Michael137 wrote:
> > > I didn't see much explanation as to why this is needed in the bug report.
> >
> >
> > The motivating example is something like:
> > ```
> > struct Info {
> > enum Mask : uintptr_t {
> > Enum
> > };
> > }
> > ```
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > `expr Info::Mask::Enum`.
> > > Is there a reason we need to complete nested types within a type? Seems
> > > like we can put that off until later. Right now if you parse a member
> > > function, any types it needs will be parsed lazily and only if needed,
> > > which is ok. If we start completing all types within types without ever
> > > needing to actually use then, it will make debugging slower and cause our
> > > memory usage to grow.
> >
> >
> > LLDB will first resolve the `Info` structure (and all its member fields,
> > but not nested types). When clang looks for `Mask`, it wants to find it in
> > a direct lookup into the `Info` DeclContext. But that lookup will fail
> > because we never created the decls for the nested type. There is no
> > fallback to the external source in such a case unfortunately so LLDB never
> > has the chance to correct this. There was no obvious point to which we
> > could defer completing the nested type when completing the outer. Maybe
> > adding that external source fallback? But I might've missed something
> > obvious. A potential option would be to limit the completion performed in
> > this patch to enums, though a general solution would be nice.
>
> What we do for classes is we create forward declarations for them at first.
> As soon as someone wants to know more, we complete the type via the external
> source interface. This works for expressions since this is what precomiled
> headers do, it also works with our CompilerType as we can ask for the forward
> type, the layout type, or the complete type. I wonder if we can make this
> work for enums as well.
>
> I am worried about a class that contains many types. Usually when we need to
> know something about a type, like in your example above, we will get a
> callback to the ClangExternalASTSourceCallbacks methods:
>
> ```
> void FindExternalLexicalDecls(
> const clang::DeclContext *DC,
> llvm::function_ref<bool(clang::Decl::Kind)> IsKindWeWant,
> llvm::SmallVectorImpl<clang::Decl *> &Result) override;
>
> bool FindExternalVisibleDeclsByName(const clang::DeclContext *DC,
> clang::DeclarationName Name) override;
> ```
>
> So if you typed Info::Mask::Enum, the expression parser will first ask about
> "Info" with no containing decl context (or maybe it specifies the translation
> unit decl context to indicate it is a root type) and we will return the
> "struct Info". Then it would call FindExternalVisibleDeclsByName() with the
> DeclContext set to "struct Info" and we will find "Mask" inside of this type,
> then it will ask about "Enum" within the "Info::Mask" type and it should be
> in the type definition. Maybe some isn't working correctly for enums in this
> lookup mechanism?
Yup that was my understanding too. Though last I stepped through clang lookup
for this case the call back into LLDB to get `Mask` in the `Info` DeclContext
wasn't being done. It has been a while since I looked at this so I'll double
check when I get the chance
https://github.com/llvm/llvm-project/pull/66879
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