Index: clang/lib/Sema/SemaTemplate.cpp
===================================================================
--- clang/lib/Sema/SemaTemplate.cpp
+++ clang/lib/Sema/SemaTemplate.cpp
@@ -1531,11 +1531,11 @@
 
   CheckValidDeclSpecifiers();
 
-  if (TInfo->getType()->isUndeducedType()) {
-    Diag(D.getIdentifierLoc(),
-         diag::warn_cxx14_compat_template_nontype_parm_auto_type)
-      << QualType(TInfo->getType()->getContainedAutoType(), 0);
-  }
+  if (const auto *T = TInfo->getType()->getContainedDeducedType())
+    if (isa<AutoType>(T))
+      Diag(D.getIdentifierLoc(),
+           diag::warn_cxx14_compat_template_nontype_parm_auto_type)
+          << QualType(TInfo->getType()->getContainedAutoType(), 0);
 
   assert(S->isTemplateParamScope() &&
          "Non-type template parameter not in template parameter scope!");
Index: clang/test/SemaTemplate/temp_arg_nontype_cxx20.cpp
===================================================================
--- clang/test/SemaTemplate/temp_arg_nontype_cxx20.cpp
+++ clang/test/SemaTemplate/temp_arg_nontype_cxx20.cpp
@@ -306,3 +306,11 @@
     f<B>(A<B(0)>()); // OK
   }
 }
+
+namespace GH57362 {
+template <int num>
+class TemplateClass {};
+
+template <TemplateClass nttp> // ok, no diagnostic expected
+void func() {}
+}
Index: clang/test/SemaTemplate/temp_arg_nontype_diagnostic_cxx1z.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaTemplate/temp_arg_nontype_diagnostic_cxx1z.cpp
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 -Wpre-c++17-compat %s
+
+namespace GH57362 {
+template <auto n> // expected-warning {{non-type template parameters declared with 'auto' are incompatible with C++ standards before C++17}}
+struct A{};
+
+template <decltype(auto) n> // expected-warning {{non-type template parameters declared with 'decltype(auto)' are incompatible with C++ standards before C++17}}
+struct B{};
+}