Make `request_parser` movable without changing body type

[xbreak]

Hi,

Version boost 1.80/330

Question/Request

Is there a technical motivation why request_parser is not move-constructible without changing body type? If not, could this be allowed?

Motivation

I have a use-case where an initial HTTP session object reads header (async_read_header) using request_parser<empty_body> and then delegates the rest of the handling to another classes (such as reading body if there is any) . Here I was thinking of passing request_parser by value, but that is currently not possible.

Since it's allowed to change body types before body parsing has begun, this (seemingly) can be worked around by needlessly changing body type:

#include <boost/beast/http.hpp>

namespace http = boost::beast::http;

void allowed() {
    http::request_parser<http::empty_body> req0;
    http::request_parser<http::string_body> req1{std::move(req0)};
    http::request_parser<http::empty_body> req2{std::move(req1)};
}

void disallowed() {
    http::request_parser<http::empty_body> req0;
    http::request_parser<http::empty_body> req1{std::move(req0)}; // error: use of deleted function
}

But moving without changing body type yields:

<source>: In function 'void disallowed()':
<source>:13:64: error: use of deleted function 'boost::beast::http::parser<isRequest, Body, Fields>::parser(boost::beast::http::parser<isRequest, Body, Fields>&&) [with bool isRequest = true; Body = boost::beast::http::empty_body; Allocator = std::allocator<char>]'
   13 |     http::request_parser<http::empty_body> req1{std::move(req0)}; // error: use of deleted function
      |                                                                ^
In file included from /opt/compiler-explorer/libs/boost_1_80_0/boost/beast/http.hpp:27,
                 from <source>:1:
/opt/compiler-explorer/libs/boost_1_80_0/boost/beast/http/parser.hpp:91:5: note: declared here
   91 |     parser(parser&& other) = delete;
      |     ^~~~~~
Compiler returned: 1

https://godbolt.org/z/zEaqsqevc

[madmongo1]

Hmm... no idea. @vinniefalco ?

[klemens-morgenstern]

Sounds like it should be allowed to me/

[vinniefalco]

you could always put it in a unique_ptr... I am concerned that playing with the move constructor will break people who are using it to transform the body type. If you are 112% certain you can make it movable without breaking anything, then ok.

[xbreak]

Thanks, yes there are various options.

This is not critical thing to us, but I thought the behavior was a bit... odd. In the interest of not keeping low priority things open indefinitely I'll simply close it.