Add define BOOST_PARSER_DISABLE_TRACE to disable trace mode at compile time.

The trace feature doubles the compile time, even if never used.
This patch introduces the preprocessor define BOOST_PARSER_DISABLE_TRACE
to deactivate this feature at compile time.

.github/workflows/macos-15.yml

@@ -1,4 +1,4 @@
-name: macos-13 - Clang 14
+name: macos-15 - Clang
 
 on:
   push:
@@ -14,9 +14,9 @@ jobs:
   build:
     strategy:
       matrix:
-        cxx_std: [17]
+        cxx_std: [17, 20]
 
-    runs-on: macos-13
+    runs-on: macos-15
 
     steps:
       - uses: actions/checkout@v4

.github/workflows/ubuntu.yml

@@ -18,6 +18,13 @@ jobs:
         compiler_version: [g++-10, g++-11]
         cxx_std: [17, 20]
         os: [ubuntu-22.04]
+        disable_trace: [false]
+        include:
+          # Test with trace disabled
+          - compiler_version: g++-11
+            cxx_std: 20
+            os: ubuntu-22.04
+            disable_trace: true
 
     runs-on: ${{ matrix.os }}
 
@@ -30,7 +37,11 @@ jobs:
         run: |
           mkdir build
           cd build
-          cmake .. -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }} -DCMAKE_CXX_COMPILER=${{ matrix.compiler_version }} -DCXX_STD=${{ matrix.cxx_std }}
+          if [ "${{ matrix.disable_trace }}" = "true" ]; then
+            cmake .. -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }} -DCMAKE_CXX_COMPILER=${{ matrix.compiler_version }} -DCXX_STD=${{ matrix.cxx_std }} -DDISABLE_TRACE=true
+          else
+            cmake .. -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }} -DCMAKE_CXX_COMPILER=${{ matrix.compiler_version }} -DCXX_STD=${{ matrix.cxx_std }}
+          fi
 
       - name: Build
         run: cd build ; make -j4

.github/workflows/windows.yml

@@ -17,6 +17,12 @@ jobs:
       matrix:
         cxx_std: [17, 20, 23]
         os: [windows-2022]
+        disable_trace: [false]
+        include:
+          # Test with trace disabled
+          - cxx_std: 20
+            os: windows-2022
+            disable_trace: true
 
     runs-on: ${{ matrix.os }}
 
@@ -24,13 +30,19 @@ jobs:
       - uses: actions/checkout@v4
 
       - name: Configure CMake
-        # Configure CMake in a 'build' subdirectory. `CMAKE_BUILD_TYPE` is only required if you are using a single-configuration generator such as make.
+        # Configure CMake in a 'build' subdirectory. Visual Studio is a multi-config generator, so we don't use CMAKE_BUILD_TYPE.
         # See https://cmake.org/cmake/help/latest/variable/CMAKE_BUILD_TYPE.html?highlight=cmake_build_type
-        run: cmake -B build -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }} -DCXX_STD=${{ matrix.cxx_std }}
+        run: |
+          if ("${{ matrix.disable_trace }}" -eq "true") {
+            cmake -B build -DCXX_STD=${{ matrix.cxx_std }} -DDISABLE_TRACE=true
+          } else {
+            cmake -B build -DCXX_STD=${{ matrix.cxx_std }}
+          }
+        shell: pwsh
 
       - name: Build
         working-directory: build
-        run: cmake --build . -- /p:CL_MPcount=4
+        run: cmake --build . --config ${{ env.BUILD_TYPE }} -- /p:CL_MPcount=4
 
       - name: Test
         working-directory: build/test

CMakeLists.txt

@@ -64,6 +64,12 @@ if (BUILD_WITH_HANA)
   add_definitions(-DBOOST_PARSER_USE_HANA_TUPLE)
 endif()
 
+set(DISABLE_TRACE false CACHE BOOL
+    "Disable parser trace functionality (defines BOOST_PARSER_DISABLE_TRACE).")
+if (DISABLE_TRACE)
+  add_definitions(-DBOOST_PARSER_DISABLE_TRACE)
+endif()
+
 
 ##################################################
 # Dependencies

README.md

@@ -67,6 +67,6 @@ Develop status:
 
 [![Windows MSVC](https://github.com/tzlaine/parser/actions/workflows/windows.yml/badge.svg?branch=develop)](https://github.com/tzlaine/parser/actions/workflows/windows.yml)
 
-[![macos-13 - Clang 14](https://github.com/tzlaine/parser/actions/workflows/macos-13.yml/badge.svg?branch=develop)](https://github.com/tzlaine/parser/actions/workflows/macos-13.yml)
+[![macos-15 - Clang](https://github.com/tzlaine/parser/actions/workflows/macos-15.yml/badge.svg?branch=develop)](https://github.com/tzlaine/parser/actions/workflows/macos-15.yml)
 
 [![License](https://img.shields.io/badge/license-boost-brightgreen.svg)](LICENSE_1_0.txt)

doc/parser.qbk

@@ -148,6 +148,7 @@
 [def _RULES_               [macroref BOOST_PARSER_DEFINE_RULES `BOOST_PARSER_DEFINE_RULES`]]
 [def _AGGR_SIZE_           [macroref BOOST_PARSER_MAX_AGGREGATE_SIZE `BOOST_PARSER_MAX_AGGREGATE_SIZE`]]
 [def _SUBRNG_              [macroref BOOST_PARSER_SUBRANGE `BOOST_PARSER_SUBRANGE`]]
+[def _DISABLE_TRACE_       [macroref BOOST_PARSER_DISABLE_TRACE `BOOST_PARSER_DISABLE_TRACE`]]
 
 [def __p_                  [globalref boost::parser::_p `_p`]]
 

doc/tutorial.qbk

@@ -3766,6 +3766,22 @@ Some things to be aware of when looking at _Parser_ trace output:
   produces that value.  In these cases, you'll see the resolved value of the
   parse argument.
 
