From 0e97be35f91ac1bb8ee01b647c6f7ef263324036 Mon Sep 17 00:00:00 2001
From: Russell Yanofsky <russ@yanofsky.org>
Date: Thu, 17 Dec 2020 16:56:07 -0500
Subject: [PATCH] Avoid depending on argument default constructors

CTransaction default constructor was removed in
faac31521bb7ecbf999541cf918d3750ff589de4
https://github.com/bitcoin/bitcoin/pull/20588 which exposed two places
where libmultiprocess was inadvertently assuming default constructors
were defined.

In both places, it would never actually call the default constructor
at runtime, just rely on it being present during compile time code
generation.

In the first case, compiler would fail while trying to figure out the type of
`std::invoke_result_t<EmplaceFn>` where EmplaceFn called
`make_shared<CTransaction>()`. This was fixed by just the moving expression
into an unexpanded template method.

In the second case code was calling Optional<CTransaction>::emplace() to
default-initialize CTransaction& output-arguments. It was fixed by
moving into a constexpr-if that would be known to be false for output
arguments at compile time.
---
 include/mp/proxy-types.h | 31 ++++++++++++-------------------
 1 file changed, 12 insertions(+), 19 deletions(-)

diff --git a/include/mp/proxy-types.h b/include/mp/proxy-types.h
index 92bb578..743b210 100644
--- a/include/mp/proxy-types.h
+++ b/include/mp/proxy-types.h
@@ -189,25 +189,14 @@ struct ReadDestEmplace
     template <typename UpdateFn>
     void update(UpdateFn&& update_fn)
     {
-        update(std::forward<UpdateFn>(update_fn), Priority<1>());
-    }
-
-    template <typename UpdateFn,
-        typename Enable =
-            std::enable_if_t<!std::is_const_v<std::remove_reference_t<std::invoke_result_t<EmplaceFn>>>, UpdateFn>>
-    void update(UpdateFn&& update_fn, Priority<1>)
-    {
-        update_fn(construct());
-    }
-
-    template <typename UpdateFn>
-    void update(UpdateFn&& update_fn, Priority<0>)
-    {
-        std::remove_cv_t<LocalType> temp;
-        update_fn(temp);
-        construct(std::move(temp));
+        if constexpr (std::is_const_v<std::remove_reference_t<std::invoke_result_t<EmplaceFn>>>) {
+            std::remove_cv_t<LocalType> temp;
+            update_fn(temp);
+            construct(std::move(temp));
+        } else {
+            update_fn(construct());
+        }
     }
-
     EmplaceFn& m_emplace_fn;
 };
 
@@ -1073,7 +1062,11 @@ void DefaultPassField(TypeList<LocalType>, ServerContext& server_context, Fn&& f
             param.emplace(std::forward<decltype(args)>(args)...);
             return *param;
         }));
-    if (!param) param.emplace();
+    if constexpr (Accessor::in) {
+        assert(param);
+    } else {
+        if (!param) param.emplace();
+    }
     fn.invoke(server_context, std::forward<Args>(args)..., static_cast<LocalType&&>(*param));
     auto&& results = server_context.call_context.getResults();
     MaybeBuildField(std::integral_constant<bool, Accessor::out>(), TypeList<LocalType>(), invoke_context,