[clang] [clang][ASTImporter] New fix for default template parameter values. (PR #101836)

[Balázs Kéri via cfe-commits]

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@@ -5968,11 +5962,21 @@ ASTNodeImporter::VisitTemplateTypeParmDecl(TemplateTypeParmDecl *D) {
   }
 
   if (D->hasDefaultArgument()) {
+    // Default argument can be "inherited" when it has a reference to the
+    // previous declaration (of the default argument) which is stored only once.
+    // Here we import the default argument in any case, and the inherited state
+    // is updated later after the parent template was created. If the
+    // inherited-from object would be imported here it causes more difficulties
+    // (parent template may not be created yet and import loops can occur).
     Expected<TemplateArgumentLoc> ToDefaultArgOrErr =
         import(D->getDefaultArgument());
     if (!ToDefaultArgOrErr)
       return ToDefaultArgOrErr.takeError();
-    ToD->setDefaultArgument(ToD->getASTContext(), *ToDefaultArgOrErr);
+    // The just called import process can trigger import of the parent template
+    // which can update the default argument value to "inherited". This should
+    // not be changed.
+    if (!ToD->hasDefaultArgument())
+      ToD->setDefaultArgument(ToD->getASTContext(), *ToDefaultArgOrErr);
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balazske wrote:

The order of objects (or the declaration chain) after the import can be different than the original order, probably for any `Decl`. This is because declarations have often recursive or circular references to each other. I think this is true for templates too (and the "templated" declarations in the templates). The redeclarations are imported all the time currently, only if a definition is imported it is mapped to an existing definition if possible. It was an old decision to not merge any (non-definition) declarations at import because it creates only even more difficulties.

https://github.com/llvm/llvm-project/pull/101836