Partial fix for #12697 FN ignoredReturnValue with "pure" and "const" function attributes (#6380)

Author: chrchr-github

lib/checkfunctions.cpp

@@ -278,7 +278,8 @@ void CheckFunctions::checkIgnoredReturnValue()
             if ((!tok->function() || !Token::Match(tok->function()->retDef, "void %name%")) &&
                 tok->next()->astOperand1()) {
                 const Library::UseRetValType retvalTy = mSettings->library.getUseRetValType(tok);
-                const bool warn = (tok->function() && tok->function()->isAttributeNodiscard()) || // avoid duplicate warnings for resource-allocating functions
+                const bool warn = (tok->function() && (tok->function()->isAttributeNodiscard() || tok->function()->isAttributePure() || tok->function()->isAttributeConst())) ||
+                                  // avoid duplicate warnings for resource-allocating functions
                                   (retvalTy == Library::UseRetValType::DEFAULT && mSettings->library.getAllocFuncInfo(tok) == nullptr);
                 if (mSettings->severity.isEnabled(Severity::warning) && warn)
                     ignoredReturnValueError(tok, tok->next()->astOperand1()->expressionString());

test/testfunctions.cpp

@@ -1434,6 +1434,16 @@ class TestFunctions : public TestFixture {
               "    delete *v.begin();\n"
               "}\n");
         ASSERT_EQUALS("", errout_str());
+
+        check("int __attribute__((pure)) p_foo(int);\n" // #12697
+              "int __attribute__((const)) c_foo(int);\n"
+              "void f() {\n"
+              "    p_foo(0);\n"
+              "    c_foo(0);\n"
+              "}\n");
+        ASSERT_EQUALS("[test.cpp:4]: (warning) Return value of function p_foo() is not used.\n"
+                      "[test.cpp:5]: (warning) Return value of function c_foo() is not used.\n",
+                      errout_str());
     }
 
     void checkIgnoredErrorCode() {