Index: clang-tidy/misc/DefinitionsInHeadersCheck.cpp
===================================================================
--- clang-tidy/misc/DefinitionsInHeadersCheck.cpp
+++ clang-tidy/misc/DefinitionsInHeadersCheck.cpp
@@ -127,6 +127,12 @@
          "function definitions in header files can lead to ODR violations")
         << FD << FixItHint::CreateInsertion(
                      FD->getReturnTypeSourceRange().getBegin(), "inline ");
+    // Output notes for full function template specializations.
+    if (FD->getTemplateSpecializationKind() != TSK_Undeclared)
+      diag(FD->getLocation(), "this is a full function template specilization "
+                              "which behaves as a regular function, so the ODR "
+                              "still applies",
+           DiagnosticIDs::Note);
   } else if (const auto *VD = dyn_cast<VarDecl>(ND)) {
     // Static data members of a class template are allowed.
     if (VD->getDeclContext()->isDependentContext() && VD->isStaticDataMember())
Index: test/clang-tidy/misc-definitions-in-headers.hpp
===================================================================
--- test/clang-tidy/misc-definitions-in-headers.hpp
+++ test/clang-tidy/misc-definitions-in-headers.hpp
@@ -29,6 +29,7 @@
 template <>
 int CA::f3() {
 // CHECK-MESSAGES: :[[@LINE-1]]:9: warning: function 'f3<int>' defined in a header file;
+// CHECK-MESSAGES: :[[@LINE-2]]:9: note: this is a full function template specilization
 // CHECK-FIXES: inline int CA::f3() {
   int a = 1;
   return a;
@@ -93,6 +94,7 @@
 template <>
 int f3() {
 // CHECK-MESSAGES: :[[@LINE-1]]:5: warning: function 'f3<int>' defined in a header file;
+// CHECK-MESSAGES: :[[@LINE-2]]:5: note: this is a full function template specilization
 // CHECK-FIXES: inline int f3() {
   int a = 1;
   return a;