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

[Corentin Jabot 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++) {
+      const NamedDecl *Param = TemplateParams->getParam(I);
+      if (Param->isParameterPack())
+        break;
+      const TemplateArgument *Arg =
+          SemaRef.getASTContext().getDefaultTemplateArgumentOrNone(Param);
+      assert(Arg && "expected a default argument");
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cor3ntin wrote:

It returns null if there is no default argument, but we except all of these parameters to have default arguments

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