[clang] [Sema] Fix an out-of-bounds crash when diagnosing bad conversion for a function with a parameter pack. (PR #92721)

[via cfe-commits]

================
@@ -11298,8 +11298,14 @@ static void DiagnoseBadConversion(Sema &S, OverloadCandidate *Cand,
   Expr *FromExpr = Conv.Bad.FromExpr;
   QualType FromTy = Conv.Bad.getFromType();
   QualType ToTy = Conv.Bad.getToType();
-  SourceRange ToParamRange =
-      !isObjectArgument ? Fn->getParamDecl(I)->getSourceRange() : SourceRange();
+  SourceRange ToParamRange;
+  if (!isObjectArgument) {
+    if (I < Fn->getNumParams())
+      ToParamRange = Fn->getParamDecl(I)->getSourceRange();
+    else
+      // parameter pack case.
+      ToParamRange = Fn->parameters().back()->getSourceRange();
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cor3ntin wrote:

Can we assert the parameter is of a pack?
Should we put that in a function in FunctionDecl?

What about
```
void f(auto..., int a = 0)
void f(auto..., auto...);
```
(we probably want to find the first pack)




https://github.com/llvm/llvm-project/pull/92721