+[heading Compile-time trace disabling]
+
+While trace mode is very useful for debugging, it does have some overhead.
+Most parser templates are instantiated twice: Once with tracing enabled,
+once with tracing disabled.
+If you want to completely disable trace functionality at compile time, you can define
+`BOOST_PARSER_DISABLE_TRACE` before including any Boost.Parser headers:
+
+[teletype]``
+#define BOOST_PARSER_DISABLE_TRACE
+#include <boost/parser/parser.hpp>
+``
+
+When this define is set, all trace-related code is compiled out,
+reducing the compile time.
+
 [endsect]
 
 [section Memory Allocation]
@@ -3983,7 +3999,8 @@ This defines a RAII trace object that will produce the verbose trace requested
 by the user if they passed `_trace_::on` to the top-level parse.  It only has
 effect if `detail::enable_trace(flags)` is `true`.  If trace is enabled, it
 will show the state of the parse at the point at which it is defined, and then
-again when it goes out of scope.
+again when it goes out of scope. By defining `BOOST_PARSER_DISABLE_TRACE`,
+the trace feature can be disabled globally at compile time.
 
 [important For the tracing code to work, you must define an overload of
 `detail::print_parser` for your new parser type/template.  See

include/boost/parser/config.hpp

@@ -36,6 +36,10 @@
     disable the use of concepts, define this macro. */
 #    define BOOST_PARSER_DISABLE_CONCEPTS
 
+/** Boost.Parser will generate code to trace the execution of each and every
+    parser by default.  To disable all trace code, define this macro. */
+#    define BOOST_PARSER_DISABLE_TRACE
+
 /** Define this macro to use `boost::hana::tuple` instead of `std::tuple`
     throughout Boost.Parser. */
 #    define BOOST_PARSER_USE_HANA_TUPLE
@@ -92,6 +96,12 @@
 #    define BOOST_PARSER_USE_CONCEPTS 0
 #endif
 
+#if defined(BOOST_PARSER_DISABLE_TRACE)
+#    define BOOST_PARSER_DO_TRACE 0
+#else
+#    define BOOST_PARSER_DO_TRACE 1
+#endif
+
 #if defined(__cpp_lib_ranges) && BOOST_PARSER_USE_CONCEPTS
 #    define BOOST_PARSER_SUBRANGE std::ranges::subrange
 #else

include/boost/parser/detail/printing.hpp

@@ -593,11 +593,14 @@ namespace boost { namespace parser { namespace detail {
     }
 
     template<typename Context, typename T>
-    auto resolve(Context const & context, T const & x);
+    decltype(auto) resolve(Context const & context, T const & x);
 
     template<typename Context>
     auto resolve(Context const &, nope n);
 
+    template<typename DependentType, bool DoTraceMacro>
+    constexpr bool trace_disabled = !DoTraceMacro;
+
     template<
         bool DoTrace,
         typename Iter,
@@ -606,6 +609,8 @@ namespace boost { namespace parser { namespace detail {
         typename Attribute>
     struct scoped_trace_t
     {
+        static_assert(!trace_disabled<Iter, BOOST_PARSER_DO_TRACE>);
+
         scoped_trace_t(
             std::ostream & os,
             Iter & first,
@@ -681,6 +686,9 @@ namespace boost { namespace parser { namespace detail {
         flags f,
         Attribute const & attr)
     {
+        if constexpr (!BOOST_PARSER_DO_TRACE)
+            return;
+
         if constexpr (Context::do_trace) {
             std::stringstream oss;
             detail::print_parser(context, parser, oss);
@@ -695,6 +703,9 @@ namespace boost { namespace parser { namespace detail {
     template<typename Context, typename Attribute>
     auto final_trace(Context const & context, flags f, Attribute const & attr)
     {
+        if constexpr (!BOOST_PARSER_DO_TRACE)
+            return;
+
         if (!detail::do_trace(f))
             return;
 

include/boost/parser/error_handling.hpp

@@ -90,6 +90,9 @@ namespace boost { namespace parser {
         os << ":\n";
 
         std::string underlining(std::distance(position.line_start, it), ' ');
+        std::transform(position.line_start, it,
+            underlining.begin(),
+            [](auto c) { return c == '\t' ? '\t' : ' ';});
         detail::trace_input(os, position.line_start, it, false, 1u << 31);
         if (it == last) {
             os << '\n' << underlining << "^\n";

include/boost/parser/parser.hpp

@@ -1020,7 +1020,7 @@ namespace boost { namespace parser {
             bool Callable = is_detected_v<callable, T const &, Context const &>>
         struct resolve_impl
         {
-            static auto call(Context const &, T const & x) { return x; }
+            static auto& call(Context const &, T const & x) { return x; }
         };
 
         template<typename Context, typename T>
@@ -1033,7 +1033,7 @@ namespace boost { namespace parser {
         };
 
