[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;
+    for (unsigned I = TemplateParams->getMinRequiredArguments();
+         I < TemplateParams->size(); I++) {
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ojhunt wrote:

Actually why not simply iterate? TemplateParaemterList seems to support it?

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