[coroutines] Fix assertion during -Wuninitialized analysis

[EricWF]

Summary

@rsmith Is there a better place to put this test?

[GorNishanov]

LBTM (Looks brilliant to me)

lib/Sema/SemaCoroutine.cpp

VarDecl *Sema::buildCoroutinePromise(SourceLocation Loc) {
 
   auto *VD = VarDecl::Create(Context, FD, FD->getLocation(), FD->getLocation(),
                              &PP.getIdentifierTable().get("__promise"), T,
                              Context.getTrivialTypeSourceInfo(T, Loc), SC_None);
   CheckVariableDeclarationType(VD);
   if (VD->isInvalidDecl())
     return nullptr;
   ActOnUninitializedDecl(VD);
+  FD->addDecl(VD);
   assert(!VD->isInvalidDecl());
   return VD;
 }
 
 /// Check that this is a context in which a coroutine suspension can appear.
 static FunctionScopeInfo *checkCoroutineContext(Sema &S, SourceLocation Loc,
                                                 StringRef Keyword,
                                                 bool IsImplicit = false) {

test/SemaCXX/coreturn.cpp

-// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts -fsyntax-only -Wignored-qualifiers -Wno-error=return-type -verify -fblocks -Wno-unreachable-code -Wno-unused-value
+// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts -fsyntax-only -Wignored-qualifiers -Wno-error=return-type -verify -fblocks -Wall -Wextra -Wno-error=unreachable-code
 #include "Inputs/std-coroutine.h"
 
 using std::experimental::suspend_always;
 using std::experimental::suspend_never;
 
 struct awaitable {
   bool await_ready();
   void await_suspend(std::experimental::coroutine_handle<>); // FIXME: coroutine_handle

test/SemaCXX/coroutine-uninitialized-warning-crash.cpp

+// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts -fsyntax-only -Wall -Wextra -Wuninitialized  -fblocks
+#include "Inputs/std-coroutine.h"
+
+using namespace std::experimental;
+
+
+struct A {
+  bool await_ready() { return true; }
+  int await_resume() { return 42; }
+  template <typename F>
+  void await_suspend(F) {}
+};
+
+
+struct coro_t {
+  struct promise_type {
+    coro_t get_return_object() { return {}; }
+    suspend_never initial_suspend() { return {}; }
+    suspend_never final_suspend() { return {}; }
+    A yield_value(int) { return {}; }
+    void return_void() {}
+    static void unhandled_exception() {}
+  };
+};
+
+coro_t f(int n) {
+  if (n == 0)
+    co_return;
+  co_yield 42;
+  int x = co_await A{};
+}
+
+template <class Await>
+coro_t g(int n) {
+  if (n == 0)
+    co_return;
+  co_yield 42;
+  int x = co_await Await{};
+}
+
+int main() {
+  f(0);
+  g<A>(0);
+}