         template<typename Context, typename T>
-        auto resolve(Context const & context, T const & x)
+        decltype(auto) resolve(Context const & context, T const & x)
         {
             return resolve_impl<Context, T>::call(context, x);
         }
@@ -2168,7 +2168,10 @@ namespace boost { namespace parser {
         {
             if (!gen_attrs || !x)
                 return;
-            c.insert(c.end(), x->begin(), x->end());
+            c.insert(c.end(),
+                std::make_move_iterator(x->begin()),
+                std::make_move_iterator(x->end())
+                );
         }
 
         template<typename Container, typename T>
@@ -2851,7 +2854,7 @@ namespace boost { namespace parser {
                                   last,
                                   parse_error<I>(first, "end of input")) ==
                                   error_handler_result::rethrow) {
-                    throw;
+                    throw parse_error<I>(first, "end of input");
                 }
                 if constexpr (std::is_same_v<T, bool>)
                     retval = false;
@@ -3177,6 +3180,7 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this,
                 first,
@@ -3185,6 +3189,7 @@ namespace boost { namespace parser {
                 detail::in_apply_parser(flags) ? detail::disable_trace(flags)
                                                : flags,
                 retval);
+#endif
 
             if constexpr (detail::is_optional_v<Attribute>) {
                 detail::optional_type<Attribute> attr;
@@ -3341,8 +3346,10 @@ namespace boost { namespace parser {
             Attribute & retval) const
         {
             //[ opt_parser_trace
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
             //]
 
             //[ opt_parser_skip
@@ -3505,8 +3512,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             use_parser_t<Iter, Sentinel, Context, SkipParser> const use_parser{
                 first, last, context, skip, flags, success};
@@ -3529,7 +3538,9 @@ namespace boost { namespace parser {
                 else
                     use_parser.first_ = prev_first;
             };
-            detail::hl::for_each(parsers_, try_parser); // TODO: -> fold-expr
+            std::apply([&try_parser](auto&&... args) {
+                ((try_parser(args)), ...);
+            }, parsers_);
 
             if (!done)
                 success = false;
@@ -3627,8 +3638,10 @@ namespace boost { namespace parser {
                 decltype(detail::hl::transform(parsers_, use_parser));
             result_t retval{};
 
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first_, last, context, flags, retval);
+#endif
 
             call_impl(
                 first,
@@ -3663,8 +3676,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first_, last, context, flags, retval);
+#endif
 
             Iter first = first_;
             use_parser_t<Iter, Sentinel, Context, SkipParser> const use_parser{
@@ -3711,7 +3726,7 @@ namespace boost { namespace parser {
                     if constexpr (detail::is_struct_compatible_v<
                                       Attribute,
                                       result_t>) {
-                        detail::assign(retval, temp_retval);
+                        detail::assign(retval, std::move(temp_retval));
                     } else {
                         detail::assign(
                             retval,
@@ -4290,12 +4305,13 @@ namespace boost { namespace parser {
         {
             Iter first = first_;
 
-            auto temp_result =
-                make_temp_result(first, last, context, skip, flags, success);
+            using temp_result_t =
+                decltype(make_temp_result(first, last, context, skip, flags, success));
 
-            std::decay_t<decltype(parser::get(temp_result, llong<0>{}))>
+            std::decay_t<decltype(parser::get(std::declval<temp_result_t>(), llong<0>{}))>
                 retval{};
 
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this,
                 first_,
@@ -4304,10 +4320,11 @@ namespace boost { namespace parser {
                 detail::in_apply_parser(flags) ? detail::disable_trace(flags)
                                                : flags,
                 retval);
+#endif
 
-            std::decay_t<decltype(parser::get(temp_result, llong<1>{}))>
+            std::decay_t<decltype(parser::get(std::declval<temp_result_t>(), llong<1>{}))>
                 indices;
-            std::decay_t<decltype(parser::get(temp_result, llong<2>{}))>
+            std::decay_t<decltype(parser::get(std::declval<temp_result_t>(), llong<2>{}))>
                 merged;
             call_impl(
                 first,
@@ -4326,7 +4343,7 @@ namespace boost { namespace parser {
             // A 1-tuple is converted to a scalar.
             if constexpr (detail::hl::size(retval) == llong<1>{}) {
                 using namespace literals;
-                return parser::get(retval, 0_c);
+                return parser::get(std::move(retval), 0_c);
             } else {
                 return retval;
             }
@@ -4347,6 +4364,7 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this,
                 first_,
@@ -4355,16 +4373,17 @@ namespace boost { namespace parser {
                 detail::in_apply_parser(flags) ? detail::disable_trace(flags)
                                                : flags,
                 retval);
+#endif
 
             Iter first = first_;
 
-            auto temp_result =
-                make_temp_result(first, last, context, skip, flags, success);
+            using temp_result_t =
+                decltype(make_temp_result(first, last, context, skip, flags, success));
             using temp_result_attr_t =
-                std::decay_t<decltype(parser::get(temp_result, llong<0>{}))>;
-            std::decay_t<decltype(parser::get(temp_result, llong<1>{}))>
+                std::decay_t<decltype(parser::get(std::declval<temp_result_t>(), llong<0>{}))>;
+            std::decay_t<decltype(parser::get(std::declval<temp_result_t>(), llong<1>{}))>
                 indices;
-            std::decay_t<decltype(parser::get(temp_result, llong<2>{}))> merged;
+            std::decay_t<decltype(parser::get(std::declval<temp_result_t>(), llong<2>{}))> merged;
 
