don't slice std:: exceptions in exception_wrapper::handle when no handler is found

Summary:
Given an exception_wrapper `ew` holding a `std::runtime_error`, the following assertion fails:

```
try {
  ew.handle();
} catch( std::runtime_error const & ) {
} catch( std::exception const & ) {
  assert(false);
}
```

Reviewed By: simpkins

Differential Revision: D9199924

fbshipit-source-id: b6f90ccf269a1b58f0dd07723451ff34cde1d529

folly/ExceptionWrapper-inl.h

@@ -606,8 +606,8 @@ inline void exception_wrapper::handle_(
   // This continuation gets evaluated if CatchFns... does not include a
   // catch-all handler. It is a no-op.
   auto continuation = [](StdEx* ex) { return ex; };
-  if (StdEx* e = impl(continuation)) {
-    throw *e; // Not handled. Throw.
+  if (nullptr != impl(continuation)) {
+    this_.throw_exception();
   }
 }
 

folly/test/ExceptionWrapperTest.cpp

@@ -636,6 +636,37 @@ TEST(ExceptionWrapper, handle_std_exception_unhandled) {
   EXPECT_TRUE(handled);
 }
 
+TEST(ExceptionWrapper, handle_std_exception_propagated) {
+  auto ep = std::make_exception_ptr(std::runtime_error{"hello world"});
+  exception_wrapper const ew_eptr(ep, from_eptr<std::runtime_error>(ep));
+  exception_wrapper const ew_small(std::runtime_error{"hello world"});
+  exception_wrapper const ew_big(BigRuntimeError{"hello world"});
+
+  try {
+    ew_eptr.handle();
+  } catch (const std::runtime_error&) {
+    SUCCEED();
+  } catch (const std::exception&) {
+    ADD_FAILURE();
+  }
+
+  try {
+    ew_small.handle();
+  } catch (const std::runtime_error&) {
+    SUCCEED();
+  } catch (const std::exception&) {
+    ADD_FAILURE();
+  }
+
+  try {
+    ew_big.handle();
+  } catch (const std::runtime_error&) {
+    SUCCEED();
+  } catch (const std::exception&) {
+    ADD_FAILURE();
+  }
+}
+
 TEST(ExceptionWrapper, handle_non_std_exception_small) {
   auto ep = std::make_exception_ptr(42);
   exception_wrapper const ew_eptr1(ep);