invoke internal storage via reference

This prevents the code from erroneously copying the storage which causes
surprising behavior as noted here:
https://github.com/boostorg/compat/issues/23

.github/workflows/ci.yml

@@ -183,18 +183,24 @@ jobs:
           - toolset: clang
             compiler: clang++-20
             cxxstd: "03,11,14,17,20,23,2c"
-            container: ubuntu:25.04
+            container: ubuntu:24.04
             os: ubuntu-latest
             install: clang-20
           - toolset: clang
-            cxxstd: "03,11,14,17,20,2b"
-            os: macos-13
+            compiler: clang++-21
+            cxxstd: "03,11,14,17,20,23,2c"
+            container: ubuntu:25.10
+            os: ubuntu-latest
+            install: clang-21
           - toolset: clang
             cxxstd: "03,11,14,17,20,2b"
             os: macos-14
           - toolset: clang
-            cxxstd: "03,11,14,17,20,2b"
+            cxxstd: "03,11,14,17,20,23,2c"
             os: macos-15
+          - toolset: clang
+            cxxstd: "03,11,14,17,20,23,2c"
+            os: macos-26
 
     runs-on: ${{matrix.os}}
 
@@ -321,9 +327,9 @@ jobs:
         include:
           - os: ubuntu-22.04
           - os: ubuntu-24.04
-          - os: macos-13
           - os: macos-14
           - os: macos-15
+          - os: macos-26
 
     runs-on: ${{matrix.os}}
 
@@ -370,9 +376,9 @@ jobs:
         include:
           - os: ubuntu-22.04
           - os: ubuntu-24.04
-          - os: macos-13
           - os: macos-14
           - os: macos-15
+          - os: macos-26
 
     runs-on: ${{matrix.os}}
 
@@ -429,9 +435,9 @@ jobs:
         include:
           - os: ubuntu-22.04
           - os: ubuntu-24.04
-          - os: macos-13
           - os: macos-14
           - os: macos-15
+          - os: macos-26
 
     runs-on: ${{matrix.os}}
 

include/boost/compat/move_only_function.hpp

@@ -247,7 +247,7 @@ bool is_nullary_arg( F f )
 template<bool NoEx, class R, class ...Args>
 struct mo_invoke_function_holder
 {
-    static R invoke_function( storage s, Args&&... args) noexcept( NoEx )
+    static R invoke_function( storage const& s, Args&&... args) noexcept( NoEx )
     {
         auto f = reinterpret_cast<R(*)( Args... )>( s.pfn_ );
         return compat::invoke_r<R>( f, std::forward<Args>( args )... );
@@ -257,7 +257,7 @@ struct mo_invoke_function_holder
 template<ref_quals RQ, bool Const, bool NoEx, class F, class R, class ...Args>
 struct mo_invoke_object_holder
 {
-    static R invoke_object( storage s, Args&&... args ) noexcept( NoEx )
+    static R invoke_object( storage const& s, Args&&... args ) noexcept( NoEx )
     {
         using T = remove_reference_t<F>;
         using cv_T = conditional_t<Const, add_const_t<T>, T>;
@@ -274,7 +274,7 @@ struct mo_invoke_object_holder
 template<ref_quals RQ, bool Const, bool NoEx, class F, class R, class ...Args>
 struct mo_invoke_local_holder
 {
-    static R invoke_local( storage s, Args&&... args ) noexcept( NoEx )
+    static R invoke_local( storage const& s, Args&&... args ) noexcept( NoEx )
     {
         using T = remove_reference_t<F>;
         using cv_T = conditional_t<Const, add_const_t<T>, T>;
@@ -285,7 +285,7 @@ struct mo_invoke_local_holder
             >
         >;
 
-        return compat::invoke_r<R>( static_cast<cv_ref_T>( *static_cast<cv_T*>( s.addr() ) ), std::forward<Args>( args )... );
+        return compat::invoke_r<R>( static_cast<cv_ref_T>( *static_cast<cv_T*>( const_cast<storage&>( s ).addr() ) ), std::forward<Args>( args )... );
     }
 };
 
@@ -485,9 +485,9 @@ struct move_only_function_base
 
     detail::storage s_;
 #if defined(__cpp_noexcept_function_type)
-    R ( *invoke_ )( detail::storage, Args&&... ) noexcept( NoEx ) = nullptr;
+    R ( *invoke_ )( detail::storage const&, Args&&... ) noexcept( NoEx ) = nullptr;
 #else
-    R ( *invoke_ )( detail::storage, Args&&... ) = nullptr;
+    R ( *invoke_ )( detail::storage const&, Args&&... ) = nullptr;
 #endif
     void ( *manager_ )( op_type, detail::storage&, detail::storage* ) = &manage_empty;
 };

test/move_only_function_test.cpp

@@ -13,7 +13,8 @@
 #include <boost/core/lightweight_test.hpp>
 #include <boost/core/lightweight_test_trait.hpp>
 
-#include <functional>
+#include <array>
+#include <cstdint>
 #include <memory>
 #include <type_traits>
 
@@ -845,11 +846,39 @@ static void test_conv()
     }
 }
 
+static void test_mutable_lambda()
+{
+    {
+        // Within SBO limits.
+        int captured = 0;
+        move_only_function<int()> func = [captured]() mutable { return ++captured; };
+
+        BOOST_TEST_EQ( func(), 1 );
+        BOOST_TEST_EQ( func(), 2 );
+
+        move_only_function<int()> func2(std::move(func));
+        BOOST_TEST_EQ( func2(), 3 );
+    }
+
+    {
+        // Too large for SBO.
+        std::array<std::uint8_t, 256> captured = {{}};
+        move_only_function<int()> func = [captured]() mutable { return ++captured[0]; };
+
+        BOOST_TEST_EQ( func(), 1 );
+        BOOST_TEST_EQ( func(), 2 );
+
+        move_only_function<int()> func2(std::move(func));
+        BOOST_TEST_EQ( func2(), 3 );
+    }
+}
+
 int main()
 {
     test_call();
     test_traits();
     test_conv();
+    test_mutable_lambda();
 
     return boost::report_errors();
 }