diff --git a/lib/symboldatabase.cpp b/lib/symboldatabase.cpp
index 6e3eda8fb5f..715c1c320e5 100644
--- a/lib/symboldatabase.cpp
+++ b/lib/symboldatabase.cpp
@@ -737,6 +737,33 @@ void SymbolDatabase::createSymbolDatabaseFindAllScopes()
                 } else {
                     tok = tok->link();
                 }
+            } else if (Token::Match(tok, "%name% (")) {
+                if (Token::simpleMatch(tok->linkAt(1), ") ;")) {
+                    const Token *funcStart = nullptr;
+                    const Token *argStart = nullptr;
+                    const Token *declEnd = nullptr;
+                    if (isFunction(tok, scope, &funcStart, &argStart, &declEnd)) {
+                        if (declEnd && declEnd->str() == ";") {
+                            bool newFunc = true; // Is this function already in the database?
+                            auto range = scope->functionMap.equal_range(tok->str());
+                            for (std::multimap<std::string, const Function*>::const_iterator it = range.first; it != range.second; ++it) {
+                                if (it->second->argsMatch(scope, it->second->argDef, argStart, emptyString, 0)) {
+                                    newFunc = false;
+                                    break;
+                                }
+                            }
+                            // save function prototype in database
+                            if (newFunc) {
+                                Function function(tok, scope, funcStart, argStart);
+                                if (function.isExtern()) {
+                                    scope->addFunction(std::move(function));
+                                    tok = declEnd;
+                                }
+                            }
+                            continue;
+                        }
+                    }
+                }
             }
             // syntax error?
             if (!scope)
@@ -5902,6 +5929,10 @@ const Function* SymbolDatabase::findFunction(const Token* const tok) const
     // find the scope this function is in
     const Scope *currScope = tok->scope();
     while (currScope && currScope->isExecutable()) {
+        if (const Function* f = currScope->findFunction(tok)) {
+            if (f->isExtern())
+                return f;
+        }
         if (currScope->functionOf)
             currScope = currScope->functionOf;
         else
diff --git a/test/testsymboldatabase.cpp b/test/testsymboldatabase.cpp
index 670ca8f6a22..7a4c7622b0c 100644
--- a/test/testsymboldatabase.cpp
+++ b/test/testsymboldatabase.cpp
@@ -270,6 +270,7 @@ class TestSymbolDatabase : public TestFixture {
         TEST_CASE(memberFunctionOfUnknownClassMacro2);
         TEST_CASE(memberFunctionOfUnknownClassMacro3);
         TEST_CASE(functionLinkage);
+        TEST_CASE(externalFunctionsInsideAFunction); // #12420
 
         TEST_CASE(classWithFriend);
 
@@ -2361,6 +2362,22 @@ class TestSymbolDatabase : public TestFixture {
         ASSERT(f && f->function() && !f->function()->isExtern() && f->function()->retDef->str() == "void");
     }
 
+    void externalFunctionsInsideAFunction() {
+        GET_SYMBOL_DB("void foo( void )\n"
+                      "{\n"
+                      "    extern void bar( void );\n"
+                      "    bar();\n"
+                      "}\n");
+
+        ASSERT(db && errout.str().empty());
+
+        const Token *f = Token::findsimplematch(tokenizer.tokens(), "bar");
+        ASSERT(f && f->function() && f->function()->isExtern() && f == f->function()->tokenDef && f->function()->retDef->str() == "void");
+
+        const Token *call = Token::findsimplematch(f->next(), "bar");
+        ASSERT(call && call->function() == f->function());
+    }
+
     void classWithFriend() {
         GET_SYMBOL_DB("class Foo {}; class Bar1 { friend class Foo; }; class Bar2 { friend Foo; };");
         // 3 scopes: Global, 3 classes