[clang] [Clang] Fix an integer overflow issue in computing CTAD's parameter depth (PR #128704)

[Younan Zhang via cfe-commits]

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@@ -377,8 +377,12 @@ struct ConvertConstructorToDeductionGuideTransform {
         if (NestedPattern)
           Args.addOuterRetainedLevels(NestedPattern->getTemplateDepth());
         auto [Depth, Index] = getDepthAndIndex(Param);
+        assert(Depth ||
+               cast<ClassTemplateSpecializationDecl>(FTD->getDeclContext())
+                   ->isExplicitSpecialization());
         NamedDecl *NewParam = transformTemplateParameter(
-            SemaRef, DC, Param, Args, Index + Depth1IndexAdjustment, Depth - 1);
+            SemaRef, DC, Param, Args, Index + Depth1IndexAdjustment,
+            Depth ? Depth - 1 : 0);
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zyn0217 wrote:

Oh I realized we sometimes still want to use `Depth - 1` even if it is an explicit specialization (because it could be nested as in the first case, and the transformed depth should be 0) - so I think this is good enough?

https://github.com/llvm/llvm-project/pull/128704