[clang] Diagnose potential size confusion with VLA params (PR #129772)

[via cfe-commits]

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@@ -10333,6 +10334,74 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
       }
     }
 
+    // Loop over the parameters to see if any of the size expressions contains
+    // a DeclRefExpr which refers to a variable from an outer scope that is
+    // also named later in the parameter list.
+    // e.g., int n; void func(int array[n], int n);
+    SmallVector<const DeclRefExpr *, 2> DRESizeExprs;
+    llvm::for_each(Params, [&](const ParmVarDecl *Param) {
+      // If we have any size expressions we need to check against, check them
+      // now.
+      for (const auto *DRE : DRESizeExprs) {
+        // Check to see if this parameter has the same name as one of the
+        // DeclRefExprs we wanted to test against. If so, then we found a
+        // situation where an earlier parameter refers to the name of a later
+        // parameter, which is (currently) only valid if there's a variable
+        // from an outer scope with the same name.
+        if (const auto *SizeExprND = dyn_cast<NamedDecl>(DRE->getDecl())) {
+          if (SizeExprND->getIdentifier() == Param->getIdentifier()) {
----------------
cor3ntin wrote:

```suggestion
        if (const auto *SizeExprND = dyn_cast<NamedDecl>(DRE->getDecl()); SizeExprND && SizeExprND->getIdentifier() == Param->getIdentifier()) {
```

https://github.com/llvm/llvm-project/pull/129772