tomlplusplus: Failure to compile with intel compiler on linux

Environment

toml++ version and/or commit hash:
toml.hpp v2.3.0 [9be51e4]

Compiler:
icpc (ICC) 19.1.0.166 20191121 icpc (ICC) 19.0.0.117 20180804

gcc backend: 7.4.0 (also tried 9.3.0)

C++ standard mode (e.g. 17, 20, ‘latest’):
17

Target arch (e.g. x64):
x64 (linux)

Library configuration overrides:
None

Relevant compilation flags:
-std=c++17

Describe the bug

Compiling a simple test program with icpc fails, complaining about issues with TOML_ALWAYS_INLINE. The same example works fine when compiling with gcc (the same version used as the backend for icpc). Other versions of gcc on the backend similarly fail.

In file included from test.cpp(3):
% icpc test.cpp -std=c++17
toml.hpp(1119): error #77: this declaration has no storage class or type specifier
  	TOML_ALWAYS_INLINE
... [[hundreds of lines of errors]]

Steps to reproduce (or a small repro code sample)

#include <iostream>
#include <fstream> //required for parse_file()
#include "toml.hpp"

int main(int argc, char** argv)
{
    toml::table tbl;
    try
    {
        tbl = toml::parse_file(argv[1]);
        std::cout << tbl << "\n";
    }
    catch (const toml::parse_error& err)
    {
        std::cerr << "Parsing failed:\n" << err << "\n";
        return 1;
    }

    return 0;
}

Additional information

Forcing #define TOML_ALWAYS_INLINE __forceinline will compile with no errors, but fail to link.

/tmp/icpcJYElhL.o: In function `toml::v2::default_formatter<char>::print(toml::v2::table const&)':
test.cpp:(.text._ZN4toml2v217default_formatterIcE5printERKNS0_5tableE[_ZN4toml2v217default_formatterIcE5printERKNS0_5tableE]+0x50): undefined reference to `toml::v2::node::type() const'
... [[et al.]]