Fix #13268 FP: unusedFunction: [[maybe_unused]] (#6954)

Author: chrchr-github

Diff for: lib/checkunusedfunctions.cpp

@@ -98,6 +98,10 @@ void CheckUnusedFunctions::parseTokens(const Tokenizer &tokenizer, const Setting
 
             FunctionUsage &usage = mFunctions[stripTemplateParameters(func->name())];
 
+            if (func->retDef && (func->retDef->isAttributeUnused() || func->retDef->isAttributeMaybeUnused())) {
+                usage.usedOtherFile = true;
+            }
+
             if (!usage.lineNumber)
                 usage.lineNumber = func->token->linenr();
 

Diff for: test/testunusedfunctions.cpp

@@ -501,6 +501,10 @@ class TestUnusedFunctions : public TestFixture {
         // #10661
         check("extern \"C\" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t dataSize) { return 0; }\n");
         ASSERT_EQUALS("", errout_str());
+
+        check("[[maybe_unused]] void f() {}\n" // #13268
+              "__attribute__((unused)) void g() {}\n");
+        ASSERT_EQUALS("", errout_str());
     }
 
     void initializer_list() {