             auto max_ = [](auto result, auto x) {
                 if constexpr (decltype(result)::value < decltype(x)::value) {
@@ -4562,7 +4581,7 @@ namespace boost { namespace parser {
                     using just_out = detail::remove_cv_ref_t<decltype(out)>;
                     if constexpr (detail::is_nope_v<attr_t>) {
                         // nothing to do
-                    } if constexpr (
+                    } else if constexpr (
                         (!out_container ||
                          !std::is_same_v<just_x, just_out>) &&
                         std::is_assignable_v<just_out &, just_x &&> &&
@@ -4578,7 +4597,9 @@ namespace boost { namespace parser {
 
             auto const parsers_and_indices =
                 detail::hl::zip(parsers_, indices, merged, backtracking{});
-            detail::hl::for_each(parsers_and_indices, use_parser);
+            std::apply([&use_parser](auto&&... args) {
+                ((use_parser(args)), ...);
+            }, parsers_and_indices);
         }
 
         template<bool AllowBacktracking, typename Parser>
@@ -4699,8 +4720,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             auto const initial_first = first;
             auto attr = parser_.call(
@@ -4765,8 +4788,10 @@ namespace boost { namespace parser {
             detail::flags flags,
             bool & success) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, detail::global_nope);
+#endif
             auto attr =
                 parser_.call(first, last, context, skip, flags, success);
             if (success && detail::gen_attrs(flags))
@@ -4790,8 +4815,11 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
+
             auto attr =
                 parser_.call(first, last, context, skip, flags, success);
             if (success && detail::gen_attrs(flags))
@@ -4818,8 +4846,10 @@ namespace boost { namespace parser {
             detail::flags flags,
             bool & success) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, detail::global_nope);
+#endif
 
             parser_.call(
                 first,
@@ -4846,8 +4876,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             parser_.call(
                 first,
@@ -4897,8 +4929,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             auto const initial_first = first;
             parser_.call(
@@ -4962,8 +4996,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             auto const initial_first = first;
             parser_.call(
@@ -5038,8 +5074,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             parser_.call(
                 first,
@@ -5098,8 +5136,10 @@ namespace boost { namespace parser {
             auto context = context_;
             ++context.no_case_depth_;
 
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             parser_.call(first, last, context, skip, flags, success, retval);
         }
@@ -5145,8 +5185,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             if constexpr (detail::is_nope_v<SkipParser>) {
                 parser_.call(
@@ -5209,8 +5251,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             auto first_copy = first;
             parser_.call(
@@ -5376,8 +5420,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             auto [trie, _0] = detail::get_trie(context, ref());
             auto const lookup = context.no_case_depth_
@@ -5451,8 +5497,10 @@ namespace boost { namespace parser {
             tag_type * const tag_ptr = nullptr;
             auto const rule_context = detail::make_rule_context(
                 context, tag_ptr, retval, locals, params);
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, rule_context, flags, retval);
+#endif
 
             bool dont_assign = false;
             if constexpr (in_recursion) {
@@ -5481,7 +5529,7 @@ namespace boost { namespace parser {
                     dont_assign);
                 if (success && !dont_assign) {
                     if constexpr (!detail::is_nope_v<decltype(attr)>)
-                        detail::assign(retval, attr);
+                        detail::assign(retval, std::move(attr));
                 }
             }
 
@@ -5545,11 +5593,14 @@ namespace boost { namespace parser {
                 locals_type locals = detail::make_locals<locals_type>(context);
                 auto params = detail::resolve_rule_params(context, params_);
                 tag_type * const tag_ptr = nullptr;
+                attr_type attr{};
                 auto const rule_context = detail::make_rule_context(
-                    context, tag_ptr, retval, locals, params);
+                    context, tag_ptr, attr, locals, params);
 
+#if BOOST_PARSER_DO_TRACE
                 [[maybe_unused]] auto _ = detail::scoped_trace(
                     *this, first, last, rule_context, flags, retval);
+#endif
 
                 bool dont_assign = false;
                 parse_rule(
@@ -5561,9 +5612,22 @@ namespace boost { namespace parser {
                     flags,
                     success,
                     dont_assign,
-                    retval);
-                if (!success || dont_assign)
-                    retval = Attribute_();
+                    attr);
+
+                if (dont_assign)
+                    return;
+
+                if (!success)
+                    attr = attr_type{};
+
+                if constexpr (detail::is_nope_v<decltype(attr)>) {
+                    return;
+                } else if constexpr (
+                    container<Attribute_> && container<attr_type>) {
+                    detail::move_back(retval, attr, detail::gen_attrs(flags));
+                } else {
+                    detail::assign(retval, std::move(attr));
+                }
             }
         }
 
@@ -6726,8 +6790,10 @@ namespace boost { namespace parser {
             detail::flags flags,
             bool & success) const noexcept
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, detail::global_nope);
+#endif
             BOOST_PARSER_SUBRANGE const where(first, first);
             auto const predicate_context = detail::make_action_context(
                 context, detail::global_nope, where);
@@ -6754,8 +6820,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
             BOOST_PARSER_SUBRANGE const where(first, first);
             auto const predicate_context = detail::make_action_context(
                 context, detail::global_nope, where);
@@ -6805,8 +6873,10 @@ namespace boost { namespace parser {
             detail::flags flags,
             bool & success) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, detail::global_nope);
+#endif
             if (first != last)
                 success = false;
             return {};
@@ -6827,8 +6897,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
             if (first != last)
                 success = false;
         }
@@ -6857,8 +6929,10 @@ namespace boost { namespace parser {
             detail::flags flags,
             bool &) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, detail::global_nope);
+#endif
             return detail::resolve(context, attr_);
         }
 
