[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:

About template parameters decl context: It is the top-level TU initially, but the constructor of the template changes it to the template itself (this process is called "adopting" the template parameter). The parameters are created before the template because they are passed to the constructor.

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