Fix a diagnoses-valid bug with using declarations

The following was found by a customer and is accepted by the other primary
C++ compilers, but fails to compile in Clang:

namespace sss {
double foo(int, double);
template <class T>
T foo(T); // note: target of using declaration
}  // namespace sss

namespace oad {
void foo();
}

namespace oad {
using ::sss::foo;
}

namespace sss {
using oad::foo; // note: using declaration
}

namespace sss {
double foo(int, double) { return 0; }
template <class T>
T foo(T t) { // error: declaration conflicts with target of using
  return t;
}
}  // namespace sss

I believe the issue is that MergeFunctionDecl() was calling
checkUsingShadowRedecl() but only considering a FunctionDecl as a
possible shadow and not FunctionTemplateDecl. The changes in this patch
largely mirror how variable declarations were being handled by also
catching FunctionTemplateDecl.

clang/lib/Sema/SemaDecl.cpp

@@ -3165,6 +3165,7 @@ static bool haveIncompatibleLanguageLinkages(const T *Old, const T *New) {
 
 template<typename T> static bool isExternC(T *D) { return D->isExternC(); }
 static bool isExternC(VarTemplateDecl *) { return false; }
+static bool isExternC(FunctionTemplateDecl *) { return false; }
 
 /// Check whether a redeclaration of an entity introduced by a
 /// using-declaration is valid, given that we know it's not an overload
@@ -3289,10 +3290,20 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD,
         return true;
       }
 
-      // Check whether the two declarations might declare the same function.
-      if (checkUsingShadowRedecl<FunctionDecl>(*this, Shadow, New))
-        return true;
-      OldD = Old = cast<FunctionDecl>(Shadow->getTargetDecl());
+      // Check whether the two declarations might declare the same function or
+      // function template.
+      if (FunctionTemplateDecl *NewTemplate =
+              New->getDescribedFunctionTemplate()) {
+        if (checkUsingShadowRedecl<FunctionTemplateDecl>(*this, Shadow,
+                                                         NewTemplate))
+          return true;
+        OldD = Old = cast<FunctionTemplateDecl>(Shadow->getTargetDecl())
+                         ->getAsFunction();
+      } else {
+        if (checkUsingShadowRedecl<FunctionDecl>(*this, Shadow, New))
+          return true;
+        OldD = Old = cast<FunctionDecl>(Shadow->getTargetDecl());
+      }
     } else {
       Diag(New->getLocation(), diag::err_redefinition_different_kind)
         << New->getDeclName();

clang/test/CXX/modules-ts/basic/basic.def.odr/p6/global-vs-module.cpp

@@ -9,7 +9,7 @@ struct str; // expected-note {{previous declaration is here}}
 using type = int;
 
 template<typename> extern int var_tpl; // expected-note {{previous declaration is here}}
-template<typename> int func_tpl(); // expected-note-re {{{{previous declaration is here|target of using declaration}}}}
+template<typename> int func_tpl(); // expected-note {{previous declaration is here}}
 template<typename> struct str_tpl; // expected-note {{previous declaration is here}}
 template<typename> using type_tpl = int; // expected-note {{previous declaration is here}}
 
@@ -26,7 +26,7 @@ using ::func;
 using ::str;
 using ::type;
 using ::var_tpl;
-using ::func_tpl; // expected-note {{using declaration}}
+using ::func_tpl;
 using ::str_tpl;
 using ::type_tpl;
 #endif
@@ -41,8 +41,7 @@ struct str; // expected-error {{declaration of 'str' in module M follows declara
 using type = int;
 
 template<typename> extern int var_tpl; // expected-error {{declaration of 'var_tpl' in module M follows declaration in the global module}}
-// FIXME: Is this the right diagnostic in the -DUSING case?
-template<typename> int func_tpl(); // expected-error-re {{{{declaration of 'func_tpl' in module M follows declaration in the global module|conflicts with target of using declaration}}}}
+template<typename> int func_tpl(); // expected-error {{declaration of 'func_tpl' in module M follows declaration in the global module}}
 template<typename> struct str_tpl; // expected-error {{declaration of 'str_tpl' in module M follows declaration in the global module}}
 template<typename> using type_tpl = int; // expected-error {{declaration of 'type_tpl' in module M follows declaration in the global module}}
 

