diff --git a/externals/simplecpp/simplecpp.cpp b/externals/simplecpp/simplecpp.cpp
index 81b5e7317..772aed07d 100644
--- a/externals/simplecpp/simplecpp.cpp
+++ b/externals/simplecpp/simplecpp.cpp
@@ -1110,9 +1110,9 @@ unsigned int simplecpp::TokenList::fileIndex(const std::string &filename)
 namespace simplecpp {
     class Macro {
     public:
-        explicit Macro(std::vector<std::string> &f) : nameTokDef(NULL), variadic(false), valueToken(NULL), endToken(NULL), files(f), tokenListDefine(f) {}
+        explicit Macro(std::vector<std::string> &f) : nameTokDef(NULL), variadic(false), valueToken(NULL), endToken(NULL), files(f), tokenListDefine(f), valueDefinedInCode_(false) {}
 
-        Macro(const Token *tok, std::vector<std::string> &f) : nameTokDef(NULL), files(f), tokenListDefine(f) {
+        Macro(const Token *tok, std::vector<std::string> &f) : nameTokDef(NULL), files(f), tokenListDefine(f), valueDefinedInCode_(true) {
             if (sameline(tok->previous, tok))
                 throw std::runtime_error("bad macro syntax");
             if (tok->op != '#')
@@ -1128,7 +1128,7 @@ namespace simplecpp {
                 throw std::runtime_error("bad macro syntax");
         }
 
-        Macro(const std::string &name, const std::string &value, std::vector<std::string> &f) : nameTokDef(NULL), files(f), tokenListDefine(f) {
+        Macro(const std::string &name, const std::string &value, std::vector<std::string> &f) : nameTokDef(NULL), files(f), tokenListDefine(f), valueDefinedInCode_(false) {
             const std::string def(name + ' ' + value);
             std::istringstream istr(def);
             tokenListDefine.readfile(istr);
@@ -1136,12 +1136,13 @@ namespace simplecpp {
                 throw std::runtime_error("bad macro syntax");
         }
 
-        Macro(const Macro &macro) : nameTokDef(NULL), files(macro.files), tokenListDefine(macro.files) {
+        Macro(const Macro &macro) : nameTokDef(NULL), files(macro.files), tokenListDefine(macro.files), valueDefinedInCode_(macro.valueDefinedInCode_) {
             *this = macro;
         }
 
         void operator=(const Macro &macro) {
             if (this != &macro) {
+                valueDefinedInCode_ = macro.valueDefinedInCode_;
                 if (macro.tokenListDefine.empty())
                     parseDefine(macro.nameTokDef);
                 else {
@@ -1151,6 +1152,10 @@ namespace simplecpp {
             }
         }
 
+        bool valueDefinedInCode() const {
+            return valueDefinedInCode_;
+        }
+
         /**
          * Expand macro. This will recursively expand inner macros.
          * @param output     destination tokenlist
@@ -1821,6 +1826,9 @@ namespace simplecpp {
 
         /** usage of this macro */
         mutable std::list<Location> usageList;
+
+        /** was the value of this macro actually defined in the code? */
+        bool valueDefinedInCode_;
     };
 }
 
@@ -2612,7 +2620,7 @@ void simplecpp::preprocess(simplecpp::TokenList &output, const simplecpp::TokenL
             const Macro &macro = macroIt->second;
             const std::list<Location> &usage = macro.usage();
             for (std::list<Location>::const_iterator usageIt = usage.begin(); usageIt != usage.end(); ++usageIt) {
-                MacroUsage mu(usageIt->files);
+                MacroUsage mu(usageIt->files, macro.valueDefinedInCode());
                 mu.macroName = macro.name();
                 mu.macroLocation = macro.defineLocation();
                 mu.useLocation = *usageIt;
diff --git a/externals/simplecpp/simplecpp.h b/externals/simplecpp/simplecpp.h
index 62f7a5866..df1d4d2fe 100644
--- a/externals/simplecpp/simplecpp.h
+++ b/externals/simplecpp/simplecpp.h
@@ -269,10 +269,11 @@ namespace simplecpp {
 
     /** Tracking how macros are used */
     struct SIMPLECPP_LIB MacroUsage {
-        explicit MacroUsage(const std::vector<std::string> &f) : macroLocation(f), useLocation(f) {}
+        explicit MacroUsage(const std::vector<std::string> &f, bool macroValueKnown_) : macroLocation(f), useLocation(f), macroValueKnown(macroValueKnown_) {}
         std::string macroName;
         Location    macroLocation;
         Location    useLocation;
+        bool        macroValueKnown;
     };
 
     /**
diff --git a/lib/preprocessor.cpp b/lib/preprocessor.cpp
index 88831ea6c..681d7a85d 100644
--- a/lib/preprocessor.cpp
+++ b/lib/preprocessor.cpp
@@ -786,6 +786,8 @@ bool Preprocessor::validateCfg(const std::string &cfg, const std::list<simplecpp
             continue;
         const std::string macroName(define.substr(0, define.find('(')));
         for (const simplecpp::MacroUsage &mu : macroUsageList) {
+            if (mu.macroValueKnown)
+                continue;
             if (mu.macroName != macroName)
                 continue;
             bool directiveLocation = false;
diff --git a/test/testpreprocessor.cpp b/test/testpreprocessor.cpp
index 7f085bb35..3df51a192 100644
--- a/test/testpreprocessor.cpp
+++ b/test/testpreprocessor.cpp
@@ -40,6 +40,7 @@ public:
     TestPreprocessor()
         : TestFixture("TestPreprocessor")
         , preprocessor0(settings0, this) {
+        settings0.addEnabled("information");
     }
 
     class OurPreprocessor : public Preprocessor {
@@ -231,7 +232,8 @@ private:
         TEST_CASE(getConfigsU6);
         TEST_CASE(getConfigsU7);
 
-        TEST_CASE(validateCfg);
+        TEST_CASE(validateCfg1);
+        TEST_CASE(validateCfg2);
 
         TEST_CASE(if_sizeof);
 
@@ -2209,11 +2211,11 @@ private:
     }
 
 
-    void validateCfg() {
+    void validateCfg1() {
         Preprocessor preprocessor(settings0, this);
 
         std::vector<std::string> files(1, "test.c");
-        simplecpp::MacroUsage macroUsage(files);
+        simplecpp::MacroUsage macroUsage(files, false);
         macroUsage.useLocation.fileIndex = 0;
         macroUsage.useLocation.line = 1;
         macroUsage.macroName = "X";
@@ -2224,6 +2226,19 @@ private:
         ASSERT_EQUALS(false, preprocessor.validateCfg("A=42;X", macroUsageList));
         ASSERT_EQUALS(true, preprocessor.validateCfg("X=1", macroUsageList));
         ASSERT_EQUALS(true, preprocessor.validateCfg("Y", macroUsageList));
+
+        macroUsageList.front().macroValueKnown = true; // #8404
+        ASSERT_EQUALS(true, preprocessor.validateCfg("X", macroUsageList));
+    }
+
+    void validateCfg2() {
+        const char filedata[] = "#ifdef ABC\n"
+                                "#endif\n"
+                                "int i = ABC;";
+
+        std::map<std::string, std::string> actual;
+        preprocess(filedata, actual, "file.cpp");
+        ASSERT_EQUALS("[file.cpp:3]: (information) Skipping configuration 'ABC' since the value of 'ABC' is unknown. Use -D if you want to check it. You can use -U to skip it explicitly.\n", errout.str());
     }
 
     void if_sizeof() { // #4071