Fix #14075 (False positive: unusedStructMember, member usage by alignas is ignored) (#7842)

This should be reviewed extra carefully; I used ai to fix the ticket.

Author: danmar

lib/checkunusedvar.cpp

@@ -1628,6 +1628,17 @@ void CheckUnusedVar::checkStructMemberUsage()
                         break;
                     }
                 }
+                // Member referenced in alignas
+                if (tok->hasAttributeAlignas()) {
+                    const std::vector<std::string> alignasExpressions = tok->getAttributeAlignas();
+                    use = std::any_of(alignasExpressions.cbegin(),
+                                      alignasExpressions.cend(),
+                                      [&var](const std::string& alignasExpr){
+                        return alignasExpr == var.name();
+                    });
+                    if (use)
+                        break;
+                }
                 if (tok->variable() != &var)
                     continue;
                 if (tok != var.nameToken()) {

test/testunusedvar.cpp

@@ -71,6 +71,7 @@ class TestUnusedVar : public TestFixture {
         TEST_CASE(structmember26); // #13345
         TEST_CASE(structmember27); // #13367
         TEST_CASE(structmember28);
+        TEST_CASE(structmember29); // #14075
         TEST_CASE(structmember_macro);
         TEST_CASE(structmember_template_argument); // #13887 - do not report that member used in template argument is unused
         TEST_CASE(classmember);
@@ -2013,6 +2014,15 @@ class TestUnusedVar : public TestFixture {
         ASSERT_EQUALS("[test.cpp:2:18]: (style) struct member 'S::a' is never used. [unusedStructMember]\n", errout_str());
     }
 
+    void structmember29() { // #14075 - alignas false positive
+        checkStructMemberUsage("struct S {\n"
+                               "    static constexpr size_t cDataAlign = 8;\n"
+                               "    static constexpr size_t cDataSize = 128;\n"
+                               "    alignas(cDataAlign) std::array<uint8_t, cDataSize> storage{};\n"
+                               "};\n");
+        ASSERT_EQUALS("[test.cpp:4:56]: (style) struct member 'S::storage' is never used. [unusedStructMember]\n", errout_str());
+    }
+
     void structmember_macro() {
         checkStructMemberUsageP("#define S(n) struct n { int a, b, c; };\n"
                                 "S(unused);\n");