[tcling] Suppress -Wunused-result diagnostics in wrappers generated by TClingCallFunc

core/metacling/src/TClingCallFunc.cxx

@@ -982,7 +982,7 @@ void TClingCallFunc::make_narg_call_with_return(const unsigned N, const string &
    //    new (ret) (return_type) ((class_name*)obj)->func(args...);
    // }
    // else {
-   //    ((class_name*)obj)->func(args...);
+   //    (void)(((class_name*)obj)->func(args...));
    // }
    //
    const FunctionDecl *FD = GetDecl();
@@ -1085,8 +1085,9 @@ void TClingCallFunc::make_narg_call_with_return(const unsigned N, const string &
          for (int i = 0; i < indent_level; ++i) {
             callbuf << kIndentString;
          }
+         callbuf << "(void)(";
          make_narg_call(type_name, N, typedefbuf, callbuf, class_name, indent_level);
-         callbuf << ";\n";
+         callbuf << ");\n";
          for (int i = 0; i < indent_level; ++i) {
             callbuf << kIndentString;
          }

core/metacling/test/TClingCallFuncTests.cxx

@@ -1,3 +1,4 @@
+#include "ROOTUnitTestSupport.h"
 #include "TInterpreter.h"
 
 #include "gtest/gtest.h"
@@ -259,3 +260,36 @@ TEST(TClingCallFunc, DISABLED_OverloadedTemplate)
                            }
                            )cpp");
 }
+
+TEST(TClingCallFunc, FunctionWrapperNodiscard)
+{
+   gInterpreter->Declare(R"cpp(
+                           struct TClingCallFunc_Nodiscard1 {
+                           #if __cplusplus >= 201703L
+                           [[nodiscard]]
+                           #endif
+                             bool foo(int) { return false; }
+                           };
+                           )cpp");
+
+   ClassInfo_t *FooNamespace = gInterpreter->ClassInfo_Factory("TClingCallFunc_Nodiscard1");
+   CallFunc_t *mc = gInterpreter->CallFunc_Factory();
+   Longptr_t offset = 0;
+
+   gInterpreter->CallFunc_SetFuncProto(mc, FooNamespace, "foo", "int", &offset);
+   std::string wrapper = gInterpreter->CallFunc_GetWrapperCode(mc);
+
+   {
+      using ::testing::Not;
+      using ::testing::HasSubstr;
+      FilterDiagsRAII RAII([] (int /*level*/, Bool_t /*abort*/,
+                               const char * /*location*/, const char *msg) {
+         EXPECT_THAT(msg, Not(HasSubstr("-Wunused-result")));
+      });
+      ASSERT_TRUE(gInterpreter->Declare(wrapper.c_str()));
+   }
+
+   // Cleanup
+   gInterpreter->CallFunc_Delete(mc);
+   gInterpreter->ClassInfo_Delete(FooNamespace);
+}

test/unit_testing_support/ROOTUnitTestSupport.h

@@ -18,6 +18,9 @@
 #ifndef ROOT_UNITTESTSUPPORT_H
 #define ROOT_UNITTESTSUPPORT_H
 
+#include "TError.h"
+#include "TInterpreter.h"
+
 #include "gtest/gtest.h"
 #include "gmock/gmock.h"
 
@@ -27,6 +30,27 @@ using testing::StrEq;
 using testing::internal::GetCapturedStderr;
 using testing::internal::CaptureStderr;
 using testing::internal::RE;
+
+/// \brief Allows a user function to filter ROOT/cling diagnostics, e.g.
+/// ```c++
+/// FilterDiagsRAII RAII([] (int level, Bool_t abort,
+///                          const char *location, const char *msg) {
+///       EXPECT_THAT(msg, Not(HasSubstr("-Wunused-result")));
+///    });
+/// ```
+class FilterDiagsRAII {
+   ErrorHandlerFunc_t fPrevHandler;
+public:
+   FilterDiagsRAII(ErrorHandlerFunc_t fn) : fPrevHandler(::GetErrorHandler()) {
+      ::SetErrorHandler(fn);
+      gInterpreter->ReportDiagnosticsToErrorHandler();
+   }
+   ~FilterDiagsRAII() {
+      gInterpreter->ReportDiagnosticsToErrorHandler(/*enable=*/false);
+      ::SetErrorHandler(fPrevHandler);
+   }
+};
+
 class ExpectedDiagRAII {
 public:
    enum ExpectedDiagKind {