diff --git a/cli/cmdlineparser.cpp b/cli/cmdlineparser.cpp
index 924009ed018..de814f05a37 100644
--- a/cli/cmdlineparser.cpp
+++ b/cli/cmdlineparser.cpp
@@ -1257,14 +1257,17 @@ CmdLineParser::Result CmdLineParser::parseFromArgs(int argc, const char* const a
             // --std
             else if (std::strncmp(argv[i], "--std=", 6) == 0) {
                 const std::string std = argv[i] + 6;
-                // TODO: print error when standard is unknown
+                bool unknown = false;
                 if (std::strncmp(std.c_str(), "c++", 3) == 0) {
-                    mSettings.standards.cpp = Standards::getCPP(std);
+                    mSettings.standards.cpp = Standards::getCPP(std, unknown);
                 }
                 else if (std::strncmp(std.c_str(), "c", 1) == 0) {
-                    mSettings.standards.c = Standards::getC(std);
+                    mSettings.standards.c = Standards::getC(std, unknown);
                 }
                 else {
+                    unknown = true;
+                }
+                if (unknown) {
                     mLogger.printError("unknown --std value '" + std + "'");
                     return Result::Fail;
                 }
diff --git a/lib/importproject.cpp b/lib/importproject.cpp
index 336e25744c8..4f3364ae88f 100644
--- a/lib/importproject.cpp
+++ b/lib/importproject.cpp
@@ -268,7 +268,7 @@ static std::string unescape(const std::string &in)
     return out;
 }
 
-void ImportProject::fsParseCommand(FileSettings& fs, const std::string& command)
+bool ImportProject::fsParseCommand(FileSettings& fs, const std::string& command)
 {
     std::string defs;
 
@@ -309,8 +309,23 @@ void ImportProject::fsParseCommand(FileSettings& fs, const std::string& command)
             if (std::find(fs.includePaths.cbegin(), fs.includePaths.cend(), i) == fs.includePaths.cend())
                 fs.includePaths.push_back(std::move(i));
         } else if (F=='s' && startsWith(fval,"td")) {
-            ++pos;
-            fs.standard = readUntil(command, &pos, " ");
+            if (command[pos] == '=') {
+                ++pos;
+                fs.standard = readUntil(command, &pos, " ");
+            } else {
+                fs.standard = fval.substr(3);
+            }
+            bool unknown_std = false;
+            (void)Standards::getCPP(fs.standard, unknown_std);
+            if (unknown_std) (void)Standards::getC(fs.standard, unknown_std);
+            if (unknown_std) {
+                const Standards::cppstd_t gnustd = Standards::getGnuCPP(fs.standard, unknown_std);
+                if (unknown_std) {
+                    printError("unknown --std value '" + fs.standard + "'");
+                    return false;
+                }
+                fs.standard = Standards::getCPP(gnustd);
+            }
         } else if (F == 'i' && fval == "system") {
             ++pos;
             std::string isystem = readUntil(command, &pos, " ");
@@ -339,6 +354,7 @@ void ImportProject::fsParseCommand(FileSettings& fs, const std::string& command)
         }
     }
     fsSetDefines(fs, std::move(defs));
+    return true;
 }
 
 bool ImportProject::importCompileCommands(std::istream &istr)
@@ -416,7 +432,8 @@ bool ImportProject::importCompileCommands(std::istream &istr)
             return false;
         }
         FileSettings fs{std::move(path)};
-        fsParseCommand(fs, command); // read settings; -D, -I, -U, -std, -m*, -f*
+        // read settings; -D, -I, -U, -std, -m*, -f*
+        if (!fsParseCommand(fs, command)) return false;
         std::map<std::string, std::string, cppcheck::stricmp> variables;
         fsSetIncludePaths(fs, directory, fs.includePaths, variables);
         fileSettings.push_back(std::move(fs));
diff --git a/lib/importproject.h b/lib/importproject.h
index 7382b56eb8d..c112a0e068a 100644
--- a/lib/importproject.h
+++ b/lib/importproject.h
@@ -64,7 +64,7 @@ class CPPCHECKLIB WARN_UNUSED ImportProject {
         CPPCHECK_GUI
     };
 
