ilya-biryukov added inline comments.

Comment at: unittests/clangd/SyncAPI.h:1
+//===--- SyncAPI.h - Sync version of ClangdServer's API ----------*- 
sammccall wrote:
> Being able to call synchronously is really nice for tests.
> It's a bit unfortunate that to do this for each function we want to call 
> synchronously (giving them a name, writing some boilerplate that repeats the 
> args a few times).
> It would be nice if we had a primitive we could compose.
> Here's an idea that might work:
> ```Tagged<CompletionList> CompletionResults;
> Server.codeComplete(File, Pos, Opts, capture(CompletionResults), 
> OverriddenContents);```
> `capture()` returns a callback that writes into CompletionResults. It also 
> magically causes the call to block!
> (brief pause for you to consider the API before I suggest a shameful 
> implementation)
> Actually capture() returns a proxy object that converts to UniqueFunction. 
> The proxy object itself has a destructor that blocks on the UF being invoked. 
> The proxy will be destroyed at the end of the full-expression, i.e. the line. 
> e.g.
> ```template <typename T> struct CaptureProxy {
>   T &Target;
>   std::promise<T> Promise;
>   std::future<T> Future;
>   CaptureProxy(T &Target) : Target(Target) {}
>   operator UniqueFunction<void(T)>() {
>     return BindWithForward([](T Value, std::promise<T> Promise) {
>       Promise.set_value(std::move(Value));
>     }, std::move(Promise));
>   }
>   ~CaptureProxy() { Target = Future.get(); }
> };
> template <typename T> CaptureProxy<T> capture(T &Target) {
>   return CaptureProxy<T>(Target);
> }```
> This is a little bit magic, but it's just for tests :-)
> (One caveat - this needs the return value to be default-constructible. We 
> could easily use Optional instead, but these are default-constructible in 
> practice)
As discussed offline, I've opted for both:
  - implementing the helper to allow reusing the future/promise code
  - providing a function that actually returns the value synchronously.

This approach writing a small amount of wrapper code for each of the LSP 
methods in ClangdServer, but it, arguably, makes the test code nicer.


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