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

[Haojian Wu 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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hokein wrote:

IIUC, for fully explicit specializations where the template parameter list is empty `<>`, they don't contribute to the depth. This makes me think we should use `Depth` directly if `FTD->getDeclContext()` is an explicit specialization?

Would it be possible the Depth is not 0 in that case? an explicit template specialization is inside a template class?

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