@@ -6877,8 +6951,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute_ & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
             if (detail::gen_attrs(flags))
                 detail::assign_copy(retval, detail::resolve(context, attr_));
         }
@@ -6943,8 +7019,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             if (first == last) {
                 success = false;
@@ -7108,8 +7186,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             if (first == last) {
                 success = false;
@@ -7259,8 +7339,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             if (first == last) {
                 success = false;
@@ -7339,8 +7421,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             if (first == last) {
                 success = false;
@@ -7461,8 +7545,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             if (first == last) {
                 success = false;
@@ -7625,8 +7711,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             if (first == last) {
                 success = false;
@@ -7635,9 +7723,10 @@ namespace boost { namespace parser {
 
             auto const prev_first = first;
 
-            auto append = [&retval,
+            std::string temp;
+            auto append = [&temp,
                            gen_attrs = detail::gen_attrs(flags)](auto & ctx) {
-                detail::move_back(retval, _attr(ctx), gen_attrs);
+                detail::move_back(temp, _attr(ctx), gen_attrs);
             };
 
             auto quote_ch = [&]() {
@@ -7695,7 +7784,10 @@ namespace boost { namespace parser {
                 detail::disable_skip(flags),
                 success);
 
-            if (!success) {
+            if (success) {
+                if (detail::gen_attrs(flags))
+                    detail::assign(retval, std::move(temp));
+            } else {
                 retval = Attribute();
                 first = prev_first;
             }
@@ -7942,8 +8034,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             if (first == last) {
                 success = false;
@@ -8109,8 +8203,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
 
             auto compare =
                 [no_case = context.no_case_depth_](char32_t a, char32_t b) {
@@ -8198,8 +8294,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
             T attr = 0;
             auto const initial = first;
             success =
@@ -8339,8 +8437,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
             T attr = 0;
             auto const initial = first;
             success =
@@ -8454,8 +8554,10 @@ namespace boost { namespace parser {
             bool & success,
             Attribute & retval) const
         {
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
             T attr = 0;
             auto const initial = first;
             success = detail::numeric::parse_real(first, last, attr);
@@ -8521,9 +8623,9 @@ namespace boost { namespace parser {
     template<typename SwitchValue, typename OrParser>
     struct switch_parser
     {
-        switch_parser() {}
-        switch_parser(SwitchValue switch_value) : switch_value_(switch_value) {}
-        switch_parser(SwitchValue switch_value, OrParser or_parser) :
+        constexpr switch_parser() {}
+        constexpr switch_parser(SwitchValue switch_value) : switch_value_(switch_value) {}
+        constexpr switch_parser(SwitchValue switch_value, OrParser or_parser) :
             switch_value_(switch_value), or_parser_(or_parser)
         {}
 
@@ -8548,8 +8650,10 @@ namespace boost { namespace parser {
             using attr_t = decltype(or_parser_.call(
                 first, last, context, skip, flags, success));
             attr_t attr{};
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ =
                 detail::scoped_trace(*this, first, last, context, flags, attr);
+#endif
             attr = or_parser_.call(first, last, context, skip, flags, success);
             return attr;
         }
@@ -8574,8 +8678,10 @@ namespace boost { namespace parser {
                  "It looks like you tried to write switch_(val).  You need at "
                  "least one alternative, like: switch_(val)(value_1, "
                  "parser_1)(value_2, parser_2)..."));
+#if BOOST_PARSER_DO_TRACE
             [[maybe_unused]] auto _ = detail::scoped_trace(
                 *this, first, last, context, flags, retval);
+#endif
             or_parser_.call(first, last, context, skip, flags, success, retval);
         }
 
