[PATCH] D141580: [clang] Don't consider a nullptr returned from ActOnTag() as a valid result in ParseClassSpecifier.

[Haojian Wu via Phabricator via cfe-commits]

hokein added inline comments.


================
Comment at: clang/lib/Parse/ParseDeclCXX.cpp:2069-2078
+    if (auto *TagDecl = Actions.ActOnTag(
+            getCurScope(), TagType, TUK, StartLoc, SS, Name, NameLoc, attrs, AS,
+            DS.getModulePrivateSpecLoc(), TParams, Owned, IsDependent,
+            SourceLocation(), false, clang::TypeResult(),
+            DSC == DeclSpecContext::DSC_type_specifier,
+            DSC == DeclSpecContext::DSC_template_param ||
+                DSC == DeclSpecContext::DSC_template_type_arg,
----------------
hokein wrote:
> aaron.ballman wrote:
> > I think this is the wrong approach to fixing the issue. None of the other assignments to `TagOrTempResult` test for nullptr first, such as: https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseDeclCXX.cpp#L2037 and https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseDeclCXX.cpp#L2029.
> > 
> > An `ActionResult` can be both valid/invalid *and* usable/unusable (and a `DeclResult` is a kind of `ActionResult`). When it's assigned *any pointer value* (including nullptr), it's a valid `ActionResult` but it's not a usable `ActionResult` (https://github.com/llvm/llvm-project/blob/main/clang/include/clang/Sema/Ownership.h#L166).
> > 
> > I think the correct fix is to find the places assuming a valid `ActionResult` means a nonnull pointer from `get()`, such as https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseDeclCXX.cpp#L2121 and switch them over to looking for a usable `ActionResult` instead.
> Thanks for the comment!
> 
> > None of the other assignments to TagOrTempResult test for nullptr first
> 
> I think the problem here is:
> - most of the `ActOn*` methods return a `DeclResult` (and inside these method implementations, they return an Invalid `DeclResult` if the Decl is nullptr), so `TagOrTempResult = Actions.ActOn*` is a fine pattern.
> - Only two exceptions `ActOnTagDecl` and `ActOnTemplatedFriendTag`, they return a `Decl*`, so the `TagOrTempResult = Action.ActOn*` is a very suspicious pattern. (IMO, they're not intentional, likely bugs).
> 
> > such as: https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseDeclCXX.cpp#L2037 and https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseDeclCXX.cpp#L2029.
> 
> The first one is the other exception; the second one is not (ActOnExplicitInstantiation returns a `DeclResult`).
> 
> > I think the correct fix is to find the places assuming a valid ActionResult means a nonnull pointer from get(), such as https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseDeclCXX.cpp#L2121 and switch them over to looking for a usable ActionResult instead.
> 
> This is one of the solutions. But instead of justifying every places of DeclResult, I think we should fix the two exception places (by adding the nullptr check in
> https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseDeclCXX.cpp#L2037 and https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseDeclCXX.cpp#L2069), what do you think?
I have updated the patch to check nullptr for ActOnTemplatedFriendTag as well. Let me know what you think (I'm also happy to do the change).


Repository:
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https://reviews.llvm.org/D141580