[clang] [clang][ASTImporter] Allow import of similar friend template with different depth (PR #115734)

[Michael Buch via cfe-commits]

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@@ -6120,6 +6119,19 @@ ExpectedDecl ASTNodeImporter::VisitClassTemplateDecl(ClassTemplateDecl *D) {
           // see ASTTests test ImportExistingFriendClassTemplateDef.
           continue;
         }
+        // When importing a friend, it is possible that multiple declarations
+        // with same name can co-exist in specific cases (if a template contains
+        // a friend template and has a specialization). For this case the
+        // declarations should match, except that the "template depth" is
+        // different. No linking of previous declaration is needed in this case.
+        // FIXME: This condition may need refinement.
+        if (D->getFriendObjectKind() != Decl::FOK_None &&
+            FoundTemplate->getFriendObjectKind() != Decl::FOK_None &&
+            D->getFriendObjectKind() != FoundTemplate->getFriendObjectKind() &&
+            IsStructuralMatch(D, FoundTemplate, /*Complain=*/false,
+                              /*IgnoreTemplateParmDepth=*/true))
+          continue;
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Michael137 wrote:

It feels like the logic on 6103 and this here could be merged together? Should `IgnoreTemplateParmDepth` on 6104 just be changed to also be set if the `FriendObjectKind` matches? Then I *think* everything should work out as expected

https://github.com/llvm/llvm-project/pull/115734