@@ -9034,8 +9140,13 @@ namespace boost { namespace parser {
                 "fill in attr above, using the attribute generated by parser. "
                 "However, parser does not generate an attribute.");
             if (trace_mode == trace::on) {
-                return reset = detail::parse_impl<true>(
-                           first, last, parser, parser.error_handler_, attr);
+                return reset =
+                           detail::parse_impl<(true && BOOST_PARSER_DO_TRACE)>(
+                               first,
+                               last,
+                               parser,
+                               parser.error_handler_,
+                               attr);
             } else {
                 return reset = detail::parse_impl<false>(
                            first, last, parser, parser.error_handler_, attr);
@@ -9052,8 +9163,9 @@ namespace boost { namespace parser {
                 "fill in attr above, using the attribute generated by parser. "
                 "However, parser does not generate an attribute.");
             if (trace_mode == trace::on) {
-                return reset = detail::parse_impl<true>(
-                           f, l, parser, parser.error_handler_, attr);
+                return reset =
+                           detail::parse_impl<(true && BOOST_PARSER_DO_TRACE)>(
+                               f, l, parser, parser.error_handler_, attr);
             } else {
                 return reset = detail::parse_impl<false>(
                            f, l, parser, parser.error_handler_, attr);
@@ -9156,7 +9268,7 @@ namespace boost { namespace parser {
     {
         if constexpr (!detail::is_char8_iter_v<I>) {
             if (trace_mode == trace::on) {
-                return detail::parse_impl<true>(
+                return detail::parse_impl<(true && BOOST_PARSER_DO_TRACE)>(
                     first, last, parser, parser.error_handler_);
             } else {
                 return detail::parse_impl<false>(
@@ -9169,7 +9281,7 @@ namespace boost { namespace parser {
             auto const l = r.end();
             auto _ = detail::scoped_base_assign(first, f);
             if (trace_mode == trace::on) {
-                return detail::parse_impl<true>(
+                return detail::parse_impl<(true && BOOST_PARSER_DO_TRACE)>(
                     f, l, parser, parser.error_handler_);
             } else {
                 return detail::parse_impl<false>(
@@ -9275,7 +9387,8 @@ namespace boost { namespace parser {
                 "fill in attr above, using the attribute generated by parser. "
                 "However, parser does not generate an attribute.");
             if (trace_mode == trace::on) {
-                return reset = detail::skip_parse_impl<true>(
+                return reset = detail::skip_parse_impl<(
+                           true && BOOST_PARSER_DO_TRACE)>(
                            first,
                            last,
                            parser,
@@ -9303,7 +9416,8 @@ namespace boost { namespace parser {
                 "fill in attr above, using the attribute generated by parser. "
                 "However, parser does not generate an attribute.");
             if (trace_mode == trace::on) {
-                return reset = detail::skip_parse_impl<true>(
+                return reset = detail::skip_parse_impl<(
+                           true && BOOST_PARSER_DO_TRACE)>(
                            f, l, parser, skip, parser.error_handler_, attr);
             } else {
                 return reset = detail::skip_parse_impl<false>(
@@ -9408,7 +9522,7 @@ namespace boost { namespace parser {
     {
         if constexpr (!detail::is_char8_iter_v<I>) {
             if (trace_mode == trace::on) {
-                return detail::skip_parse_impl<true>(
+                return detail::skip_parse_impl<(true && BOOST_PARSER_DO_TRACE)>(
                     first, last, parser, skip, parser.error_handler_);
             } else {
                 return detail::skip_parse_impl<false>(
@@ -9421,7 +9535,7 @@ namespace boost { namespace parser {
             auto const l = r.end();
             auto _ = detail::scoped_base_assign(first, f);
             if (trace_mode == trace::on) {
-                return detail::skip_parse_impl<true>(
+                return detail::skip_parse_impl<(true && BOOST_PARSER_DO_TRACE)>(
                     f, l, parser, skip, parser.error_handler_);
             } else {
                 return detail::skip_parse_impl<false>(
@@ -9525,7 +9639,8 @@ namespace boost { namespace parser {
     {
         if constexpr (!detail::is_char8_iter_v<I>) {
             if (trace_mode == trace::on) {
-                return detail::callback_parse_impl<true>(
+                return detail::callback_parse_impl<(
+                    true && BOOST_PARSER_DO_TRACE)>(
                     first, last, parser, parser.error_handler_, callbacks);
             } else {
                 return detail::callback_parse_impl<false>(
@@ -9538,7 +9653,8 @@ namespace boost { namespace parser {
             auto const l = r.end();
             auto _ = detail::scoped_base_assign(first, f);
             if (trace_mode == trace::on) {
-                return detail::callback_parse_impl<true>(
+                return detail::callback_parse_impl<(
+                    true && BOOST_PARSER_DO_TRACE)>(
                     f, l, parser, parser.error_handler_, callbacks);
             } else {
                 return detail::callback_parse_impl<false>(
@@ -9650,7 +9766,8 @@ namespace boost { namespace parser {
     {
         if constexpr (!detail::is_char8_iter_v<I>) {
             if (trace_mode == trace::on) {
-                return detail::callback_skip_parse_impl<true>(
+                return detail::callback_skip_parse_impl<(
+                    true && BOOST_PARSER_DO_TRACE)>(
                     first,
                     last,
                     parser,
@@ -9673,7 +9790,8 @@ namespace boost { namespace parser {
             auto const l = r.end();
             auto _ = detail::scoped_base_assign(first, f);
             if (trace_mode == trace::on) {
-                return detail::callback_skip_parse_impl<true>(
+                return detail::callback_skip_parse_impl<(
+                    true && BOOST_PARSER_DO_TRACE)>(
                     f, l, parser, skip, parser.error_handler_, callbacks);
             } else {
                 return detail::callback_skip_parse_impl<false>(

test/CMakeLists.txt

@@ -82,6 +82,7 @@ add_test_executable(parser_seq_permutations_1)
 add_test_executable(parser_seq_permutations_2)
 add_test_executable(parser_or_permutations_1)
 add_test_executable(parser_or_permutations_2)
+add_test_executable(disable_trace)
 
 if (MSVC)
     add_executable(vs_output_tracing tracing.cpp)

test/disable_trace.cpp

@@ -0,0 +1,27 @@
+/**
+*   Copyright (C) 2025
+ *
+ *   Distributed under the Boost Software License, Version 1.0. (See
+ *   accompanying file LICENSE_1_0.txt or copy at
+ *   http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+#define BOOST_PARSER_DISABLE_TRACE
+
+#include <boost/core/lightweight_test.hpp>
+#include <boost/parser/parser.hpp>
+
+int main()
+{
+    namespace bp = boost::parser;
+    {
+        auto const parser =
+            bp::string("FOO") >> -(bp::string("bar") | bp::string("foo"));
+
+        auto result = bp::parse("FOOfoo", parser);
+        BOOST_TEST(result);
+        BOOST_TEST(bp::get(*result, bp::llong<0>{}) == std::string("FOO"));
+        BOOST_TEST(bp::get(*result, bp::llong<1>{}) == std::string("foo"));
+    }
+    return boost::report_errors();
+}

test/github_issues.cpp

@@ -501,6 +501,282 @@ void github_issue_285()
     BOOST_TEST(result.value().get() != nullptr);
 }
 
