[clang] [C++20][Modules] Load function body from the module that gives canonical decl (PR #111992)

[Ilya Biryukov via cfe-commits]

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@@ -10057,15 +10057,18 @@ void ASTReader::finishPendingActions() {
       // For a function defined inline within a class template, force the
       // canonical definition to be the one inside the canonical definition of
       // the template. This ensures that we instantiate from a correct view
-      // of the template.
+      // of the template. This behaviour seems to be important only for inline
+      // friend functions. For normal member functions, it might results in
+      // selecting canonical decl from module A but body from module B.
       //
       // Sadly we can't do this more generally: we can't be sure that all
       // copies of an arbitrary class definition will have the same members
       // defined (eg, some member functions may not be instantiated, and some
       // special members may or may not have been implicitly defined).
-      if (auto *RD = dyn_cast<CXXRecordDecl>(FD->getLexicalParent()))
-        if (RD->isDependentContext() && !RD->isThisDeclarationADefinition())
-          continue;
+      if (FD->getFriendObjectKind())
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ilya-biryukov wrote:

Does it mean we can still potentially have this problem for `friend` functions?
Or are there some invariants in Clang/C++ itself that stop that from happening?

If we have a small example, it'd be interesting to check that adding `friend` does not cause the problem to resurface.

https://github.com/llvm/llvm-project/pull/111992