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

[Matheus Izvekov via cfe-commits]

================
@@ -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();
----------------
mizvekov wrote:

Sure, fixing the crash is important, but in that case I would leave a FIXME and keep the bug open, with updated information. It's your call.

Though I don't think solving this is complicated: Just a linear scan on the parameter list:
* A parameter which is a pack consumes `cast<PackExpansionType>(Fn->getParamDecl(I)->getType())->getNumExpansions()` arguments, which can possibly be zero arguments. It should not be possible to find an unexpanded pack in this diagnostic.
* A parameter which is not a pack consumes one argument.

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