[analyzer] Make it a noop when initializing a field of empty record (llvm#138594)

Previously, Static Analyzer initializes empty type fields with zeroes.
This can cause problems when those fields have no unique addresses. For
example, llvm#137252.

rdar://146753089

Author: ziqingluo-90

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"
@@ -715,7 +716,11 @@ 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 =
+            cast<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;
+}