c++17 : lambda to std::function conversion failure - c++

Question

I'm currently exploring c++17 additions. After playing around with std::variant, wanted to use std::optional too, with the same example. Currently seeing that the compilation fails because of the following error:
error: no viable conversion from returned value of type
'(lambda at ./html_parser.hpp:53:9)' to function return type 'Parser<char>' (aka
'std::__1::function<std::__1::optional<std::__1::pair<char, std::__1::basic_string<char> > >
(std::__1::basic_string<char>)>')
return [=](std::string& input) -> ParserResult<char> {
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/acid/tools/include/c++/v1/functional:1627:5: note: candidate constructor not viable: no known conversion
from '(lambda at ./html_parser.hpp:53:9)' to 'std::nullptr_t' (aka 'nullptr_t') for 1st argument
function(nullptr_t) _NOEXCEPT : __f_(0) {}
^
/home/acid/tools/include/c++/v1/functional:1628:5: note: candidate constructor not viable: no known conversion
from '(lambda at ./html_parser.hpp:53:9)' to 'const
std::__1::function<std::__1::optional<std::__1::pair<char, std::__1::basic_string<char> > >
(std::__1::basic_string<char>)> &' for 1st argument
function(const function&);
^
/home/acid/tools/include/c++/v1/functional:1629:5: note: candidate constructor not viable: no known conversion
from '(lambda at ./html_parser.hpp:53:9)' to 'std::__1::function<std::__1::optional<std::__1::pair<char,
std::__1::basic_string<char> > > (std::__1::basic_string<char>)> &&' for 1st argument
function(function&&) _NOEXCEPT;
^
/home/acid/tools/include/c++/v1/functional:1631:5: note: candidate template ignored: requirement
'__callable<(lambda at ./html_parser.hpp:53:9)>::value' was not satisfied [with _Fp =
(lambda at ./html_parser.hpp:53:9)]
function(_Fp);
^
1 error generated.
To parse the HTML to give the DOM, started with declaring some parser combinators as follows:
#pragma once
#include <string>
#include <utility>
#include <functional>
#include <optional>
namespace dragon {
namespace html {
namespace parser {
template <typename ParserOutput, typename ParserInput = std::string>
using ParserResult = std::optional<std::pair<ParserOutput, ParserInput>>;
template<typename ParserOutput, typename ParserInput = std::string>
using Parser = std::function<ParserResult<ParserOutput, ParserInput>(ParserInput)>;
template <typename ParserOutput, typename ParserInput = std::string>
auto parse(Parser<ParserOutput, ParserInput> p, ParserInput i) -> ParserResult<ParserOutput, ParserInput>{
return p(i);
}
// few parser combinators.
// thenP combinator: applies the first parser, if it succeeds apply the second to the rest of
// the input left over by the first parser.
// currently just fails and returns empty!! does not provide any debugging info/msg
// as to why the parsing failed.
template<typename FirstParser, typename SecondParser>
auto thenP(FirstParser f, SecondParser s) {
return [=](std::string input) -> decltype(parse(s, std::string())) {
auto fv = parse(f, input);
if (fv) {
auto fvv = *fv;
return parse(s, fvv.second);
}
else {
return {};
}
};
}
template<typename FirstParser, typename SecondParser>
auto choiceP(FirstParser f, SecondParser s) {
return [=](std::string input) {
auto fv = parse(f, input);
if (!fv) return parse(s, input);
return fv;
};
}
auto charP(char match) -> Parser<char> {
return [=](std::string& input) -> ParserResult<char> {
if ((input.empty() == false) && (input[0] == match)) {
return std::make_pair(input[0], input.substr(1));
}
return {};
};
}
}
}
}
I'm seeing the above error, when trying to compile the simple use as shown below:
int main()
{
auto less = Parser::parser::charP('<');
auto greater = Parser::parser::charP('>');
auto lag = Parser::parser::thenP(less, greater);
auto log = Parser::parser::choiceP(less, greater);
auto lagv = lag("<>");
auto logv = log("|>");
return 0;
}
This compiles fine with Visual Studio 2017 (std=c++-latest). But Clang gives the above error. Trying to figure out the discrepancy between these two compilers. and how to fix this issue with Clang.

Answer 1

This is ill-formed:
auto charP(char match) -> Parser<char> {
return [=](std::string& input) -> ParserResult<char> { ... };
}
for the same reason that this is ill-formed:
std::function<void(int)> f = [](int& ){};
The lambda on the right is not invocable with an int, only an int&, so you can't construct a function<void(int)> out of it. MSVC has some permissive mode where it allows constructing a non-const lvalue reference from an rvalue, which is probably why it worked.