[PATCH] D146358: [clang][AST] Print name instead of type when diagnosing uninitialized subobject in constexpr variables

[Takuya Shimizu via Phabricator via cfe-commits]

hazohelet added a comment.

@aaron.ballman 
Thanks for the review! I'll add a release note.



================
Comment at: clang/lib/AST/ExprConstant.cpp:2357-2361
+    if (SubobjectDecl) {
+      Info.FFDiag(DiagLoc, diag::note_constexpr_uninitialized) << SubobjectDecl;
+      Info.Note(SubobjectDecl->getLocation(),
+                diag::note_constexpr_subobject_declared_here);
+    }
----------------
aaron.ballman wrote:
> Hmm, this breaks one of the contracts of our constexpr evaluation engine, doesn't it? My understanding is that if constexpr evaluation fails, we will have emitted a note diagnostic for why it failed. But if the caller doesn't pass a nonnull `SubobjectDecl`, we'll return `false` but we won't issue a diagnostic.
> 
> I'm surprised no tests lost notes as a result of this change, that suggests we're missing test coverage for the cases where nullptr is passed in explicitly to this function.
Yeah, I was looking into when `SubobjectDecl` can be null here. I `assert`ed the non-nullness of `SubobjectDecl` before and found that there exists two lines of code (https://github.com/llvm/llvm-project/blob/22a3f974d35da89247c0396594f2e4cd592742eb/clang/test/SemaCXX/attr-weak.cpp#L49 and https://github.com/llvm/llvm-project/blob/abf4a8cb15d4faf04ee0da14e37d7349d3bde9a1/clang/test/CodeGenCXX/weak-external.cpp#L97) in the test codes that nulls here.
It seems they are doing the same thing, doing comparison against a pointer to a `[[gnu::weak]]` member. A simple reproducing code is here: https://godbolt.org/z/qn997n85n
As you can see from the compiler explorer, there's no note emitted here before the patch.
I inserted some `printf` into the code before this patch  and confirmed `Info.FFDiag(DiagLoc, diag::note_constexpr_uninitialized) << true << Type` was actually called when compiling the reproducing code and that somehow it is ignored. FWIW, `SubobjectLoc.isValid()` was `false` here.


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  https://reviews.llvm.org/D146358/new/

https://reviews.llvm.org/D146358