-    static void fsParseCommand(FileSettings& fs, const std::string& command);
+    static bool fsParseCommand(FileSettings& fs, const std::string& command);
     static void fsSetDefines(FileSettings& fs, std::string defs);
     static void fsSetIncludePaths(FileSettings& fs, const std::string &basepath, const std::list<std::string> &in, std::map<std::string, std::string, cppcheck::stricmp> &variables);
 
diff --git a/lib/standards.h b/lib/standards.h
index 4965e432fcd..5563e631791 100644
--- a/lib/standards.h
+++ b/lib/standards.h
@@ -68,6 +68,11 @@ struct Standards {
         return "";
     }
     static cstd_t getC(const std::string &std) {
+        bool _unused;
+        return getC(std, _unused);
+    }
+    static cstd_t getC(const std::string &std, bool &unknown) {
+        unknown = false;
         if (std == "c89") {
             return Standards::C89;
         }
@@ -83,6 +88,7 @@ struct Standards {
         if (std == "c23") {
             return Standards::C23;
         }
+        unknown = true;
         return Standards::CLatest;
     }
     bool setCPP(std::string str) {
@@ -114,6 +120,11 @@ struct Standards {
         return "";
     }
     static cppstd_t getCPP(const std::string &std) {
+        bool _unused;
+        return getCPP(std, _unused);
+    }
+    static cppstd_t getCPP(const std::string &std, bool &unknown) {
+        unknown = false;
         if (std == "c++03") {
             return Standards::CPP03;
         }
@@ -135,6 +146,34 @@ struct Standards {
         if (std == "c++26") {
             return Standards::CPP26;
         }
+        unknown = true;
+        return Standards::CPPLatest;
+    }
+    static cppstd_t getGnuCPP(const std::string &std, bool &unknown) {
+        // treat gnu++XX as c++XX
+        unknown = false;
+        if (std == "gnu++03") {
+            return Standards::CPP03;
+        }
+        if (std == "gnu++11") {
+            return Standards::CPP11;
+        }
+        if (std == "gnu++14") {
+            return Standards::CPP14;
+        }
+        if (std == "gnu++17") {
+            return Standards::CPP17;
+        }
+        if (std == "gnu++20") {
+            return Standards::CPP20;
+        }
+        if (std == "gnu++23") {
+            return Standards::CPP23;
+        }
+        if (std == "gnu++26") {
+            return Standards::CPP26;
+        }
+        unknown = true;
         return Standards::CPPLatest;
     }
 };
diff --git a/test/testcmdlineparser.cpp b/test/testcmdlineparser.cpp
index 46425aa161f..b7c46dd5a6f 100644
--- a/test/testcmdlineparser.cpp
+++ b/test/testcmdlineparser.cpp
@@ -1369,8 +1369,8 @@ class TestCmdlineParser : public TestFixture {
     void stdunknown2() {
         REDIRECT;
         const char *const argv[] = {"cppcheck", "--std=cplusplus11", "file.cpp"};
-        TODO_ASSERT_EQUALS(static_cast<int>(CmdLineParser::Result::Fail), static_cast<int>(CmdLineParser::Result::Success), static_cast<int>(parser->parseFromArgs(3, argv)));
-        TODO_ASSERT_EQUALS("cppcheck: error: unknown --std value 'cplusplus11'\n", "", logger->str());
+        ASSERT_EQUALS(static_cast<int>(CmdLineParser::Result::Fail), static_cast<int>(parser->parseFromArgs(3, argv)));
+        ASSERT_EQUALS("cppcheck: error: unknown --std value 'cplusplus11'\n", logger->str());
     }
 
     void platformWin64() {