+void github_pr_290()
+{
+    namespace bp = boost::parser;
+
+    auto const pTest = bp::lit("TEST:") > -bp::quoted_string;
+
+    auto result = bp::parse("TEST: \"foo\"", pTest, bp::blank);
+    BOOST_TEST(result);
+    BOOST_TEST(*result == "foo");
+}
+
+namespace github_issue_294_ {
+    namespace bp = boost::parser;
+    struct Foo
+    {};
+    constexpr bp::rule<struct foo_parser_tag, std::shared_ptr<Foo>> foo_parser =
+        "foo_parser";
+    constexpr auto foo_parser_action = [](auto & ctx) {
+        std::shared_ptr<Foo> & val = _val(ctx);
+        val = std::shared_ptr<Foo>(new Foo{});
+    };
+    constexpr auto foo_parser_def = bp::eps[foo_parser_action];
+    struct Bar
+    {
+        std::shared_ptr<Foo> foo;
+    };
+    constexpr bp::rule<struct bar_parser_tag, std::shared_ptr<Bar>> bar_parser =
+        "bar_parser";
+    constexpr auto bar_parser_action = [](auto & ctx) {
+        std::shared_ptr<Bar> & val = _val(ctx);
+        val = std::shared_ptr<Bar>(new Bar{});
+        std::optional<std::shared_ptr<Foo>> & attr = _attr(ctx);
+        if (attr) {
+            val->foo = attr.value();
+        }
+    };
+    constexpr auto bar_parser_def =
+        (bp::lit("(") > -foo_parser > bp::lit(")"))[bar_parser_action];
+
+    BOOST_PARSER_DEFINE_RULES(bar_parser, foo_parser);
+}
+
+void github_issue_294()
+{
+    namespace bp = boost::parser;
+    using namespace github_issue_294_;
+
+    bp::parse("()", bar_parser, bp::blank);
+}
+
+namespace github_pr_297_ {
+    namespace bp = boost::parser;
+    constexpr auto bar_required_f = [](auto& ctx) -> bool {
+        const bool& argument = bp::_p<0>(ctx);
+        return argument;
+    };
+    constexpr bp::rule<struct foobar_parser_tag> foobar_parser = "foobar_parser";
+    constexpr auto foobar_parser_def =
+        bp::lit("foo")
+        >> bp::switch_(bar_required_f)
+            (true, bp::lit("bar"))
+            (false, -bp::lit("bar"));
+    BOOST_PARSER_DEFINE_RULES(foobar_parser);
+}
+
+void github_pr_297()
+{
+    namespace bp = boost::parser;
+    using namespace github_pr_297_;
+
+    {
+        const bool bar_required = true;
+        const bool result =
+            bp::parse("foo bar", foobar_parser.with(bar_required), bp::blank);
+        BOOST_TEST(result);
+    }
+    {
+        const bool bar_required = true;
+        const bool result =
+            bp::parse("foo", foobar_parser.with(bar_required), bp::blank);
+        BOOST_TEST(!result);
+    }
+    {
+        const bool bar_required = false;
+        const bool result =
+            bp::parse("foo bar", foobar_parser.with(bar_required), bp::blank);
+        BOOST_TEST(result);
+    }
+    {
+        const bool bar_required = false;
+        const bool result =
+            bp::parse("foo", foobar_parser.with(bar_required), bp::blank);
+        BOOST_TEST(result);
+    }
+}
+
+namespace github_issue_312_ {
+    /*
+     * Recursive descent parser for expressions.
+     * Supports addition (+), multiplication (*) and
+     * parethesized expressions, nothing else.
+     *
+     * Creates a tree of "evaluatable" objects which
+     * own their downstream objects in a unique_ptr
+     */
+
+    // base class for all tree nodes
+    struct evaluatable
+    {
+        virtual double evaluate() = 0;
+        virtual ~evaluatable() = default;
+    };
+
+    namespace bp = boost::parser;
+
+    // top level parser
+    constexpr bp::rule<struct expression_tag, std::unique_ptr<evaluatable>> expression_parser = "expression_parser";
+
+    /*
+     *      LITERAL EXPRESSION
+     */
+    struct literal_evaluatable : evaluatable
+    {
+        explicit literal_evaluatable(double v) : value_(v) {}
+        double evaluate() override
+        {
+            return value_;
+        }
+        double value_;
+    };
+    constexpr bp::rule<struct literal_tag, std::unique_ptr<evaluatable>> literal_parser = "literal_parser";
+    constexpr auto literal_parser_action = [](auto& ctx) {
+        std::unique_ptr<evaluatable>& val = _val(ctx);
+        double& parsed_value = _attr(ctx);
+        val = std::make_unique<literal_evaluatable>(parsed_value);
+    };
+    constexpr auto literal_parser_def =
+        bp::double_[literal_parser_action];
+
+    /*
+     *      PARENTHESIZED EXPRESSION
+     */
+    struct parenthesized_evaluatable : evaluatable
+    {
+        explicit parenthesized_evaluatable(std::unique_ptr<evaluatable>&& e) : evaluatable_(std::move(e)) {}
+        double evaluate() override
+        {
+            return evaluatable_->evaluate();
+        }
+        std::unique_ptr<evaluatable> evaluatable_;
+    };
