[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:

Default smallvector holds 4 (or 8? the size of TemplateArgument is obscured in a wall of unions and badly named structs :D) so it's fine in practice - It was more a question of "did you make a conscious choice here, or is it just because the API doesn't exist atm" :D

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