[Sema] do not destruct fields of unions

The C++ standard prohibits this implicit destructor call, leading to
incorrect reports from clang-analyzer. This causes projects that use
std::option (including llvm) to fail the cplusplus.NewDelete test
incorrectly when run through the analyzer.

Fixes #119415

Author: vtjnash

clang/lib/Analysis/CFG.cpp

@@ -2041,6 +2041,8 @@ void CFGBuilder::addImplicitDtorsForDestructor(const CXXDestructorDecl *DD) {
   }
 
   // First destroy member objects.
+  if (RD->isUnion())
+    return;
   for (auto *FI : RD->fields()) {
     // Check for constant size array. Set type to array element type.
     QualType QT = FI->getType();

clang/test/Analysis/NewDelete-checker-test.cpp

@@ -441,3 +441,31 @@ void testLeakBecauseNTTPIsNotDeallocation() {
   void* p = ::operator new(10);
   deallocate_via_nttp<not_free>(p);
 }  // leak-warning{{Potential leak of memory pointed to by 'p'}}
+
+namespace optional_union {
+  template <typename T>
+  class unique_ptr {
+    T *q;
+  public:
+    unique_ptr() : q(new T) {}
+    ~unique_ptr() {
+      delete q;
+    }
+  };
+
+  union custom_union_t {
+    unique_ptr<int> present;
+    char notpresent;
+    custom_union_t() : present(unique_ptr<int>()) {}
+    ~custom_union_t() {};
+  };
+
+  void testUnionCorrect() {
+    custom_union_t a;
+    a.present.~unique_ptr<int>();
+  }
+
+  void testUnionLeak() {
+    custom_union_t a;
+  } // leak-warning{{Potential leak of memory pointed to by 'a.present.q'}}
+}

clang/test/Analysis/dtor-array.cpp

@@ -377,3 +377,27 @@ void directUnknownSymbol() {
 }
 
 }
+
+void testUnionDtor() {
+  static int unionDtorCalled;
+  InlineDtor::cnt = 0;
+  InlineDtor::dtorCalled = 0;
+  unionDtorCalled = 0;
+  {
+      union UnionDtor {
+          InlineDtor kind1;
+          char kind2;
+          ~UnionDtor() { unionDtorCalled++; }
+      };
+      UnionDtor u1{.kind1{}};
+      UnionDtor u2{.kind2{}};
+      auto u3 = new UnionDtor{.kind1{}};
+      auto u4 = new UnionDtor{.kind2{}};
+      delete u3;
+      delete u4;
+  }
+
+  clang_analyzer_eval(unionDtorCalled == 4); // expected-warning {{TRUE}}
+  clang_analyzer_eval(InlineDtor::dtorCalled != 4); // expected-warning {{TRUE}}
+  clang_analyzer_eval(InlineDtor::dtorCalled == 0); // expected-warning {{TRUE}}
+}