suppress undefined-template warnings when the pattern is declared in a system header

[nlewycky]

Summary

The way to fix an undefined-template warning is to add lines to the header file that defines the template pattern. We should suppress the warnings when the template pattern is in a system header because we don't expect users to edit those.

[nlewycky]

Ping!

[rsmith]

Thanks, I agree with your reasoning.

lib/Sema/SemaTemplateInstantiateDecl.cpp

if (DiagnoseUninstantiableTemplate(PointOfInstantiation, Function,
     else if (TSK == TSK_ExplicitInstantiationDefinition) {
       // Try again at the end of the translation unit (at which point a
       // definition will be required).
       assert(!Recursive);
       Function->setInstantiationIsPending(true);
       PendingInstantiations.push_back(
         std::make_pair(Function, PointOfInstantiation));
     } else if (TSK == TSK_ImplicitInstantiation) {
-      if (AtEndOfTU && !getDiagnostics().hasErrorOccurred()) {
+      if (AtEndOfTU && !getDiagnostics().hasErrorOccurred() &&
+          !getSourceManager().isInSystemHeader(PatternDecl->getLocStart())) {
         Diag(PointOfInstantiation, diag::warn_func_template_missing)
           << Function;
         Diag(PatternDecl->getLocation(), diag::note_forward_template_decl);
         if (getLangOpts().CPlusPlus11)
           Diag(PointOfInstantiation, diag::note_inst_declaration_hint)
             << Function;
       }
     }
void Sema::InstantiateVariableDefinition(SourceLocation PointOfInstantiation,
   // definition (or provide a specialization for it) in another translation
   // unit.
   if (!Def && !DefinitionRequired) {
     if (TSK == TSK_ExplicitInstantiationDefinition) {
       PendingInstantiations.push_back(
         std::make_pair(Var, PointOfInstantiation));
     } else if (TSK == TSK_ImplicitInstantiation) {
       // Warn about missing definition at the end of translation unit.
-      if (AtEndOfTU && !getDiagnostics().hasErrorOccurred()) {
+      if (AtEndOfTU && !getDiagnostics().hasErrorOccurred() &&
+          !getSourceManager().isInSystemHeader(PatternDecl->getLocStart())) {
         Diag(PointOfInstantiation, diag::warn_var_template_missing)
           << Var;
         Diag(PatternDecl->getLocation(), diag::note_forward_template_decl);
         if (getLangOpts().CPlusPlus11)
           Diag(PointOfInstantiation, diag::note_inst_declaration_hint) << Var;
       }
       return;
     }

test/SemaTemplate/undefined-template.cpp

 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++14 -Wundefined-func-template %s
 
+#if !defined(INCLUDE)
 template <class T> struct C1 {
   static char s_var_1;       // expected-note{{forward declaration of template entity is here}}
   static char s_var_2;       // expected-note{{forward declaration of template entity is here}}
   static void s_func_1();    // expected-note{{forward declaration of template entity is here}}
   static void s_func_2();    // expected-note{{forward declaration of template entity is here}}
   void meth_1();             // expected-note2{{forward declaration of template entity is here}}
   void meth_2();
   template <class T1> static char s_tvar_2;      // expected-note{{forward declaration of template entity is here}}

 namespace test_24 {
   template <typename T> struct X {
     friend void g(int);
     operator int() { return 0; }
   };
   void h(X<int> x) { g(x); } // no warning for use of 'g' despite the declaration having been instantiated from a template
 }
 
+#define INCLUDE
+#include "undefined-template.cpp"
+void func_25(SystemHeader<char> *x) {
+  x->meth();
+}
+
 int main() {
   return 0;
 }
+#else
+#pragma clang system_header
+template <typename T> struct SystemHeader { T meth(); };
+#endif