsCache;
Index: clang/test/SemaCXX/Inputs/std-coroutine-exp-namespace.h
===================================================================
--- clang/test/SemaCXX/Inputs/std-coroutine-exp-namespace.h
+++ clang/test/SemaCXX/Inputs/std-coroutine-exp-namespace.h
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts -fsyntax-only -Wignored-qualifiers -Wno-error=return-type -verify -fblocks -Wno-unreachable-code -Wno-unused-value
+// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts -fsyntax-only -Wignored-qualifiers -Wno-error=return-type -verify=expected,declared -fblocks -Wno-unreachable-code -Wno-unused-value
 #ifndef STD_COROUTINE_EXPERIMENTAL_H
 #define STD_COROUTINE_EXPERIMENTAL_H
 
@@ -6,6 +6,7 @@
 namespace experimental {
 template <class Ret, typename... T>
 struct coroutine_traits { using promise_type = typename Ret::promise_type; };
+// declared-note@-1{{declared here}}
 
 template <class Promise = void>
 struct coroutine_handle {
Index: clang/test/SemaCXX/Inputs/std-coroutine.h
===================================================================
--- clang/test/SemaCXX/Inputs/std-coroutine.h
+++ clang/test/SemaCXX/Inputs/std-coroutine.h
@@ -6,6 +6,7 @@
 
 template <class Ret, typename... T>
 struct coroutine_traits { using promise_type = typename Ret::promise_type; };
+// declared-note@-1{{declared here}}
 
 template <class Promise = void>
 struct coroutine_handle {
Index: clang/test/SemaCXX/co_await-range-for-exp-namespace.cpp
===================================================================
--- clang/test/SemaCXX/co_await-range-for-exp-namespace.cpp
+++ clang/test/SemaCXX/co_await-range-for-exp-namespace.cpp
@@ -1,6 +1,6 @@
 // RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts \
-// RUN:    -fsyntax-only -Wignored-qualifiers -Wno-error=return-type -verify \
-// RUN:    -fblocks
+// RUN:    -fsyntax-only -Wignored-qualifiers -Wno-error=return-type \
+// RUN:    -verify=expected,declared -fblocks
 #include "Inputs/std-coroutine-exp-namespace.h"
 
 using namespace std::experimental;
Index: clang/test/SemaCXX/coreturn-eh-exp-namespace.cpp
===================================================================
--- clang/test/SemaCXX/coreturn-eh-exp-namespace.cpp
+++ clang/test/SemaCXX/coreturn-eh-exp-namespace.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts -fcxx-exceptions -fsyntax-only -Wignored-qualifiers -Wno-error=return-type -verify -fblocks -Wall -Wextra -Wno-error=unreachable-code
+// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts -fcxx-exceptions -fsyntax-only -Wignored-qualifiers -Wno-error=return-type -verify=expected,declared -fblocks -Wall -Wextra -Wno-error=unreachable-code
 
 #include "Inputs/std-coroutine-exp-namespace.h"
 
Index: clang/test/SemaCXX/coreturn-exp-namespace.cpp
===================================================================
--- clang/test/SemaCXX/coreturn-exp-namespace.cpp
+++ clang/test/SemaCXX/coreturn-exp-namespace.cpp
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts -fsyntax-only -Wignored-qualifiers -Wno-error=return-type -verify -fblocks -Wall -Wextra -Wno-error=unreachable-code
+// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts -fsyntax-only -Wignored-qualifiers -Wno-error=return-type -verify=expected,declared -fblocks -Wall -Wextra -Wno-error=unreachable-code
 #include "Inputs/std-coroutine-exp-namespace.h"
 
 using std::experimental::suspend_always;
Index: clang/test/SemaCXX/coroutine-final-suspend-noexcept-exp-namespace.cpp
===================================================================
--- clang/test/SemaCXX/coroutine-final-suspend-noexcept-exp-namespace.cpp
+++ clang/test/SemaCXX/coroutine-final-suspend-noexcept-exp-namespace.cpp
@@ -7,6 +7,7 @@
 namespace experimental {
 template <class Ret, typename... T>
 struct coroutine_traits { using promise_type = typename Ret::promise_type; };
+// expected-note@-1{{declared here}}
 
 template <class Promise = void>
 struct coroutine_handle {
Index: clang/test/SemaCXX/coroutine-mixed-exp-namespace.cpp
===================================================================
--- clang/test/SemaCXX/coroutine-mixed-exp-namespace.cpp
+++ clang/test/SemaCXX/coroutine-mixed-exp-namespace.cpp
@@ -1,9 +1,11 @@
-// This file is to test the mixed use of `std::experimental::coroutine*` and `std::coroutine*`
-// wouldn't make the compiler to crash and emit the diagnostic message correctly.
-// RUN: %clang_cc1 -verify -std=c++20 -fsyntax-only %s
+// This file is to test the mixed use of `std::experimental::coroutine*` and
+// `std::coroutine*` wouldn't make the compiler to crash and emit the diagnostic
+// message correctly.
+
+// RUN: %clang_cc1 -verify=expected,declared -std=c++20 -fsyntax-only %s
 
