Re-land "[analyzer] Make it a noop when initializing a field of empty record" (#138951)

The original commit assumes that
`CXXConstructExpr->getType()->getAsRecordDecl()` is always a
`CXXRecordDecl` but it is not true for ObjC programs.

This relanding changes
`cast<CXXRecordDecl>(CXXConstructExpr->getType()->getAsRecordDecl())`
to
`dyn_cast_or_null<CXXRecordDecl>(CXXConstructExpr->getType()->getAsRecordDecl())`

This reverts commit 9048c2d4f2.
rdar://146753089

clang/lib/StaticAnalyzer/Core/ExprEngineCXX.cpp

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "clang/AST/ASTContext.h"
 #include "clang/AST/AttrIterator.h"
 #include "clang/AST/DeclCXX.h"
 #include "clang/AST/ParentMap.h"
@@ -23,6 +24,7 @@
 #include "clang/StaticAnalyzer/Core/PathSensitive/SVals.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/Sequence.h"
+#include "llvm/Support/Casting.h"
 #include <optional>
 
 using namespace clang;
@@ -715,7 +717,11 @@ void ExprEngine::handleConstructor(const Expr *E,
         // actually make things worse. Placement new makes this tricky as well,
         // since it's then possible to be initializing one part of a multi-
         // dimensional array.
-        State = State->bindDefaultZero(Target, LCtx);
+        const CXXRecordDecl *TargetHeldRecord =
+            dyn_cast_or_null<CXXRecordDecl>(CE->getType()->getAsRecordDecl());
+
+        if (!TargetHeldRecord || !TargetHeldRecord->isEmpty())
+          State = State->bindDefaultZero(Target, LCtx);
       }
 
       Bldr.generateNode(CE, N, State, /*tag=*/nullptr,

clang/test/Analysis/issue-137252.cpp

@@ -0,0 +1,50 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=cplusplus -verify %s
+// RUN: %clang_analyze_cc1 -analyzer-checker=cplusplus -verify %s -DEMPTY_CLASS
+// UNSUPPORTED: system-windows
+// expected-no-diagnostics
+
+// This test reproduces the issue that previously the static analyzer
+// initialized an [[no_unique_address]] empty field to zero,
+// over-writing a non-empty field with the same offset.
+
+namespace std {
+#ifdef EMPTY_CLASS
+
+  struct default_delete {};
+  template <class _Tp, class _Dp = default_delete >
+#else
+  // Class with methods and static members is still empty:
+  template <typename T>
+  class default_delete {
+    T dump();
+    static T x;
+  };
+  template <class _Tp, class _Dp = default_delete<_Tp> >
+#endif
+  class unique_ptr {
+    [[no_unique_address]]  _Tp * __ptr_;
+    [[no_unique_address]] _Dp __deleter_;
+
+  public:
+    explicit unique_ptr(_Tp* __p) noexcept
+      : __ptr_(__p),
+        __deleter_() {}
+
+    ~unique_ptr() {
+      delete __ptr_;
+    }
+  };
+}
+
+struct X {};
+
+int main()
+{
+  // Previously a leak falsely reported here.  It was because the
+  // Static Analyzer engine simulated the initialization of
+  // `__deleter__` incorrectly.  The engine assigned zero to
+  // `__deleter__`--an empty record sharing offset with `__ptr__`.
+  // The assignment over wrote `__ptr__`.
+  std::unique_ptr<X> a(new X()); 
+  return 0;
+}