+    constexpr bp::rule<struct parenthesized_tag, std::unique_ptr<evaluatable>> parenthesized_parser = "parenthesized_parser";
+    constexpr auto parenthesized_action = [](auto& ctx) {
+        std::unique_ptr<evaluatable>& val = _val(ctx);
+        std::unique_ptr<evaluatable>& attr = _attr(ctx);
+        val = std::make_unique<parenthesized_evaluatable>(std::move(attr));
+    };
+    constexpr auto parenthesized_parser_def =
+        (
+        bp::lit('(') > expression_parser > bp::lit(')')
+        )[parenthesized_action];
+
+    /*
+     *      ATOM EXPRESSION
+     */
+    struct atom_evaluatable : evaluatable
+    {
+        explicit atom_evaluatable(std::unique_ptr<evaluatable>&& e) : evaluatable_(std::move(e)) {}
+        double evaluate() override
+        {
+            return evaluatable_->evaluate();
+        }
+        std::unique_ptr<evaluatable> evaluatable_;
+    };
+    constexpr bp::rule<struct atom_tag, std::unique_ptr<evaluatable>> atom_parser = "atom_parser";
+    constexpr auto atom_action = [](auto& ctx) {
+        std::unique_ptr<evaluatable>& val = _val(ctx);
+        std::unique_ptr<evaluatable>& attr = _attr(ctx);
+        val = std::make_unique<atom_evaluatable>(std::move(attr));
+    };
+    constexpr auto atom_parser_def =
+        (
+            parenthesized_parser
+            |
+            literal_parser
+        )[atom_action];
+
+    /*
+     *      MULTIPLICATION EXPRESSION
+     */
+    struct multiplication_evaluatable : evaluatable
+    {
+        multiplication_evaluatable(std::vector<std::unique_ptr<evaluatable>>&& e)
+            : evaluatables_(std::move(e))
+        {}
+        double evaluate() override
+        {
+            double result = 1;
+            for (const auto& e : evaluatables_) {
+                result *= e->evaluate();
+            }
+            return result;
+        }
+        std::vector<std::unique_ptr<evaluatable>> evaluatables_;
+    };
+    constexpr bp::rule<struct mult_tag, std::unique_ptr<evaluatable>> mult_parser = "mult_parser";
+    constexpr auto mult_parser_action = [](auto& ctx) {
+        std::unique_ptr<evaluatable>& val = _val(ctx);
+        std::vector<std::unique_ptr<evaluatable>>& operands = _attr(ctx);
+        val = std::make_unique<multiplication_evaluatable>(std::move(operands));
+    };
+    constexpr auto mult_parser_def =
+        (atom_parser % bp::lit('*'))[mult_parser_action];
+
+    /*
+     *      ADDITION EXPRESSION
+     */
+    struct addition_evaluatable : evaluatable
+    {
+        addition_evaluatable(std::vector<std::unique_ptr<evaluatable>>&& e)
+            : evaluatables_(std::move(e))
+        {}
+        double evaluate() override
+        {
+            double result = 0;
+            for (const auto& e : evaluatables_) {
+                result += e->evaluate();
+            }
+            return result;
+        }
+        std::vector<std::unique_ptr<evaluatable>> evaluatables_;
+    };
+    constexpr bp::rule<struct add_tag, std::unique_ptr<evaluatable>> add_parser = "add_parser";
+    constexpr auto add_parser_action = [](auto& ctx) {
+        std::unique_ptr<evaluatable>& val = _val(ctx);
+        std::vector<std::unique_ptr<evaluatable>>& operands = _attr(ctx);
+        val = std::make_unique<addition_evaluatable>(std::move(operands));
+    };
+    constexpr auto add_parser_def =
+        (mult_parser % bp::lit('+'))[add_parser_action];
+
+    constexpr auto expression_parser_action = [](auto& ctx) {
+        std::unique_ptr<evaluatable>& val = _val(ctx);
+        std::unique_ptr<evaluatable>& attr = _attr(ctx);
+        val = std::move(attr);
+    };
+
+    /*
+     *      EXPRESSION
+     */
+    constexpr auto expression_parser_def =
+        add_parser[expression_parser_action];
+
+    BOOST_PARSER_DEFINE_RULES(
+        literal_parser,
+        mult_parser,
+        add_parser,
+        expression_parser,
+        parenthesized_parser,
+        atom_parser);
+}
+
+void github_issue_312()
+{
+    namespace bp = boost::parser;
+    using namespace github_issue_312_;
+
+    auto result = bp::parse("(2 + 3) + 3.1415 * 2", expression_parser, bp::blank);
+    BOOST_TEST(result);
+    BOOST_TEST(result.value()->evaluate() == (2 + 3) + 3.1415 * 2);
+
+    result = bp::parse("((2*0.1) + 33.) * (2 + 3) + 3.1415 * 2", expression_parser, bp::blank);
+    BOOST_TEST(result);
+    BOOST_TEST(result.value()->evaluate() == ((2*0.1) + 33.) * (2 + 3) + 3.1415 * 2);
+}
+
 
 int main()
 {
@@ -516,5 +792,9 @@ int main()
     github_issue_268();
     github_issue_279();
     github_issue_285();
+    github_pr_290();
+    github_issue_294();
+    github_pr_297();
+    github_issue_312();
     return boost::report_errors();
 }