[clang] [Sema] Fix computations of "unexpanded packs" in substituted lambdas (PR #99882)

[via cfe-commits]

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@@ -353,7 +353,11 @@ Sema::DiagnoseUnexpandedParameterPacks(SourceLocation Loc,
       }
 
       if (!EnclosingStmtExpr) {
-        LSI->ContainsUnexpandedParameterPack = true;
+        // It is ok to have unexpanded packs in captures, template parameters
+        // and parameters too, but only the body statement does not store this
+        // flag, so we have to propagate it through LamdaScopeInfo.
+        if (LSI->AfterParameterList)
+          LSI->BodyContainsUnexpandedParameterPack = true;
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cor3ntin wrote:

I think this just force us to do more work - going over the lambda twice is not ideal

https://github.com/llvm/llvm-project/pull/99882