 #include "Inputs/std-coroutine-exp-namespace.h"
-#include "Inputs/std-coroutine.h"
+#include "Inputs/std-coroutine.h" // Second
 
 struct my_awaitable {
   bool await_ready() noexcept;
Index: clang/test/SemaCXX/coroutine-mixed2-exp-namespace.cpp
===================================================================
--- clang/test/SemaCXX/coroutine-mixed2-exp-namespace.cpp
+++ clang/test/SemaCXX/coroutine-mixed2-exp-namespace.cpp
@@ -1,10 +1,12 @@
-// This file is to test the mixed use of `std::experimental::coroutine_traits` and `std::coroutine_traits`
-// which is similar to coroutine-mixed-exp-namesapce. This file tests the relative order of
-// included header wouldn't affect the diagnostic messages.
-// RUN: %clang_cc1 -verify -std=c++20 -fsyntax-only %s
+// This file is to test the mixed use of `std::experimental::coroutine_traits`
+// and `std::coroutine_traits` which is similar to
+// coroutine-mixed-exp-namespace. This file tests the relative order of included
+// header wouldn't affect the diagnostic messages.
 
+// RUN: %clang_cc1 -verify=expected,declared -std=c++20 -fsyntax-only %s
+
+#include "Inputs/std-coroutine.h" // First
 #include "Inputs/std-coroutine-exp-namespace.h"
-#include "Inputs/std-coroutine.h"
 
 struct my_awaitable {
   bool await_ready() noexcept;
Index: clang/test/SemaCXX/coroutine-rvo-exp-namespace.cpp
===================================================================
--- clang/test/SemaCXX/coroutine-rvo-exp-namespace.cpp
+++ clang/test/SemaCXX/coroutine-rvo-exp-namespace.cpp
@@ -30,6 +30,7 @@
 
 template <class Ret, class... Args>
 struct coroutine_traits : public traits_sfinae_base<Ret> {};
+// expected-note@-1{{declared here}}
 } // namespace std::experimental
 
 struct suspend_never {
Index: clang/test/SemaCXX/coroutine-seh-exp-namespace.cpp
===================================================================
--- clang/test/SemaCXX/coroutine-seh-exp-namespace.cpp
+++ clang/test/SemaCXX/coroutine-seh-exp-namespace.cpp
@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -std=c++1z -fcoroutines-ts -verify %s -fcxx-exceptions -fexceptions -triple x86_64-windows-msvc -fms-extensions
 namespace std::experimental {
 template <typename... T> struct coroutine_traits;
+// expected-note@-1{{declared here}}
 
 template <class Promise = void> struct coroutine_handle {
   coroutine_handle() = default;
Index: clang/test/SemaCXX/coroutine-traits-undefined-template-exp-namespace.cpp
===================================================================
--- clang/test/SemaCXX/coroutine-traits-undefined-template-exp-namespace.cpp
+++ clang/test/SemaCXX/coroutine-traits-undefined-template-exp-namespace.cpp
@@ -6,7 +6,7 @@
 namespace std {
 namespace experimental {
 template <typename... T>
-struct coroutine_traits {
+struct coroutine_traits { // expected-note{{declared here}}
   struct promise_type {};
 };
 
Index: clang/test/SemaCXX/coroutine-unhandled_exception-warning-exp-namespace.cpp
===================================================================
--- clang/test/SemaCXX/coroutine-unhandled_exception-warning-exp-namespace.cpp
+++ clang/test/SemaCXX/coroutine-unhandled_exception-warning-exp-namespace.cpp
@@ -1,6 +1,6 @@
 // RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts \
-// RUN:    -fsyntax-only -Wignored-qualifiers -Wno-error=return-type -verify \
-// RUN:    -fblocks -Wno-unreachable-code -Wno-unused-value
+// RUN:    -fsyntax-only -Wignored-qualifiers -Wno-error=return-type \
+// RUN:    -verify=expected,declared -fblocks -Wno-unreachable-code -Wno-unused-value
 
 // RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts \
 // RUN:    -fsyntax-only -Wignored-qualifiers -Wno-error=return-type -verify \
Index: clang/test/SemaCXX/coroutine_handle-address-return-type-exp-namespace.cpp
===================================================================
--- clang/test/SemaCXX/coroutine_handle-address-return-type-exp-namespace.cpp
+++ clang/test/SemaCXX/coroutine_handle-address-return-type-exp-namespace.cpp
@@ -32,6 +32,7 @@
 
 template <class Ret, class... Args>
 struct coroutine_traits : public traits_sfinae_base<Ret> {};
+// expected-note@-1{{declared here}}
 } // namespace std::experimental
 
 struct suspend_never {
Index: clang/test/SemaCXX/coroutines-exp-namespace.cpp
===================================================================
--- clang/test/SemaCXX/coroutines-exp-namespace.cpp
+++ clang/test/SemaCXX/coroutines-exp-namespace.cpp
@@ -45,6 +45,7 @@
 
 template <class Ret, class... Args>
 struct coroutine_traits : public traits_sfinae_base<Ret> {};
+// expected-note@-1{{declared here}}
 } // namespace experimental
 } // namespace std