================
@@ -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);
----------------
balazske wrote:
The case when default argument becomes already set happens when it is changed
in function `updateTemplateParametersInheritedFrom` that is called when the
template is imported. This change can happen during the import of the default
argument object. That function calls the `setInheritedFrom` function. I was
experimenting with your idea to just call `getInheritedFrom` and import the
result and set it (and not use `updateTemplateParametersInheritedFrom`), but I
got another crash. I tried to explain this in the added comment.
The "deferring" is done in the current code from
`updateTemplateParametersInheritedFrom` that is called at end of the template
import. I wanted to ensure that a default value is always set in the template
parameter object, because until `updateTemplateParametersInheritedFrom` is
called some other import may happen that uses this value. I think this can
cause ODR detection problem. If the default argument is not set (but will be
set later from `updateTemplateParametersInheritedFrom`) this causes a gap where
the default argument is missing. If during this time something is compared to
this object it can cause false structural in-equivalence. To avoid this the
default argument is imported and set immediately (even if this object is later
discarded). Structural equivalence comparison does not compare if the argument
is "inherited" or not, but it does compare the argument value.
https://github.com/llvm/llvm-project/pull/101836
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