Fix #14371: FP redundantAssignment for nested union members

Reading another member of the same union in a subexpression of the rhs
is a use of the previous value through aliasing, so the assignment is
not redundant. Check the whole rhs expression for the union variable
instead of only the leading member-access chain.

Author: DanTheMan2000

lib/fwdanalysis.cpp

@@ -261,6 +261,8 @@ FwdAnalysis::Result FwdAnalysis::checkRecursive(const Token *expr, const Token *
                         parent = parent->astParent();
                     if (parent && parent->valueType() && Token::simpleMatch(parent->astParent(), "=") && !Token::Match(parent->astParent()->astParent(), "%assign%") && parent->astParent()->astOperand1() == parent) {
                         const Token * assignment = parent->astParent()->astOperand2();
+                        if (hasOperand(assignment, expr))
+                            return Result(Result::Type::READ);
                         while (Token::simpleMatch(assignment, ".") && assignment->varId() != expr->varId())
                             assignment = assignment->astOperand1();
                         if (assignment && assignment->varId() != expr->varId()) {

test/testother.cpp

@@ -11063,6 +11063,23 @@ class TestOther : public TestFixture {
               "    Dst.s->y = Src.s->y;\n"
               "}\n");
         ASSERT_EQUALS("", errout_str());
+
+        // Ticket #14371 "redundantAssignment when using a union"
+        check("union U {\n"
+              "    struct {\n"
+              "        unsigned int abcd;\n"
+              "    } u32;\n"
+              "    struct {\n"
+              "        unsigned short ab;\n"
+              "        unsigned short cd;\n"
+              "    } u16;\n"
+              "};\n"
+              "void f() {\n"
+              "    U m;\n"
+              "    m.u32.abcd = 1234;\n"
+              "    m.u32.abcd = 5 * m.u16.ab;\n"
+              "}\n", dinit(CheckOptions, $.inconclusive = false));
+        ASSERT_EQUALS("", errout_str());
     }
 
     void redundantVarAssignment_7133() {