[clang-tidy] Improve diagnostic message for misc-definitions-in-header.

[hokein]

Summary

Users might get confused easily when they see the check's message on
full template function speciliations.

Add a note to the output message, which mentions these kind of function
specializations are treated as regular functions.

[alexfh]

clang-tidy/misc/DefinitionsInHeadersCheck.cpp

131–135

Notes are useful for pointing at related but different locations (e.g. point to the declaration of an entity when the diagnostic is issued at a reference to the entity).

Here I would suggest just issuing a different message (e.g. diag(..., "%select{function|full function template specialization}0 %1 defined in a header file ....") << (FD->getTemplateSpecializationKind() != TSK_Undeclared) << FD << ...;) or if it seems more helpful, add a note pointing to the template being specialized.

[hokein]

Address review comment.

[hokein]

clang-tidy/misc/DefinitionsInHeadersCheck.cpp

131–135

SG. Didn't know %select before, thanks for pointing it out.

[alexfh]

LG. Thank you!

clang-tidy/misc/DefinitionsInHeadersCheck.cpp

if (const auto *MD = dyn_cast<CXXMethodDecl>(FD)) {
       }
     }
 
     diag(FD->getLocation(),
          "function %0 defined in a header file; "
          "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())
       return;
     if (VD->getTemplateSpecializationKind() == TSK_ImplicitInstantiation)
       return;
     // Ignore variable definition within function scope.
     if (VD->hasLocalStorage() || VD->isStaticLocal())

test/clang-tidy/misc-definitions-in-headers.hpp

 
 void CA::f2() { }
 // CHECK-MESSAGES: :[[@LINE-1]]:10: warning: function 'f2' defined in a header file;
 // CHECK-FIXES: inline void CA::f2() {
 
 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;
 }
 
 template <typename T>
 void CA::CAA<T>::CAB::f4() {
 // OK: member function definition of a nested template class in a class.

 T f3() {
   T a = 1;
   return a;
 }
 
 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;
 }
 
 int f5(); // OK: function declaration.
 inline int f6() { return 1; } // OK: inline function definition.
 namespace {