From 4596e8938ea6ab43e806a3dc88bc6e0d2a3bd9ed Mon Sep 17 00:00:00 2001
From: Christian Mazakas <christian.mazakas@gmail.com>
Date: Wed, 21 Jan 2026 19:36:38 -0800
Subject: [PATCH] initialize object storage using tag dispatch

Currently, the code branches on whether to use SBO or not using a
runtime check, i.e. a raw `if(x)`. This can trick the msvc optimizer
into believing the SBO path can be taken, which in the case of
sufficiently large Callables triggers a buffer overrun warning as
emplacing into the small buffer would exceed its size.

By updating the code to instead dispatch at compile-time via tags, the
msvc optimizer is no longer confused and the warning disappears as the
code path is now truly unreachable.
---
 include/boost/compat/move_only_function.hpp | 29 ++++++++++++---------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/include/boost/compat/move_only_function.hpp b/include/boost/compat/move_only_function.hpp
index 5511b52..d307a00 100644
--- a/include/boost/compat/move_only_function.hpp
+++ b/include/boost/compat/move_only_function.hpp
@@ -435,6 +435,22 @@ struct move_only_function_base
     {
     }
 
+    template<class VT, class ...CArgs>
+    void init_object( std::false_type /* use_sbo */, CArgs&& ...args )
+    {
+        s_.pobj_ = new VT( std::forward<CArgs>( args )... );
+        invoke_ = &mo_invoke_object_holder<RQ, Const, NoEx, VT, R, Args...>::invoke_object;
+        manager_ = &manage_object<VT>;
+    }
+
+    template<class VT, class ...CArgs>
+    void init_object( std::true_type /* use_sbo */, CArgs&& ...args )
+    {
+        new( s_.addr() ) VT( std::forward<CArgs>( args )... );
+        invoke_ = &mo_invoke_local_holder<RQ, Const, NoEx, VT, R, Args...>::invoke_local;
+        manager_ = &manage_local<VT>;
+    }
+
     template<class VT, class ...CArgs>
     void init( std::false_type /* is_function */, CArgs&& ...args )
     {
@@ -444,18 +460,7 @@ struct move_only_function_base
             return;
         }
 
-        if( !storage::use_sbo<VT>() )
-        {
-            s_.pobj_ = new VT( std::forward<CArgs>( args )... );
-            invoke_ = &mo_invoke_object_holder<RQ, Const, NoEx, VT, R, Args...>::invoke_object;
-            manager_ = &manage_object<VT>;
-        }
-        else
-        {
-            new( s_.addr() ) VT( std::forward<CArgs>( args )... );
-            invoke_ = &mo_invoke_local_holder<RQ, Const, NoEx, VT, R, Args...>::invoke_local;
-            manager_ = &manage_local<VT>;
-        }
+        init_object<VT>( std::integral_constant<bool, storage::use_sbo<VT>()>{}, std::forward<CArgs>( args )... );
     }
 
     template<class VT, class ...CArgs>