[clang] [Sema]Substitue parameter packs when deduced from function parameter (PR #79371)

[Gábor Spaits via cfe-commits]

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@@ -4371,6 +4397,41 @@ Sema::TemplateDeductionResult Sema::DeduceTemplateArguments(
           // corresponding argument is a list?
           PackScope.nextPackElement();
         }
+      } else if (!IsTrailingPack && !PackScope.isPartiallyExpanded() &&
+                 PackScope.isDeducedFromEarlierParameter() &&
+                 !isa<PackExpansionType>(ParamTypes[ParamIdx + 1])) {
+        // [temp.deduct.general#3]
+        // When all template arguments have been deduced
+        // or obtained from default template arguments, all uses of template
+        // parameters in the template parameter list of the template are
+        // replaced with the corresponding deduced or default argument values
+        //
+        // If we have a trailing parameter pack, that has been deduced
+        // perviously we substitute the pack here in a similar fashion as seen
+        // above with the trailing parameter packs. The main difference here is
+        // that, in this case we are not processing all of the remaining
+        // arguments. We are only process as many arguments as much we have in
+        // the already deduced parameter.
+        SmallVector<UnexpandedParameterPack, 2> Unexpanded;
+        collectUnexpandedParameterPacks(ParamPattern, Unexpanded);
+        if (Unexpanded.size() == 0)
+          continue;
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spaits wrote:

I think it could be. In that case that every template packs that are needed by the dependent name are all expanded. For example this could happen when we use explicitly specified template arguments.

https://github.com/llvm/llvm-project/pull/79371