Index: clang/docs/ReleaseNotes.rst
===================================================================
--- clang/docs/ReleaseNotes.rst
+++ clang/docs/ReleaseNotes.rst
@@ -132,6 +132,9 @@
 - no_sanitize("...") on a global variable for known but not relevant sanitizers
   is now just a warning. It now says that this will be ignored instead of
   incorrectly saying no_sanitize only applies to functions and methods.
+- No longer issue ``-Wctad-maybe-unsupported`` warnings when the template which
+  might not support CTAD is defined in a system header. This silences warnings
+  when using ``std::lock_guard`` from ``<mutex>``.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------
Index: clang/include/clang/Basic/DiagnosticGroups.td
===================================================================
--- clang/include/clang/Basic/DiagnosticGroups.td
+++ clang/include/clang/Basic/DiagnosticGroups.td
@@ -1329,8 +1329,6 @@
 // A group for cross translation unit static analysis related warnings.
 def CrossTU : DiagGroup<"ctu">;
 
-def CTADMaybeUnsupported : DiagGroup<"ctad-maybe-unsupported">;
-
 def FortifySource : DiagGroup<"fortify-source">;
 
 def MaxTokens : DiagGroup<"max-tokens"> {
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2470,7 +2470,7 @@
   InGroup<CXXPre17Compat>, DefaultIgnore;
 def warn_ctad_maybe_unsupported : Warning<
   "%0 may not intend to support class template argument deduction">,
-  InGroup<CTADMaybeUnsupported>, DefaultIgnore;
+  InGroup<DiagGroup<"ctad-maybe-unsupported">>, DefaultIgnore;
 def note_suppress_ctad_maybe_unsupported : Note<
   "add a deduction guide to suppress this warning">;
 
Index: clang/lib/Sema/SemaInit.cpp
===================================================================
--- clang/lib/Sema/SemaInit.cpp
+++ clang/lib/Sema/SemaInit.cpp
@@ -10295,16 +10295,16 @@
   //  by overload resolution for class template deduction.
   QualType DeducedType =
       SubstAutoType(TSInfo->getType(), Best->Function->getReturnType());
-  Diag(TSInfo->getTypeLoc().getBeginLoc(),
-       diag::warn_cxx14_compat_class_template_argument_deduction)
+  SourceLocation TyLoc = TSInfo->getTypeLoc().getBeginLoc();
+  Diag(TyLoc, diag::warn_cxx14_compat_class_template_argument_deduction)
       << TSInfo->getTypeLoc().getSourceRange() << 1 << DeducedType;
 
   // Warn if CTAD was used on a type that does not have any user-defined
-  // deduction guides.
-  if (!HasAnyDeductionGuide) {
-    Diag(TSInfo->getTypeLoc().getBeginLoc(),
-         diag::warn_ctad_maybe_unsupported)
-        << TemplateName;
+  // deduction guides, but do not warn if the deduction guide is in a system
+  // header on the assumption that the support is intentional there.
+  if (!HasAnyDeductionGuide &&
+      !SourceMgr.isInSystemHeader(Template->getLocation())) {
+    Diag(TyLoc, diag::warn_ctad_maybe_unsupported) << TemplateName;
     Diag(Template->getLocation(), diag::note_suppress_ctad_maybe_unsupported);
   }
 
Index: clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
===================================================================
--- clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
+++ clang/test/SemaCXX/cxx1z-class-template-argument-deduction.cpp
@@ -519,6 +519,18 @@
 TestSuppression ta("abc");
 static_assert(__is_same(decltype(ta), TestSuppression<const char *>), "");
 }
+
+// Test that the diagnostic is suppressed if within a system header.
+# 2 "test.h" 1 3
+template <class T>
+struct SysHeaderObj {
+  SysHeaderObj(T) {}
+  SysHeaderObj(T, int) {}
+};
+# 6 "test.h" 2
+
+SysHeaderObj sho(42); // Okay because SysHeaderObj is from a system header.
+
 #pragma clang diagnostic pop
 
 namespace PR41549 {