sammccall accepted this revision.
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This revision is now accepted and ready to land.

LG, suggest a tweak to capture() though.

I wonder whether we want to introduce `using Callback<T...> = 
UniqueFunction<void(T...)>` for readability at some point...

Comment at: clangd/ClangdServer.cpp:209
+  auto Action = [Contents, Pos, TaggedFS,
+                 PCHs](Path File, decltype(Callback) Callback,
+                       llvm::Expected<InputsAndPreamble> IP) {
ioeric wrote:
> ilya-biryukov wrote:
> > ioeric wrote:
> > > nit: I'd probably use a different name than `Callback` for this parameter 
> > > for clarity.
> > I actually think we should keep it this way. It's a bit tricky to grasp, 
> > but it has two important advantages:
> >   - Makes referencing `Callback` from outer scope impossible
> >   - saves us from coming up with names for that superficial variable, i.e. 
> > should it be called `InnerCallback`, `Callback2`, `C` or something else?
> > 
> > Is it that too confusing or do you feel we can keep it?
> > Makes referencing Callback from outer scope impossible. 
> For lambdas, I think we should rely on captures for this instead of the 
> parameter names.
> >saves us from coming up with names for that superficial variable, i.e. 
> >should it be called InnerCallback, Callback2, C or something else?
> I thought this is a common practice with llvm coding style :P I would vote 
> for `C` :) 
> > Is it that too confusing or do you feel we can keep it?
> It is a bit confusing when I first looked at it, but nothing is *too* 
> confusing as long as the code is correct ;) I just think it would make the 
> code easier to read.
I agree with you both :-)
Conceptually, this *is* a capture - BindWithForward simulates move-capture that 
C++11 doesn't have native syntax for. So the same name seems appropriate to me 
- you want shadowing  for the same reasons you want captures to shadow.

Comment at: unittests/clangd/SyncAPI.cpp:69
+  llvm::Expected<Tagged<SignatureHelp>> Result = Tagged<SignatureHelp>();
+  (void)(bool)Result; // Expected has to be checked.
+  Server.signatureHelp(File, Pos, capture(Result), OverridenContents);
ioeric wrote:
> ilya-biryukov wrote:
> > ioeric wrote:
> > > I'd expect this to be checked by callers. Would `return 
> > > std::move(Result);` work?
> > The reason why we need it is because `capture(Result)` writes return value 
> > of the callback to the `Result` variable. But we have to first check the 
> > default-constructed value that was there **before** calling 
> > `Server.signatureHelp`
> I think we should probably fix the behavior of `capture` since this changes 
> the behavior of `llvm::Expected` in user code - the check is there to make 
> sure users always check the status. But here we always do this for users.

I suggested `capture(T&)` was the right API because I thought it'd be used 
directly by test code (but we wrap it here) and because I thought T would be 
default-constructible without problems (but it's not).

I'd suggest changing to `capture(Optional<T>&)` instead, so the caller can just 
create an empty optional. That makes the callsites here slightly less obscure.

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