[clang] [Clang] Add default arguments to the parameter mapping (PR #192071)

[Oliver Hunt via cfe-commits]

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@@ -2105,6 +2105,25 @@ void SubstituteParameterMappings::buildParameterMapping(
       SemaRef.MarkUsedTemplateParameters(Args->arguments(),
                                          /*Depth=*/0, OccurringIndices);
   }
+
+  // If a parameter is only referenced in a default template argument,
+  // we need to add it to the mapping explicitly.
+  {
+    llvm::SmallVector<TemplateArgument> DefaultArgs;
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ojhunt wrote:

Is this intermediate vector necessary? It looks like `Sema::MarkUsedTemplateParameters(ArrayRef<TemplateArgument>,..)` is just iterating the array and calling `::MarkUsedTemplateParameters` an element at a time. It's seems like adding `MarkUsedTemplateParameter(TemplateArgument&,...)` (to match the existing Expr* version), or even more directly just calling `MarkUsedTemplateParameter({Arg},....)`

OTOH that's screaming premature optimization based on impl knowledge that could change in future given a SmallVector is unlikely to actually need to grow in practice.

I guess the question is more "is this design by choice or requirement?"

https://github.com/llvm/llvm-project/pull/192071