[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 redeclarable declarations can change during import, `ASTImporter` worked always this way. In the "order" I mean order got with `getPreviousDecl`. If we go back the decl chain of a template with default argument, the first one should have the non-inherited default argument and all others should have an inherited one. This property is preserved now, even if the "To" AST has different declaration chain than the "From" AST, always the first occurrence of a template will have the non-inherited default argument.

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