TOML_DISABLE_NOEXCEPT_NOEXCEPT workaround MSVC bug noexcept + modules

[N-Dekker]

What does this change do?

Allowed users of the tomlplusplus module to compile using MSVC.

Without TOML_DISABLE_NOEXCEPT_NOEXCEPT, MSVC appears to produce the following error:

include\toml++\impl\value.hpp(270,39): error C3878: syntax error: unexpected token ',' following 'simple-type-specifier'
missing one of: '(' '{' ?
include\toml++\impl\value.hpp(270,39):
the template instantiation context (the oldest one first) is
include\toml++\impl\parser.inl(2757,26):
while compiling class template member function 'toml::v3::value<int64_t>::value<int64_t,0>(int64_t &&) noexcept()'

Is it related to an existing bug report or feature request?

• It is another follow-up to pull request Allow tomlplusplus to be built by C++20 modules #266

Pre-merge checklist

• I've read CONTRIBUTING.md
• I've rebased my changes against the current HEAD of origin/master (if necessary)
• I've added new test cases to verify my change
• I've regenerated toml.hpp (how-to)
• I've updated any affected documentation
• I've rebuilt and run the tests with at least one of:
  • Clang 8 or higher
  • GCC 8 or higher
  • MSVC 19.20 (Visual Studio 2019) or higher
• I've added my name to the list of contributors in README.md

[N-Dekker]

This is the only use of noexcept(noexcept(<expression>)) in toml++, and while a regular MSVC build appears to be able to parse it, it fails when using the tomlplusplus module (doing a C++20 compilation).

[N-Dekker]

Honestly, MSVC does compile noexcept(noexcept(<expression>)) in other situations. I don't know exactly in which particular situations noexcept(noexcept(<expression>)) triggers an error, but this is one of them 🤷

[N-Dekker]

Still "Draft" because I haven't yet figured out if this is a known compiler bug. Otherwise, maybe it should be reported!

The added value of this particular noexcept specifier looks rather small, right? 🤷 So then, is it OK to put it inside an #if ... #endif?

[marzer]

Hmmn, might be able to work around this by extracting the inner noexcept() out into a separate typedef or template variable, then using that in the outer noexcept()?

Otherwise yeah I'm OK with a switch to turn it off.

[N-Dekker]

My feeling is that the combination of variadic templates + noexcept(noexcept(...)) + modules is more than MSVC can handle. You can use each feature, just don't combine them 😸