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

[Balázs Kéri via cfe-commits]

================
@@ -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:

I try to explain the source location problem with this code:
```
template <class P = Def> class X;
template <class P> class X {};
```
At the second `X` (definition) `P` has inherited default argument.
The import may reverse the order (this reversal looks hard to fix):
```
template <class P = Def> class X {};
template <class P> class X;
```
The default argument is now at the definition of `X`. I think this is the good behavior at import and this is how it currently works. The source location of `= Def` becomes inconsistent with the line where it originally was, and this can affect the "end" location of `<class P = Def>` too.

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