fix MSVC C++20 module build error C2872, C2039, C2873 on 'literals'

[N-Dekker]

What does this change do?

Replaced toml with TOML_NAMESPACE in the using-declarations in the exported namespace.

Fixed the errors from Visual Studio 2022 version 17.13.5 Compiler Version 19.43.34809 that appeared when building "tomlplusplus_modules", saying:

src\modules\tomlpp.cppm(22,21): error C2872: 'literals': ambiguous symbol
    src\modules\tomlpp.cppm(21,22):
    could be 'toml::literals'
    include\toml++\impl\parser.hpp(322,19):
    or       'toml::v3::literals'
src\modules\tomlpp.cppm(22,31): error C2039: '""_toml': is not a member of 'toml::literals'
    src\modules\tomlpp.cppm(21,22):
    see declaration of 'toml::literals'
src\modules\tomlpp.cppm(22,9): error C2873: '""_toml': symbol cannot be used in a using-declaration

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

• Follow-up to pull request Allow tomlplusplus to be built by C++20 modules #266 commit fea1d90 by @mikomikotaishi

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]

@marzer Do you think the other using toml::<identifier>s within the export namespace toml clause should also be replaced with using TOML_NAMESPACE::<identifier>?`

It does compile, when doing:

    using TOML_NAMESPACE::array;
    using TOML_NAMESPACE::date;
    using TOML_NAMESPACE::date_time;
    ...
    using TOML_NAMESPACE::parse_file;

Instead of the current:

tomlplusplus/src/modules/tomlpp.cppm

Lines 26 to 66 in fea1d90

	using toml::array;
	using toml::date;
	using toml::date_time;
	using toml::inserter;
	using toml::json_formatter;
	using toml::key;
	using toml::node;
	using toml::node_view;
	using toml::parse_error;
	using toml::parse_result;
	using toml::path;
	using toml::path_component;
	using toml::source_position;
	using toml::source_region;
	using toml::table;
	using toml::time;
	using toml::time_offset;
	using toml::toml_formatter;
	using toml::value;
	using toml::yaml_formatter;
	using toml::format_flags;
	using toml::node_type;
	using toml::path_component_type;
	using toml::value_flags;
	using toml::array_iterator;
	using toml::const_array_iterator;
	using toml::const_table_iterator;
	using toml::default_formatter;
	using toml::inserted_type_of;
	using toml::optional;
	using toml::source_index;
	using toml::source_path_ptr;
	using toml::table_iterator;
	using toml::at_path;
	using toml::get_line;
	using toml::operator""_toml;
	using toml::operator""_tpath;
	using toml::operator<<;
	using toml::parse;
	using toml::parse_file;

[marzer]

Oh, hmmn, yeah. I completely forgot about all of that machinery.

In theory you should be able to simplify it by replacing

export namespace toml
{
	//...
}

with

export TOML_NAMESPACE_START
{
	// ...
}

because TOML_NAMESPACE_START is shorthand for namespace toml { inline namespace <internal abi namespace>. And/or individually qualifying them, yes.

I'll be honest in saying that I know very little about modules, so I am not too sure of the right approach here.

[N-Dekker]

Thanks @marzer So far I haven't been able to get export TOML_NAMESPACE_START compiling, so for now I'm proposing to just replace toml with TOML_NAMESPACE in those using-declarations. That will at least make MSVC slightly happier.

(Doing export TOML_NAMESPACE_START might be a nicer follow-up, of course)

[N-Dekker]

For the record, when I locally did export TOML_NAMESPACE_START ... TOML_NAMESPACE_END; in "src/modules/tomlpp.cppm", the module still compiled well, but "examples/simple_parser" encountered an internal compiler error! (I locally adjusted "simple_parser.cpp" by doing import tomlplusplus; instead of #include <toml++/toml.hpp>.) It produced the following output:

------ Build started: Project: simple_parser, Configuration: Debug x64 ------
simple_parser.cpp
S:\tomlplusplus\examples\simple_parser.cpp(30,20): error C1001: Internal compiler error.
S:\tomlplusplus\examples\simple_parser.cpp(30,20): error C1001: (compiler file 'D:\a\_work\1\s\src\vctools\Compiler\CxxFE\sl\p1\cxx\function-signature.cpp', line 212)
S:\tomlplusplus\examples\simple_parser.cpp(30,20): error C1001:  To work around this problem, try simplifying or changing the program near the locations listed above.
S:\tomlplusplus\examples\simple_parser.cpp(30,20): error C1001: If possible please provide a repro here: https://developercommunity.visualstudio.com 
S:\tomlplusplus\examples\simple_parser.cpp(30,20): error C1001: Please choose the Technical Support command on the Visual C++ 
S:\tomlplusplus\examples\simple_parser.cpp(30,20): error C1001:  Help menu, or open the Technical Support help file for more information
Done building project "simple_parser.vcxproj" -- FAILED.

[N-Dekker]

@marzer @mikomikotaishi I noticed that operator""_toml and operator""_tpath are exported twice.

Here, inside toml::literals:

tomlplusplus/src/modules/tomlpp.cppm

Lines 21 to 24 in fea1d90

	inline namespace literals {
	using toml::literals::operator""_toml;
	using toml::literals::operator""_tpath;
	}

And here, at the root of the toml namespace:

tomlplusplus/src/modules/tomlpp.cppm

Lines 62 to 63 in fea1d90

	using toml::operator""_toml;
	using toml::operator""_tpath;

Was that intended?

[mikomikotaishi]

Hmm, I thought it would be necessary to re-export the contents of literals, and on my end it compiles fine - but if it is only causing more harm than benefit, perhaps it should be removed entirely? I am not entirely sure what the literals namespace is for myself.

[N-Dekker]

@mikomikotaishi Well, it just isn't clear to me why operator""_toml and operator""_tpath are exported twice (once in toml::literals and once in toml), but if you have a use case that only works when they are exported twice, can you please share it?

Anyway, this pull request already appears to fix those MSVC compile errors on literals, so technically it seems to work, exporting those operators twice.

[marzer]

@N-Dekker sorry, just coming back to this now. It looks complete, but was there anything still outstanding?

[N-Dekker]

@marzer No problem! The pull request itself is complete, I think. Unfortunately, there were still some other compile errors, when I last tried the module with MSVC. But these errors were unrelated. 🤷 I hope I was just missing an MSVC patch, when I tried!