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add TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA workaround MSVC error C2057 #247

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merged 1 commit into from
Feb 12, 2025
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add TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA workaround MSVC error C2057 #247

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1 change: 1 addition & 0 deletions README.md
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Expand Up @@ -290,6 +290,7 @@ UTF-8 decoding is performed using a state machine based on Bjoern Hoehrmann's '[
- **[@levicki](https://github.com/levicki)** - Helped design some new features
- **[@moorereason](https://github.com/moorereason)** - Reported a whole bunch of bugs
- **[@mosra](https://github.com/mosra)** - Created the awesome [m.css] used to generate the API docs
- **[@N-Dekker](https://github.com/N-Dekker)** - Added a workaround for the legacy lambda processor of MSVC 2019/2022
- **[@ned14](https://github.com/ned14)** - Reported a bunch of bugs and helped design some new features
- **[@okureta](https://github.com/okureta)** - Reported a bug
- **[@prince-chrismc](https://github.com/prince-chrismc)** - Added toml++ to ConanCenter, and fixed some typos
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3 changes: 3 additions & 0 deletions include/toml++/impl/array.hpp
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Expand Up @@ -951,7 +951,10 @@ TOML_NAMESPACE_START
static_cast<node_ref>(static_cast<Array&&>(arr)[i])
.visit(
[&]([[maybe_unused]] auto&& elem) //
// Define this macro as a workaround to compile errors caused by a bug in MSVC's "legacy lambda processor".
#if !TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA
noexcept(for_each_is_nothrow_one<Func&&, Array&&, decltype(elem)>::value)
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For what it's worth, I did try to work around the problem (locally) by introducing a variable template for_each_is_nothrow_one_v, in order to do:

    noexcept( for_each_is_nothrow_one_v<Func&&, Array&&, decltype(elem)> )

But it still did not make the old legacy lambda processor happy. 🤷

#endif
{
using elem_ref = for_each_elem_ref<decltype(elem), Array&&>;
static_assert(std::is_reference_v<elem_ref>);
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11 changes: 11 additions & 0 deletions include/toml++/impl/preprocessor.hpp
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Expand Up @@ -1241,6 +1241,17 @@ TOML_ENABLE_WARNINGS;
/// \detail Defaults to `0`.
//# }}

#ifndef TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA
#define TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA 0
#endif
//# {{
/// \def TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA
/// \brief Disable using noexcept(<condition>) in lambda definitions within the toml++ library implementation.
/// \detail This macro offers a workaround to a bug in the old "legacy lambda processor" of Visual C++, which
/// caused compile errors like "error C2057: expected constant expression", when it encountered such lambda's.
/// These compile errors were reported by Kevin Dick, Jan 19, 2024, at https://github.com/marzer/tomlplusplus/issues/219
//# }}

/// @}
//#====================================================================================================================
//# CHARCONV SUPPORT
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3 changes: 3 additions & 0 deletions include/toml++/impl/table.hpp
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Expand Up @@ -930,7 +930,10 @@ TOML_NAMESPACE_START
static_cast<node_ref>(*kvp.second)
.visit(
[&]([[maybe_unused]] auto&& v) //
// Define this macro as a workaround to compile errors caused by a bug in MSVC's "legacy lambda processor".
#if !TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA
noexcept(for_each_is_nothrow_one<Func&&, Table&&, decltype(v)>::value)
#endif
{
using value_ref = for_each_value_ref<decltype(v), Table&&>;
static_assert(std::is_reference_v<value_ref>);
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10 changes: 10 additions & 0 deletions toml.hpp
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Expand Up @@ -1133,6 +1133,10 @@ TOML_ENABLE_WARNINGS;
#define TOML_ENABLE_FLOAT16 0
#endif

#ifndef TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA
#define TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA 0
#endif

#if !defined(TOML_FLOAT_CHARCONV) && (TOML_GCC || TOML_CLANG || (TOML_ICC && !TOML_ICC_CL))
// not supported by any version of GCC or Clang as of 26/11/2020
// not supported by any version of ICC on Linux as of 11/01/2021
Expand Down Expand Up @@ -6799,7 +6803,10 @@ TOML_NAMESPACE_START
static_cast<node_ref>(static_cast<Array&&>(arr)[i])
.visit(
[&]([[maybe_unused]] auto&& elem) //
// Define this macro as a workaround to compile errors caused by a bug in MSVC's "legacy lambda processor".
#if !TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA
noexcept(for_each_is_nothrow_one<Func&&, Array&&, decltype(elem)>::value)
#endif
{
using elem_ref = for_each_elem_ref<decltype(elem), Array&&>;
static_assert(std::is_reference_v<elem_ref>);
Expand Down Expand Up @@ -8161,7 +8168,10 @@ TOML_NAMESPACE_START
static_cast<node_ref>(*kvp.second)
.visit(
[&]([[maybe_unused]] auto&& v) //
// Define this macro as a workaround to compile errors caused by a bug in MSVC's "legacy lambda processor".
#if !TOML_DISABLE_CONDITIONAL_NOEXCEPT_LAMBDA
noexcept(for_each_is_nothrow_one<Func&&, Table&&, decltype(v)>::value)
#endif
{
using value_ref = for_each_value_ref<decltype(v), Table&&>;
static_assert(std::is_reference_v<value_ref>);
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