[coroutines] Fix fallthrough warning on try/catch

Summary:
The test case added in this diff would incorrectly warn that control
flow may fall through without returning. Here's a standalone example:
https://godbolt.org/z/dCwXEi

The same program, but using `return` instead of `co_return`, does not
produce a warning: https://godbolt.org/z/mVldqQ

The issue was in how Clang analysis would structure its representation
of the control-flow graph. Specifically, when constructing the CFG,
`CFGBuilder::Visit` had special handling of a `ReturnStmt`, in which it
would place object destructors in the same CFG block as a `return` statement,
immediately after it. Doing so would allow the logic in
`lib/Sema/AnalysisBasedWarning.cpp` `CheckFallThrough` to work properly in the
program that used `return`, correctly determining that no "plain edges" preceded
the exit block of the function.

Because a `co_return` statement would not enjoy the same treatment when
it was being built into the control-flow graph, object destructors
would not be placed in the same CFG block as the `co_return`, thus
resulting in a "plain edge" preceding the exit block of the function,
and so the warning logic would be triggered.

Add special casing for `co_return` to Clang analysis, thereby
remedying the mistaken warning.

Test Plan: `check-clang`

Reviewers: GorNishanov, tks2103, rsmith

Reviewed By: GorNishanov

Subscribers: EricWF, lewissbaker, cfe-commits

Differential Revision: https://reviews.llvm.org/D54075

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@346074 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 45 insertions, 0 deletions

diff --git a/test/SemaCXX/coreturn-eh.cpp b/test/SemaCXX/coreturn-eh.cpp
new file mode 100644
index 0000000000..79065736c0
--- /dev/null
+++ b/test/SemaCXX/coreturn-eh.cpp
@@ -0,0 +1,45 @@
+// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++14 -fcoroutines-ts -fcxx-exceptions -fsyntax-only -Wignored-qualifiers -Wno-error=return-type -verify -fblocks -Wall -Wextra -Wno-error=unreachable-code
+// expected-no-diagnostics
+
+#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
+  void await_resume();
+} a;
+
+struct object { ~object() {} };
+
+struct promise_void_return_value {
+  void get_return_object();
+  suspend_always initial_suspend();
+  suspend_always final_suspend();
+  void unhandled_exception();
+  void return_value(object);
+};
+
+struct VoidTagReturnValue {
+  struct promise_type {
+    VoidTagReturnValue get_return_object();
+    suspend_always initial_suspend();
+    suspend_always final_suspend();
+    void unhandled_exception();
+    void return_value(object);
+  };
+};
+
+template <typename T1>
+struct std::experimental::coroutine_traits<void, T1> { using promise_type = promise_void_return_value; };
+
+VoidTagReturnValue test() {
+  object x = {};
+  try {
+    co_return {};
+  } catch (...) {
+    throw;
+  }
+}