Fix #13345: FP unusedStructMember when member is only used in offsetof() (danmar#7041)

Author: ludviggunne

lib/checkunusedvar.cpp

@@ -1549,6 +1549,14 @@ void CheckUnusedVar::checkStructMemberUsage()
                     use = true;
                     break;
                 }
+                // Member referenced in offsetof
+                if (Token::Match(tok, ("offsetof ( struct| " + scope.className + " , %name%").c_str())) {
+                    tok = tok->astSibling()->astOperand2();
+                    if (tok->str() == var.name()) {
+                        use = true;
+                        break;
+                    }
+                }
                 if (tok->variable() != &var)
                     continue;
                 if (tok != var.nameToken()) {

test/testunusedvar.cpp

@@ -70,6 +70,7 @@ class TestUnusedVar : public TestFixture {
         TEST_CASE(structmember23);
         TEST_CASE(structmember24); // #10847
         TEST_CASE(structmember25);
+        TEST_CASE(structmember26); // #13345
         TEST_CASE(structmember_macro);
         TEST_CASE(classmember);
 
@@ -1962,6 +1963,14 @@ class TestUnusedVar : public TestFixture {
                       errout_str());
     }
 
+    void structmember26() { // #13345
+        checkStructMemberUsage("struct foobar {\n"
+                               "   char unused;\n"
+                               "};\n"
+                               "size_t offset_unused = offsetof(struct foobar, unused);\n");
+        ASSERT_EQUALS("", errout_str());
+    }
+
     void structmember_macro() {
         checkStructMemberUsageP("#define S(n) struct n { int a, b, c; };\n"
                                 "S(unused);\n");