[clang] [analyzer] Ignoring `T v=v; ` idiom for uninitialized variable checker and dead store checker (PR #187530)

[Aaron Ballman via cfe-commits]

================
@@ -620,6 +620,21 @@ void ExprEngine::VisitDeclStmt(const DeclStmt *DS, ExplodedNode *Pred,
     return;
   }
 
+  // Self-assignment initialization in variable declaration,
+  // i.e., `int x = x;`,
+  // is a C idiom to suppress warnings of unused variables.
+  // This filter will not match variables of C++ record types, but will match
+  // C++ references. Allow references continuing here to make the undefined
+  // value checker report self-assignments of C++ references.
+  if (const Expr *EI = VD->getInit()) {
+    if (const DeclRefExpr *DR = dyn_cast<DeclRefExpr>(EI->IgnoreImpCasts()))
----------------
AaronBallman wrote:

The kind of situation where this comes up is macro expansions because it's idiomatic to wrap macro arguments in parens. Consider:
```
#define INIT(x) (x)

int v = INIT(v);
```
That ends up expanding to the parenthesized form. I don't think it's the most critical thing to support, but given that it should be low effort, it seems like a nice-to-have.

https://github.com/llvm/llvm-project/pull/187530