clang/test/CXX/modules-ts/basic/basic.def.odr/p6/module-vs-global.cpp

@@ -10,9 +10,9 @@ struct str; // expected-error {{declaration of 'str' in the global module follow
 using type = int;
 
 template<typename> extern int var_tpl; // expected-error {{declaration of 'var_tpl' in the global module follows declaration in module M}} expected-note@global-vs-module.cpp:43 {{previous}}
-template<typename> int func_tpl(); // expected-error {{declaration of 'func_tpl' in the global module follows declaration in module M}} expected-note@global-vs-module.cpp:45 {{previous}}
-template<typename> struct str_tpl; // expected-error {{declaration of 'str_tpl' in the global module follows declaration in module M}} expected-note@global-vs-module.cpp:46 {{previous}}
-template<typename> using type_tpl = int; // expected-error {{declaration of 'type_tpl' in the global module follows declaration in module M}} expected-note@global-vs-module.cpp:47 {{previous}}
+template<typename> int func_tpl(); // expected-error {{declaration of 'func_tpl' in the global module follows declaration in module M}} expected-note@global-vs-module.cpp:44 {{previous}}
+template<typename> struct str_tpl; // expected-error {{declaration of 'str_tpl' in the global module follows declaration in module M}} expected-note@global-vs-module.cpp:45 {{previous}}
+template<typename> using type_tpl = int; // expected-error {{declaration of 'type_tpl' in the global module follows declaration in module M}} expected-note@global-vs-module.cpp:46 {{previous}}
 
 typedef int type;
 namespace ns { using ::func; }

clang/test/CXX/modules-ts/basic/basic.def.odr/p6/module-vs-module.cpp

@@ -34,9 +34,9 @@ struct str; // expected-error {{declaration of 'str' in module N follows declara
 using type = int;
 
 template<typename> extern int var_tpl; // expected-error {{declaration of 'var_tpl' in module N follows declaration in module M}} expected-note@global-vs-module.cpp:43 {{previous}}
-template<typename> int func_tpl(); // expected-error {{declaration of 'func_tpl' in module N follows declaration in module M}} expected-note@global-vs-module.cpp:45 {{previous}}
-template<typename> struct str_tpl; // expected-error {{declaration of 'str_tpl' in module N follows declaration in module M}} expected-note@global-vs-module.cpp:46 {{previous}}
-template<typename> using type_tpl = int; // expected-error {{declaration of 'type_tpl' in module N follows declaration in module M}} expected-note@global-vs-module.cpp:47 {{previous}}
+template<typename> int func_tpl(); // expected-error {{declaration of 'func_tpl' in module N follows declaration in module M}} expected-note@global-vs-module.cpp:44 {{previous}}
+template<typename> struct str_tpl; // expected-error {{declaration of 'str_tpl' in module N follows declaration in module M}} expected-note@global-vs-module.cpp:45 {{previous}}
+template<typename> using type_tpl = int; // expected-error {{declaration of 'type_tpl' in module N follows declaration in module M}} expected-note@global-vs-module.cpp:46 {{previous}}
 
 typedef int type;
 namespace ns { using ::func; }

clang/test/SemaCXX/using-decl-templates.cpp

@@ -101,3 +101,33 @@ struct Derived : Base<false> { // expected-note {{requested here}}
   using Base<false>::Base; // OK. Don't diagnose that 'Base' isn't a base class of Derived.
 };
 } // namespace DontDiagnoseInvalidTest
+
+namespace func_templ {
+namespace sss {
+double foo(int, double);
+template <class T>
+T foo(T);
+} // namespace sss
+
+namespace oad {
+void foo();
+}
+
+namespace oad {
+using sss::foo;
+}
+
+namespace sss {
+using oad::foo;
+}
+
+namespace sss {
+double foo(int, double) { return 0; }
+// There used to be an error with the below declaration when the example should
+// be accepted.
+template <class T>
+T foo(T t) { // OK
+  return t;
+}
+} // namespace sss
+} // namespace func_templ