[clang] [Analyzer][CFG] Correctly handle rebuilt default arg and default init expression (PR #91879)

[Balazs Benics via cfe-commits]

================
@@ -1964,39 +1964,55 @@ void ExprEngine::Visit(const Stmt *S, ExplodedNode 
*Pred,
     case Stmt::CXXDefaultArgExprClass:
     case Stmt::CXXDefaultInitExprClass: {
       Bldr.takeNodes(Pred);
-      ExplodedNodeSet PreVisit;
-      getCheckerManager().runCheckersForPreStmt(PreVisit, Pred, S, *this);
+      ExplodedNodeSet CheckedSet;
+      getCheckerManager().runCheckersForPreStmt(CheckedSet, Pred, S, *this);
 
       ExplodedNodeSet Tmp;
-      StmtNodeBuilder Bldr2(PreVisit, Tmp, *currBldrCtx);
+      StmtNodeBuilder Bldr2(CheckedSet, Tmp, *currBldrCtx);
 
-      const Expr *ArgE;
-      if (const auto *DefE = dyn_cast<CXXDefaultArgExpr>(S))
+      bool HasRewrittenInit = false;
+      const Expr *ArgE = nullptr;
+      if (const auto *DefE = dyn_cast<CXXDefaultArgExpr>(S)) {
         ArgE = DefE->getExpr();
-      else if (const auto *DefE = dyn_cast<CXXDefaultInitExpr>(S))
+        HasRewrittenInit = DefE->hasRewrittenInit();
+      } else if (const auto *DefE = dyn_cast<CXXDefaultInitExpr>(S)) {
         ArgE = DefE->getExpr();
-      else
+        HasRewrittenInit = DefE->hasRewrittenInit();
+      } else
         llvm_unreachable("unknown constant wrapper kind");
 
-      bool IsTemporary = false;
-      if (const auto *MTE = dyn_cast<MaterializeTemporaryExpr>(ArgE)) {
-        ArgE = MTE->getSubExpr();
-        IsTemporary = true;
-      }
+      if (HasRewrittenInit) {
+        for (auto *I : CheckedSet) {
+          ProgramStateRef state = (*I).getState();
+          const LocationContext *LCtx = (*I).getLocationContext();
+          SVal Val = state->getSVal(ArgE, LCtx);
+          state = state->BindExpr(S, LCtx, Val);
+          Bldr2.generateNode(S, I, state);
+        }
+      } else {
+        // If it's not rewritten, the contents of these expressions are not
+        // actually part of the current function, so we fall back to constant
+        // evaluation.
+        bool IsTemporary = false;
+        if (const auto *MTE = dyn_cast<MaterializeTemporaryExpr>(ArgE)) {
+          ArgE = MTE->getSubExpr();
+          IsTemporary = true;
+        }
+
+        std::optional<SVal> ConstantVal = svalBuilder.getConstantVal(ArgE);
+        if (!ConstantVal)
+          ConstantVal = UnknownVal();
----------------
steakhal wrote:

```suggestion
```
I'd just drop this and use `value_or()` at the only use-site instead.

https://github.com/llvm/llvm-project/pull/91879
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