[clang] [clang] Fix high memory consumption during pack deduction (PR #88637)

[via cfe-commits]

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@@ -831,7 +831,7 @@ class PackDeductionScope {
     if (IsPartiallyExpanded)
       PackElements += NumPartialPackArgs;
     else if (IsExpanded)
-      PackElements += *FixedNumExpansions;
+      PackElements += FixedNumExpansions.value_or(1);
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Sirraide wrote:

> I assumed the template parameter not being a pack is a valid case and not a bug.

Yeah, I’m unfortunately not familiar enough w/ this part of Clang to comment on whether that’s the case or not; I’d have to look into this a bit more.

CC @AaronBallman, @erichkeane 

https://github.com/llvm/llvm-project/pull/88637