[clang] [clang][analyzer] Fix the false positive ArgInitializedness warning on unnamed bit-field (PR #145066)

[Balazs Benics via cfe-commits]

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@@ -2122,8 +2122,21 @@ SVal RegionStoreManager::getBindingForField(RegionBindingsConstRef B,
   if (const std::optional<SVal> &V = B.getDirectBinding(R))
     return *V;
 
-  // If the containing record was initialized, try to get its constant value.
+  // UnnamedBitField is always Undefined unless using memory operation such
+  // as 'memset'.
+  // For example, for code
+  //    typedef struct {
+  //      int i  :2;
+  //      int    :30;  // unnamed bit-field
+  //    } A;
+  //    A a = {1};
+  // The bits of the unnamed bit-field in local variable a can be anything.
   const FieldDecl *FD = R->getDecl();
+  if (FD->isUnnamedBitField()) {
+      return UndefinedVal();
+  }
+
+  // If the containing record was initialized, try to get its constant value.
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steakhal wrote:

I think the idea was that a copy in C++ means a member-wise copy. And there anonym bitfields don't count.
Consequently, the padding bits may or may not get copied in a copy operation, thus they wanted to having it Undef.
I don't think it's important. We can drop this part.

https://github.com/llvm/llvm-project/pull/145066