[clang] [clang][Sema] Fix crash when diagnosing candidates with parameter packs (PR #93079)

[Matheus Izvekov via cfe-commits]

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@@ -11298,8 +11298,9 @@ 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 && I < Fn->getNumParams())
+    ToParamRange = Fn->getParamDecl(I)->getSourceRange();
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mizvekov wrote:

It seems like the problem here may be that `I` is an index into an argument-as-written list, which doesn't take into account that mapping into the parameter list needs to consider packs.

I think this could do the wrong thing if there are other parameters after the pack.

How does your patch handle the following example?
```C++
template <typename ...a> int b(a..., int);
 int d() { return b<int, int>(0, 0, d); }
```

https://github.com/llvm/llvm-project/pull/93079